TESTING FOR C E M E N T - A D M I X T U R E I N C O M P A T I B I L I T Y

EDGAR FURTADO

A Thesis Submitted in Conformity With the Requirements For the Degree o f Master o f Applied Science in Civil Engineering

Concrete Materials Group Graduate Department of Civil Engineering

University o f Toronto

O Copyright by Edgar Furtado 1999

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TESTING FOR CEMENT-ADMIXI'URE INCOMPATIBILITY

Edgar Furtado (MA&. - 1999)

Department of C i d Engineering Universic). of Toronto

Although chefnicd admi~tures are not an essentiai component in the production

of concrete they do prove to be induable ingredients whm produckg a h g h qualirg

concrete. Chernical admixtures are incorporated into concrete to yield both economic

and physiral hece5rr. However, it is important that an understandmg of their

compatibility with certain cernents, or \\ith other adxnisunrres, be established before they

are used in practical applications.

The aLn of this project is to determine the cornpatibility of commerady

availabie \vz ter reducers, superpias acizers, and air en training agents. The compa tibility of

admktures is determined by the loss of workability of cernent pastes and concretes with

respect to t h e . This project utilises small scaie laboratory tests, inducimg Kantro's

miniature Slump test and the Foam Index test as proposed by VH. Dodson.

1 w-ould E e to express my gratitude to Professor R. D. Hooton, P.Eng., of the Departxnent of Cilil Engineering at the University o f Toronto for his assistance and guidance during the course o f this thesis.

I would also like to thank Don Lamb and Cam Monroe o f mast ter Builders Technologies Ltd., Jim Peel and Gerta Campbel of W-R Grace & Co. and Vito Debendictis and BPan Salazr Gom The Eudid Chernid Company; each for conmbuting their t h e , the product information that \vas needed, as well as for the products that they donated for use in this thesis.

For their help in the laboratories and in the technical aspects o f this project 1 would iike to acknotv1edge the assisrance of U d a Nyd;o, Dr. Amr El-Dieb and Kyle S tanis h.

iii

. . Absttact . . ~ . . . . , . . . C f C f C f C f C f . . . ~ C f C f . . C f C f C f C f C f C f C f C f C f C f . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . u -.. Acknowledgemen ts ........................................................................................................................ rrr .-* List Of Tables ................... ... ................................................................................................ vru

List O f Illustrations ................................................................. iu - - List of Abbreviations and Symbois ............................................................................................ ,uu

CHAPTER 1. INTRODUCTION

Background ....................................................................................................................... 1 3 Objective .............................e............................................................................................. -

Plan of Organisation ....................................................................................................... 3

CHAPTER 2. LITERATURE REVIEW

............................................................................................. 2-1. O r d i n q Portland Cernent 5 2.2. Eady Reaaion Chemistry and Stiffening .............................................~~...................... 6

........................................................................................................... 2.3. Wockability ........ ., 9 2.4. Chernical Admiunires and Their Use in Coacrete ...............................-................... 10

2.4.1. Air Entraining Agents ....................................................................................... II 24.1 -1. Factors Influenciag Air Entrainment .................-..-.--..---.--.-........ 13

3-42. Water-reducing Admtvtures ............ .. ............................................................... 15 2.4.2.1. Lignosulfonates .................................................................................... 17

............................................................................................... 3.4.3. Superplasticizers 18 2.4.3.1. How Superphsticizers Work In Concrete ................................... 19

TABLE OF CONTENTS v

2.5. Slump Loss and S t i f f k g of Coacrete ..................................................................... 21 ...................... 25.1. Incornpatibility .... ................................................................. 22

................ .......*...................... 2.6. Factors That Lnfluence SIump Loss ,... 2 3 26.1. Cernent Properes ........................................................................................... 2 4

26.1.1. The Effect of C3A on Early S tiffening ....................................... 2 5 2.6.1 .S. Effect of Sulphate Content ................................................................ 26

2.6.l.2,l. Influence o f Water reducers & Superpkticzer on the .......................................................................... Rok of Sulphates 28

26.1.2.2. Suiphate Form ............................................................................... 30 36.2 3 . Fineness of the Cement and Excessive Fines .................. ....... 31 2.6.1.4. ALkali Content ................................................................................. 3 2

. . 2.6.2. Superplaseiuzer Propemes ............................................................................... 34 2.6.2.1. Degree of Suifonation ......................................................................... 35 2.6.2.2 Effect o f Molecular Weight ............................................................... 35

2.6.3. DelayedAddition ............................................................................................... 36 2.6.4. Temperature ....................................................................................................... 37 26.5. Reduced Cernent Paste and Water Content ............................................ 38 36.6. Initial SIump ...................................................................................................... 38

. . 2.6.7. Proloaged ~Mtxmg ........................................................................................ 3 9 2.7. Influmces of Admixtures on the Hydration and SetMg Properties o f

................... ........................................................................................... Cements .. 3 9 2.7.1. Set Retardation ................................................................................................... 39 2.7.2. Ettringite Formation ......................................................................................... 40

CHAPTER 3 . EXPERIMENTAL PROCEEDURES

3.2 Introduction ................... .- ....................................................................................... 4 2 3.2 Mateds ..................................................................................................................... 4 . 3

3.2.1 Cments ...................... .. .......................................................................... 4 3 .......................................................................... 3.2.1.1 Cement Propedes 44

3.2.2 Air En training Admi.xtures ......................... .....,.. .................................. 4 6 ISSUES IN ADMIXTURE COMPATIBIL1TV

TABLE OF CONTENTS vi

3.23 WaterReducers .............................................................................................. 46 - 3.2.4 Superplasticizers .................................................................... .. ................... 41

........................................................................................... 3 -3 Aggrega te ................... .... 48

3.4 Foam Index Test .................................................................................................... 4 8 .............................. 3.5 Mini-Slump Cone Test .. ............................................................ 50

................................................. 3-51 Inadequaties of the hIini-slump cone test 52 ........................................................................... 3.52 hLini-Slump Test Program 53

3.6 MeasuDng Heat Rise of Hydrating Cernent Pas tes .............................................. 55 3-61 Cali'bration of the Thennocouple Apparatus ........................... ........ 57

- 3.1 Coacrete Trial Mixes .................................................................................................... 5 8 3.8 ~Meauring Soluble Alk ali. Contents .................... ... ............................................ 61

. .

............................................................................................. 3.9 Determining Air Content 61

CHAPTER 4 . OBSERVATIONS & RESULTS

............................................................................................................................. 4.1 TesMg 63 ....................................................................................................... 3.2 Foam index Tests 63 .................................................................................................... 3.3 LLIUll-Slump Resuin 6 5 . . 43.1 Superplasttazer-Cemenr interactions ........ ....... .................................. 65

............................................................ 4.32 Influence of Air Entraining Agents 68 4.4 Optimum SP Dosage .................................................................................................... 63 4.5 Slump Loss In Concrete ............................................................................................... 75 4.6 hfid-slurnp w . Concrete Slump .............................................................................. 79 4.7 Heat Rise Li Cement Pastes ....................... .... ...................................................... 8 3

........................................................ 4.7.1 The Influences of the Water Reducer 83 ......................................................... 4.7.2 The Influences of Superplasticizers 84 ...................................................... 4.7.3 Influence of Air-Entrainiog Agents 8 6

4.8 Soluble Alkali Contents of Cernent ........................................................................ 91

ISSUES IN AOMUCTURE COMPATIB1LITV

TABLE OF CONTENTS vii

CHAPTER 5 . DISCUSSION

5.1 Product Protection ......................................... .. 93 ...................................................................... 5 2 Change of Cements In Concrete Tests 93

5.3 Cement Properties ......................................................................................................... 94 5.4 rI-Entmimnen t ...................... ... ................. ... ........................... 9 4 5.5 High-rUkali Cernent & Melamine Sulfonate SP Incompatibility ........................... 95 5.6 Low-alkali Cement/SP Interactions -,... ..,.......-- -- ..-.-. ......--. -. 96 5.7 hfodifications for the Mini-Slump Test ..................................................................... 96 5.8 TreaMg Eady StiffeaiLlg Problems ............................................................................ 97

CHAPTER 6 . CONCLUSIONS & RECOMMENDATIONS

6.1 Conclusions ................................................................................................................ 9 8

6.1.1 Evaluation of the Mini-Slump Cone Test ................. ....... ...................... 98 6.1.2 Influence of Akalis ..................................................................................... 99 6.1 -3 The Influence of Admixtures ................................................................. 1 0 0

6.2 Recommendations ...................................................................................................... 101

REFERENCES ........................................................ .. .... .. ...................................... 103

APPENDICIES

Appeadi,~ A bLi~ Designs: mi ni-Slump & Heat Developmwt Tests ............. 109

Appendiv B Mid-Slump Results: Loss of workability aith Time .................. 1 14

Appendiv C Heat Dew.lopment: Temperature vs . T h e Plots ....................... 136

Appendiv D Grain Size Distributions for Fine and Coarse Aggregates ......... 157

&SUES IN ADMIXTURE COMPATiBlLlTY

Table 3.1 .

Table 3.2 .

Table 3.3 .

Table 3.4 .

Table 3-5

Table 3-6

Table 3-7

Table 4-1

Table 4-2

Table 4-3

Physical and chernical properties of cemena ................................................. 35

............ Active agents and physicd and properties of air entraining agents 46

Active agents and physical propertes O E water-reducers ............................ 47

Active agents and physicd and propertks of superplastizea ................... 47

iblLving regiment for cement pastes prepared in a kitchen blender .......... -54

Concrete mi.?? design and composition ...................................................... 5 9

Miving regime for coacrete teal mixes ............................................................ 60

Foam Indes results on plain cemen ts ....................................................... 64

Foam Indes results on cements with calcium lignosulfonate WR. ............. 64

Soluble alkali contents of cements ................... .. ........................................... 91

Figure 2-1.

Figure 3-1.

Figure 3-2.

Figure 3-3.

Figure 3-4

Figure 3-5

Figure 4-1

Figure 4-2

Figure 4-3

Figure 4-4

Rate of heat evolution of orciinary podand cerneut ....................................... G

hn illustration of the foam index test ............................... .... . . The muir-slump cone .......................................................................................... 51

Thermocouple apparatus .................................................................................... 55

Typical temperature profde of hydrating cernent pas te ............................... 5G

........................... Calibrated temperature cuve for insulated themiocouple 57

MiPi-slump vs. time for low-allrali TlO/melamine SP combination (No AEA) ............................................................................................................. 66

Adhi-slump rs. time for low-alkali TlO/naphthalene SP combination (No A U ) ............................................................................................................ 66

hfni-sl~~~~p n. rime for high-akali TlO/melamine SP combination (No AEA) ............................................................................................................. 67

Mini-slump r s - time for hi&-alliali TlO/naphthalene SP combinaaoa (No AEA) ..................................... ... ......................... 67

Figures 4-5 to 4-8 5 & 60 min mini-slump vs. SP dose for LAPC, melamine SP and various AEA agents ........................ .. ........... .... 7 1

Figures 4-9 to 4-12 5 & 60 min mini-slump 1%- SP dose for LAPC, naphthalene ...................................................... SP and Fanous AEA agents 7 2

UST OF ILLUSTRATIONS x

Figures 4-W to 4-16 5 & 60 min mini-shmp vs. SP dose for HAPC melamine SP and d o u s riEA agents ....................................................................... 73

Figures 4-17 to 4-20 5 & 60 min mini-slump vs. SP dose for HAPC, naphthalene SP and ratious M2.A agents ...- .-.................................................-.--....- 74

Figure 4-21

Figure 4-22

Figure 4-23

Figure 4-24

Figure 4-25

Figure 4-26

Figure 4-27

Figure 4-28

Figure 4-29

Figure 4-30

Figure 4-3 1

Estimated slump as a functioa of water content for various sizes of

Slump loss in concrete over time using high-alkali Tl0 cernent and varied SP type and dose ........................... . .-....... ... .... . . . . . . . . 78

Slump loss in conaete orer time using low-alkali Tl0 cement and varied SP tvpe ................... .. ..-*-..-..........-.....................-......-.-.-..--..-.---.--.--.-----.-.. 78

Cernent paste mini-slump area vs. concrete slump high-dld Tl0 cement ..*.................................-......*....-....*-- * .*.....*...-.......*-...---..--..-.. -*--...--...---..-.--- 8 1

Cernent paste mini-slump area vs. conuete slump for the l o w - W

TI0 cement .........,.... .. ..-.-....-.-.. .-- . . . 82

Cernent paste mini-slump area vs. concrete slump for the low-akaL

Tl0 cement ...-..........-.....................-..... -- .......................................................... -.-.. 82

Time to peak temperature vs- SP dose fol: the low-alM Type 10 cernent with melamine SP & Ca LignosuKonate W R ..................................... 87

Time to peak temperature vs. SP dose for the low-alkali Type 10 cement with naphthalene SP & Ca iignosuifoaate W R ........................-..-m.- -87

Time to peak temperanue vs. SP dose for the high-alkali Tl0 with melamine SP & Ca iignosulfonate WR ,...... . .......-...-.--.....--......-...-.-.-.........--.... 88

Time to peak temperature vs. SP dose for the high-alkali Tl0 with naphthalene SP & Ca lignosulfonate WR .................................................. 88

ISSUES IN ADMIXTURE COMPATlBlLlTY

LIST OF ILLUSTRATlONS xi

Figure 4-32

Figure 4-33

Figure 4-34

Figure 4-35

Peak temperature vs- SP dose for the low-alkali Type 10 cement with ............................................................ melamine SP & Ca iignosulfonate WR 89

Peak temperature vs. SP dose for the low-alkaii Type 10 cernent with ....................................................... naphthalene SP & Ca lignosulfonate W R 89

Peak temperature 1s. SP dose for the hi&-ahb Tl0 with meLunine .............................. ................. SP & Ca lignosdfonate WR -.-.--....---..--.-.-.- 9

Peak temperature m. SP dose for the hi&-&di Tl0 mith ....................................................... naphthaiene SP & Ca lignosulfonate WR W

ISSUES IN ADMIXIURE COMPATlBlLlTY

AND SYMBOLS

AEA

CA

C A CJAF

C2s c3s

CLS W C W C OPC MW

M-Type N-Type

PNS PMS SCM

SP w/c W R

Ar En training Agent Coarse Aggregate

Tricalcium Nwninate - 3Ca0.A1203

Tetracalcium A l i o f e r t e - 4CaOai-0,aFq03

Dicalaum Silicate - 2CaOmSi0,

TficalUurn Silicate - 3CaO*SiOl Calcium Lignosulfonate Wh-Alkali Poaland Cemerit L o w - L W Portland Cement Ordinaq Portland Cernent Molecular Weigh t Melamine Type Superplas ticizer Naphthalene Type Superplasticizer PoIynaphthalene Sulfonate Polymelamuie Sul fonate Supplementary Cernenting Materials Superplas ticizer Water Cernent Ratio Water Reducer

xii

Irnprowxnent to the durability and mechanicd behaviour of concrete can be achieved by reduung the interstitial void space, this requLes that the individual cernent must be moved doser to one another vover, 1998). The production of these high- strength or durable concretes can cypically be achieved implemenog a low water/cementitious materials (\-/cm) ratio. Unfortunately, the use of a low %-/cm ratio requires thar either workability be compromised, or hgh canent content be used to maintain a workable concrete, however, neither is a desirable option (Hover, 1998). Fortunately, the development of commercial chemicai admi~tures has made it possible to increase concre te w-orkability Li low- w-/ cm concre tes, thus making high- per forniance concretes a reaiiv.

The use of chemicai admixtures has become cornmon place Li the production of concre te; concretes absent of admi~tures tend to be the exception today. i\dmiunires are used to impart some benefiual influence ont0 concrete whether it is to be in its fresh or

hardened state. Typicdy, ad-tues are used in combination with others so as to achieve a combined benefig and are generally successfd when used together. Each

INTRODUCTION 2

interacts nith the various constituents of cements, and influences the hydration reactions in di ffering wztys.

Under c d circumstances compatibility between admisures, or admixtures

and cernent, may be of concern. Predicthg the compatibility of admixtures in combination wiith cement is an almost impossible kat to perform by chemical analyses alone. Admiunires and cements are both complex in their nature. Portland cements and chemical adrnktures are multi-component m a t d s , m-hich undergo cornplex chernical reactions during the hydration of ponkad cernent paste.

Cases have been reported o admi~trrre-cernent incompatibiliry, sometimes resulting in estreme cases of set retardauon or flash set (Dodson and Haydeo, 1989; Johnston, 1987; Aitn et ai., 1994; TuthiU et al., 1961). In either ciccumstance, cement- admiunire incornpatibility is a major problem in the concrete industry that affects the efficiency of concrete piacng, the quality of conccete, and may resuit in changes made to the scheduled consmction. The resultlig costs incuned may be esueme. Fortunately

when the one of the components of the concrete is replaced, most typicdy to a different

type or source of superpiastizer or cernent, the problem disappears (Dodson, 1990; Helmuth et al., t 995).

Compatibility tests have traditionaily been performed using small t i a l &es of concrete. Howerer, these &al mixes remain to be both costly and materiai intensive. Recent advancements have led to the daelopment of smd-scale tests on cernent pastes

and mortan.

Typicaliy retardes, wxter reducers, accelerators, and air-entraining agents (AEA) and superplasticizers (SP) are compatible when used in concert with cernent, and ofieten rcsults in an enhanceci combined effea The induceci high w-orhbility of lou- w/m concrete contaking a superplasticizer is maintained generally for about 30-bOmLiutes, and this rapid decrease in slump is terrned slump loss. However, in some cases se- rnay be sigdcantiy reduced or offset by the incorporation of these admiunires. It is

ISSUES IN ADMIXTURE COMPATlBlLlTY

INTRODUCTION 3

essential that each admiunire combination be evaluated before it can be practicdy u dis ed .

The general scope of this smdy is to investigate the eects that chemical admisme incompatibility has on the siump loss of superpiaticized high-perfomiaace (low W/C) conuete. This investigation emphasises the use of s d scale laboratory tests ro measure the rheological behaviour of cernent pas tes, O f ciiffixent alkali contents, under the influence of chemical a c h k s ~ ~ e s ; as weU as th& ability to predict the behaviour of

admixnire enhanced concrete. The tests used in ~ h e contesx of this study include Kantro's Miniature-Slump Test, Dodson's Foam Indei Test and the measurement of the

heat of hydration of the cernent pastes; a bief investigation also caMed out to characterise the efiect of the soluble alkali content on the eady hydration properties of three cornmerciaiiy aniiable cements. The &dmgs of these tests are discussed hereafier in this paper.

1.3 PLAN OF ORGANIZATION The remaliiry: sections of this thesis can be divided into nvo portions: the kt

being a literature rex-iea- of the cruciai concepts that are at the heart of chemical adrnkx~es and the compatibility issues assoaated uith them, and the second portion consisting of the elxperimenta.i work performed in this paper. The literature review is contained in Chapter Two, and i provided to give the reader the understanding and knowledge necessary to oliow the w o d c perfomied in the following chapters. Issues covered indude the influence of tricalaum duminate (Cd) content, content and form of sulphate and alkali content on early hydraaoa reactions of portland cernent paste with and without the influence of admistufes.

The experhental portion of this work commences in Chapter Three with a description of the mateeah used and the experimentai procedures used in the resting

program. Chapter Four presents the results and hndings obtained in the Iaborato. These results are then discussed Li the Efth chapter of this uiritten work.

ISSUES IN ADMIXTURE COMQATlBlCrrY

Chapter SLY completes this report and draws conclusions kom the enaie body of this work, and goes as far as to discuss several recommendations on how- this work could be improved if amended.

ISSUES IN ADMUCTURE COMPATIBILITY

Before continung on into the topic OF chemical admi~nires and th& interactions with ordliary portland cements (OPC) it is needed to reriew some of the processes assouated with the hydration, and early age behaviour, of plain cernent paste. Podmd cemen t paste is the most actire component of any mortar or concrete; and is a complez misme of mulaple inorganic components such as the aluminate phases (CA, GiW) and silicates phases (C,S, CzS), and to a lesser degree gypsum and other sulphate orms (C~SO~~SH~O) , allralis (NazO, &O), etc (Jolicoeur and Simard, 1998). Naturally, more components can be found in blended cemenu. When xater is introduced to portland cernent a series of chemical ceactions are teggered that lead to the formation of hydration products and inter-partide bondkg which results in a.dense stable ma&.

2.2 -Y REACTION CHEMISTRY AND STIFFENING

Upon initiai contact w+th water, the multiple phases and components of the cernent undergo a variecy of chemicai reactions, as weil as peciods of varieci reactivi- rates. The hydration of cernent undergoes three distinct phases; the hnt is an initiai period of rapid chemical reaction. This is then followed by what is ofien termed as a 'dormant' or 'induction' phase where the rates of reactioo are slowed. The induction phase then leads =a\- to a second heightened rate of reacrion which, readies a peak and then subsides once again, this second raised rate of reaction denotes the chemical processes associated with the se* of the cemen& while the hardening process occun during the h a 1 sequence of the reactions. These changing rates of reactions are reflected in the derelopment of heat due to a s&es of exothennic reactions. A typical representation of this heat production is illustrateci in Figure 2-1.

c rn L a - I II 1 1 1 IV ';' so- c - O -t 40 - 3

Cu I a 1 l I 1 1 1 1 1 1 1 1 1 1 I

4 8 12 7 6 20 24 28 Tirne - hrs

Figure 2-1, Rate of heat evolution of o rd inq portland cernent- With the (i) initial reactions, (II) induction, (III) acceIerated or setting, and 0 hardeaiag phases.

Cernent hydrahon begins Lnmediately upon contact with water. Within several minutes, the easily solubilized components (Nac, KC, Ca*, S 0 4 , OH-) of the cernent are dissolved into the aqueous phase, and initiai hydration reaaions commence (Jolicoeur

ISSUES IN ADMIXTURE COMPAflBlLlTY

LITERATURE REWON 7

and Simard, 1998). The hydration process of cernent inoolres the series of compler chernical readons presented in Equations 2-1 to 2-8. iMuch of these iaitiat reactions involve the rapidly hydrating tricalaum aluminate (Cd) phase and its interactions aith the sulphates present in the cemenb and results in a rapid production of heat, depicted as the hrst peak in Figure 2-1. A thin layer of hydration products (in the fomi of caiciurn

sulp hoaluminates consiscing of e ttringite 3CaOeAL~0+3CaSO~.3W20) is fomied as the surface hydration O cemen t particles, inrolving reactions \cith the sulp hates present, continues. Eventually the cernent parcides become fulty coated with a protectke layer of hydration products that hinders the diffusion of reacting species in and out of the reaction interphase, thus sharply reduciq the rate of the various reactions (Hehuth et al., 1 995; Jolicoeur and Simard, 1998; Stein, 196 1). This perod of high initial reactions O ften has a duration of only minutes.

The initial phase of high reactiiitg is followed by a p&od of ktency n o d y referred to as the 'domiant' or 'induction' p e r d und the onset of s e t h g (Helmuth et al., 1995; Jolicoeur and Sirnard, 1998). Although the reactions initiateci in the tirst phase c o n ~ u e d u ~ g the induction pend , iictie emingite is produced duxing this latent period ailowing the cernent paste to maintain most of i plastiaty. Solidification (semng) starts after about ca-O to t h e hours, due to the formation of calcium silicate hydrate (Meyer

-

and Perenchio, 1979). The only no table event at this stage is a progressive thid- of the surface gel layer. Any loss in the consistency at this stage is m d y attributable to the physicai coagulation of the cernent particles rather than to any chernical process (Ramachanran, 1995).

The induction pePod may be shorrened in durauoa if there is an insuffiuent sulphate content in the cernent as excessive nudeation and growth of C-A-H products rnay occur (flash set); if, on the other hand this concentration is too high (hemihydrate, alkali sulphates), massire nudeaion and g r 0 6 of gypsu. a p a k may be obsemed. (false set) (Jolicoeur and S i d , 1998).

The end of the induction p e r d is m k e d by a sharp increase Li the reaction rate of cernent, in general, indicated by the second pronounced temperature rise (Stein, 1 96 1). The number and energy of the interactions bemeen the growing particles of the system also increase, rapidlv convertkg the system into a stiff maais (Jolicoeur and Sirnard, 1998). The ormation of an internal structure results when the deposition of hydration products on the surfaces of cernent grains are allowed to the corne Lit0 contact with neighbouing gains (Guo, 1994). Cernent grains bond to one anocher by the intenvea~ing of the C S hydration products, this graduaily orms an internal structure that as it c o n ~ u e s to grow ~~~ bring about the stiffening, and e v e n d y the set of concrete. Ultimately, producing a strong durable matris of cement hydrates.

Thc accelerated acti~ty in the cernent paste has been acknowledged to be die result of tncalcium silicate (CS) hydration (Jolicoeur and Simard, 1998; Stein, 1961). Several effects have been considered to explain the onset of the acceleration period (Jolicoeur and Simard, 1 998):

Disruption of the hydrate protective layer by physico-chernical transformations of the hydrates; Breakdown of the protective laye by osrnotic efects; Nudeation and growth of C-SH products, and; CH nudeation and grow-th.

ISSUES IN ADMIXTURE COMPAflBlLllY

The second peak of heat evolution in Figure 2-1 often denotes the hydraaon of the C S phase. In some cements there may be a thLd less pronounced peak in this curve, the result of renewed Cs4 hpdraaon once all the SO, is reacted, the gypsum is depleted and ettarigite formation has conduded, and cypically occurs withk a several days Lom the tint contact wth water (Stein, 1961). This renewed C d hydration may occur simultaneously sith CjS h$ration and the two wiu appear as one peak temperature rise.

When desaibing the qualities of Gesh concrete the term \vorkability7 is ofien applied, yet it laclis an =act definirion. Workability' can be described in terms of i a qu;ilitative components, such as flow, compaction, stability, ease of fnishing, and i o abiliv to be pumped. Each of these components is a rheological quality of some complesiy, how-ever, each of these characteristics is determined by the ease with which an applied stress results in some f o m of plastic deformation (Kantro, 1980; hIantegazza and hlberti, 1994). The ability of a cernent paste to resist a gii-en shear suain is amibuted to the intemai Giction of the solid particies, the Fiscosity of the liquid medium and forces of electrostaac attraction or repulsion benveen the partides (Guo, 1994). The term workability can chus be described as a hnction of both yield stress and plastic vkcosity (Kantro, 1980).

Workability has traditionaUy been rneasured in terms of slump, although the

slump test is more a rneasure of consistency than of worlrability. Studies have pointed out that slump is dkectly related to the yield value of the conuete, or cement paste

(Kantro, 1980). Hence, slump can be used as an indirect methoci for rneasuing consistency. Due to the formation of hydration products when cement is reacted Mth

water, the workability of the concrete is progressively reduced, resulting in the loss of slump with tirne (LMantegazza and Alberti, 1994).

ISSUES IN ADMUCTURE COMPATlBlLlTV

LITERATURE REVlEW 10

2.4 CHEMICAL ADMIXTURES AND THELR USE IN CONCRETE

Production of high-swngth concrete or durable concre te resistant to severe

environmental conditions can not be reaked by the use of plain portland cernent concrete, chernicai admi~tures must be incorporated into the N e d e defines an

admiunire as a ccchemical product which, except in speual cases, is added to the concrete mis in quanaties no larger than 5% by mass of cernent dung mixing or dueiig an additional mi* operation pdor to the placing of concrete, for the purpose of achieving a certain modification, or moditications to the n o d properties of concrete" (Nede, 1 997).

The most p i c d reasons for which admixtures are used in practice are:

To reduce the cost ofconstruction;

To achieve the properties in concrete more effectively than by other means;

To ensure the quality of concrete dung the stages of misin& transporting, plang and cuPng in adrerse w-eather conditions;

To improve the durability of concrete esposed to adverse weather

conditions.

Most of the commeral chemicai admixtures are composed of orgaaic compounds, and perfonn rarous hnctions through a variety of both phpical and

chernicai interactions with the hydrating cernent phases- The addition of chernical admi~nires further cornplicates the already elaborate behaviour of hydrating cement paste. Ir is important to understand the specific effects and consequences of acLmk~e- cernent interactions prior to incorporation into concrete. To optimise the hnctional properties of admixtures in a given cernent appropciate understandkg of their mode of action must be reached.

ISSUES IN ADMIXTURE COMPAflBlLlTY

The majority of chemicai admixtures can be dassified, according to L I S T ~ C 494-92, as:

TYPe A

Type B Type C Type D Type E

T F G

Water-reducing

Retarding AcceIefaeing

Water-reducing and retard@ High-range mater-reducing or superplas tizing, and High-range w-ater-reducing and retarding or

superplas ticizing and retarding

Howeever7 this papa s h d ody investigate the influences of Air En- Agents (.-WA), water reducers (WR), and or superplasticizers (SP) when used in concert in the production of concrete.

As k i r name impiies, an AEA is used to purposely entrain microscopic air bubbles in to concrete. It has been proven rime and h e again that air-entrainment has the effect of drasticallv improving the durability of concrete elrposed to moistue dukg cycles of freezing and thawing (Kosmath et al., 1995). The mechanics of how the entrained au void svstern works w d not be discussed here since it is not relevant to the issues at hand. However, ifs influence on early aged concrete and its interaction with admistures ltili be covered.

The majority of traditionai air entraining admisnues are created Gom the by- products of the pulp and paper industy, refnernent of petroleum or the processing of animal fats and hides (Dodson, 1990)- The main forms of air entraining agents can be classified as (a) saits of fatty acids derived Erom animal and vegetable fats and oiIs @) alkali salts of wood resins (c) allrali salts of sulphated and sulfonated organic compounds (Neville, 1997). It is important to noie that AEA's do not generate air, rather they simply stabilise the bubbles that are formed during the miviog- AEA are comprised of long

ISSUES IN ADMIXTURE COMQATlBlUlY

LITERATURE REVIEW 12

chained soluble molecules that hnction as surfa-ts and act to d u c e the surface tension of the \iater. The hEA adsorbs onto the cernent partides, causing them to

become hydrophobie, so chat the bubbles that are generated during the mixing process become adhered to the cernent (Ramachandran, 1995). This process stabilises the entrained air and prevents th& coalescence.

Once incorporated into the mixture the entraid air becomes co~ered by a s heath of LE molecules that repel one another, resulthg in the u d o r m dispersion of the m u a l i e d air (Neville, 1997). The film that encapnilates the ai. void must be able to resist intemal and =temal pressures, deterioration and resist coalescence while the concrete

remains in its green state (Dodson, 190) , it must also not impair the physical properties of the concrete. Howerer, even if they were to possess some deomentai propertes they are used in such s d amounts that the effects would be negligible.

The incorporation of an air-enrraining agent into a concrete mk not O+ exhibits beneficid qualities in concrete's hardened state, but also &parts some beneficial traits while in its plastic state. Air-enrraining agents have been found to have the ability to improve workabiliy of fresh concrete, the presence of the microscopie air void .stem has the effect of acting as 'bd bearings' that help to tluidise the concrete to a Limited estent. The enaainment of e v q I0/o air dows for a water reduction of 1h, up to 16%

of entrained air (Dodson, 1990). Enuained air dso benefits kesh conaete by eliminating or mliimising segregation and subsequent bleeding by at least nvo mechanisms (Dodson, IWO):

1. It provides a certain degree of buoyancy to the aggregates, reducing

their rate of sedimen ta tion; and 2. It reduces the effective volume through which the difierential

movemen t of water may occur.

ISSUES IN ADMMTURE COMPATlBlLlTY

When air-entrainkg agents are used with other admiunires, the interaction between the admixtures and th& interaction with cernent become important for compatibiliy purposes (&machandran, 1995). When used with a wxter-reducer, there is the chance that less AEA wili be required to achiere a desired air content in concrete, even if the \WR has no air enrrainlig propertes. Water reducers alter the physicd or chemical environment so as to permit the AEA to operate effisiendy (Neville, 1997). It should be noted that some water-redung admixtures have air-entrahing characte&ics, most noted wodd be those tbat are based upon Lignosulfonates (Ramachandtan, 1995).

Unfortunately, some combinaaons of chemical adrnixtures are capable of

producing detemental results. The incorporation of some superplas ticizers into air-

entralied concrete may produce an unstable roid system. Typically, the irnpro~ed

compaction and fluidity achieved with a superplasticizer can faciltate the escape of some air or coalesce the air bubbles to sorne estent (Nede, 1997; Okkenhaug and Gjom, 1 992; Rarnachandran, 1995). The entrained air is thus more easily .1--rked out of the concrete d u k g compaction, vibration and h i s h m g , causing a decrease in the air content and to ta1 air void surface areas, resulting in an inuease in the spacing factor. The extent of die superplasticizer's effect on the air void system depends on the dosage used (Dhir and Yap, 1983). In g e n d the addition of an SP to concrete inmeases the bubble spacing, and sornetimes to unacceptable lemls. Research has shown that the freeze-thaw

durability of air-encralied superplasticked conaete, in spite of increased bubble spa* factor, not to be adversely affected when compared with that of the air-entrained

reference concrete (MacInnis and Racic, 1986; IMalhotra, 1981). The type of superplas ticizer used also has an influence on the air void system. It has been noted that

by using a melamine-based admixture, as opposed to a naphthalene-based agent, a better and more stable air void system can be produced (Okkenhaug and Gjorr, 1992). This illustrates the importance of using a compatible combination of materials in the production of concrete.

ISSUES IN ADMlXTlJRE COMPATlBlLlN

Aside kom the influences of other admi~hires on the entrainment of au, other factors that influence the ML4 requkmeot are (Dodson, 1990; N e d e , 1997; Ramachandran, 1935):

1. Fineness of portiand cernent - As the heness of a cernent increases, the ability for an air-enelitrainiog agent to act becomes inhibited. It has been specdated that the presence of the ultra-fine Lacnon has a tendency to dismpt

the proteciive 61m around the entrained air void, reducing the effdveness of an hEA to protect the air voids Gom codescing. This has the effect of raishg the required amount of AEA to produce the desired air void sysrem.

2. Alkali content of concrete - aUialis tend to depress the solubility of calcium ions in the aqueous phase of concrete. The calcium-anioaic AEX 61ms that surround the air voids in the keshly mived concrete are probably thinner (or possibly more soluble) than those which would develop in concretes of low alkali content. This reduces the stability of the air void system.

3. The amount of coarse and fine aggregate - as the mauimum size of coarse aggregate (CA) inueases the air requirement deueases; the hnc aggregate portion of the rnkture that serves as a screen to trap the air during mishg. The fine aggregate provides interstices that contain paste and air bubbles, so it generates the air elfiuendy. Thus, as the portion of sand in the total aggregate is increased, so is the air content of the concrete.

4. A high proportion of ultra-fine material in the aggregate - The presence of finely divided materiais causes a reduction Li the air content of concrete and an increase in the required dosage of air-entraining agent,

5. Temperature - a higher temperature of the conaete resula in a lower air content, and vice versa.

ISSUES IN ADMIXTURE COMPATIBILITY

-

6. Mixes with high cernent contents and vey low w/c ratios (0.30 co 032) require extremely high admi,xture dosages.

7. Contamination by oil or grease (even in very s 4 amounts) contributes to poor entrainmen t of air.

8. Use of fly ashes with high carbon content - The hW may be adsorbed ont0 the surfaces of carbon partides thus increasing the content of agent required.

2.4.2 WATER-REDUCING ADMILCTURES

As implied by th& m e , the function of these admivtures is to reduce the amount of water needed in concrete, usualiv in the range of 5 to lQO/o, to achieve a aven wvorkability (Kosmatka et ai., 1995; N e d e , 1997; Ramachandran, 1995). Genedy s-ater-reduog admixtures are water-soluble macromolecular substances which, when present in the cement paste, may be adsorbed at the surfaces of cernent grains to form an inten-ening iiquid layer benveen adjacent grains thicker than what could be maintruneci in the case of pure water. This lowers the intemal fricaon and reduces the yield stress of the paste, thus a 'lubricating' effect can be wimessed dispersing the cement particles, and they remain dispersed &ter mising (Guo, 1994; Helmuth et al., 1995). Some researchers have suggested that the plasticking eect may be due to a retardation of enriogite formation, and a decrease in the interlocklig effect of ettrngite panides, thus produang decreased miter dernand (Ramachandran, 1995); or by irnposing a net n e e e electic charge to the cernent partides produchg an electrostatic repulsion beween partides. What ever the mechanism may be, the liberated w-ater that is fked Gom the flocculated sys tem now becomes available for lubrication (Nede , 1997).

Water reducers can be employai to ob tain diffrrnt physical effects on the plas tic s tate of concre te; they c m be used to reduce the quantity of mixing \%mer required to produce concrete of a given slump, reduce w-ater-cementhg materiais ratio, d u c e cernent and .riater content, or increase slump (Kosmatka et al., 1995). Due to slow

ISSUES IN ADMWTURE COMPATlBlUTV

LITERATURE REVlEW 16

hydration dwng the induction phase, the effects of a water-reducer dimliish with time

as the admixture is slow-ly removed Gom solution by sorpaon on the hydratai products (Helmuth, 1995). The efficiency of a witer-reducer tends to be greatest when used in concretes made Gom cements of low aLkali and/or low- C d contents (Docison, 19W)- However, the effects of alkalis and C d content are topics that wiU be discussed later.

It shouid be expected that that the interacon of a chernical admivture that is

physicdy adsorbed on to the reactive components of a cernent, may significantly alter

the hydration sequence of cernent pastes and concretes dwing the induction p&od (Jolicoeur and Sirnard, 1998; Meyer and Perenchio, 1979). The use of a water-reducer must \t-airrant some cauaon since that most are capable of retarding the hydration of the cernent. This ability of a w-ater reducer tends to be the result of the presence of a sugar or carbohydrate in the water-reducer- The retardation of the time of set may be e-xtreme if the recomrnended addition rate is esceeded (Kosrnada et ai., 1995).

The dispersing capaaty of these admkniies can contribute to accelerated hydration of the cernent and lead to an increased rate of slurnp loss. Under non-water- reduced conditions, the mixing of portland cernent and -ter causes tiny bubbles of warer to be encapsulated by cernent partides, rendering a Gacaon of the mishg amer unai-ailable for workabiiity, placing and hishing (Dodson, 1990). By dispersing the cernent particles some of this entrapped water becomes liberated and d e s additional

surfaces OF cernent partides anilable for hydration (Dodsoa, 199; Nede, 1997). A second resdt of the increased dispersal oE the cement partides is that the surfaces of cernent particies formerly abutting each other are now- arailable for eady hydraton. This liberates approximately 12-20% of the cernent surface for direct contact Mth a-ater (Dodson, 1930).

The problems of accelerated set can be overcome by incread dosage, or re- dosing, howexr, this may result in unacceptable set retardation, if at all possible, a slight delay to the addition of the adrniuture in the mi- process c m enhance performance (Nede, 1997).

ISSUES IN ADMUC~URECOM~A~~~L~TY

Most water-reducers in production today are based on lignosulfonates, the hrst polyrneric wvater-reducer that n;as used by the coacrete industry. They are manufacwed Gom one of the wastes produced by the pulp and paper industry, maste Liquor whose composition indudes about 20-309'0 of Iignin (Ramachandran, 1995). This liquor is primarily a solution/suspmsion of sulfonation products of LignLi, decomposition products of ceilulose and lignin, various sugars, and sulphates (Ramachandran, 1995). When hrst introduced to the industry, lignodonates were reiatively inexpensive and could be esploited wvthout much cost Unfortunately, due to the non-udormity of the raw materials used variances in sugaz content and lignin decomposition products produced problems associated aith excessive set retardaaon and air entrainment (Aitan et al. 1994; Ward et al., 1980). The reliability of the performance of these admixtures has become more predictable as the producers of these products have drastically irnproved th& quality control (Ward et al., 1980).

hlthough the probiems assoated with excessive retardation in the eariy

generations of iignosdfonates have been minimise4 they stiu rnay occur when used at hqgh dosages. The presence of sugars and other contaminants in commercial Lignosulfonates can be biamed for this, since that these compounds are difficuit to remove completely (Atciu, et al., 1994). Under certain circumstances, a high enough dose of a iignosulfonate admixture may pemianently suppress hydration of CS and inhibit strength developmenk pYticularly in cements with rery low C d content and lowv alkalis (Johnston, 1987). The use of lignosulfonates should be avoided for the production of high slump loa- w/c concrete; the problern is so serere that the Cernent

and Concrete Association in Bntain have categorked lignosulfonates as a category TV retarder, indicating that they act as cernent destroyers when used at faidy high doses (Johnston, 1987).

Lignosulfonate-based acimisaes s d maintain some air-entraining characteristics (Nedle, 1997; Okkenhaug and Gjorv, 1992; Ramachaadran, 1995), therefore, the dose of any riEA should be reduced to avoid excessive air entrainment.

ISSUES IN ADMIXTURE COMPATlBlUTY

LITERATURE REWEW 18

Superplas ticizers, are additives that are water-soluble dispering agents. When

incorporated into concrete these admixtures act rnuch in the same way as ordioary water reducers, oniy nith far p a t e r efficiency. In compacison to o r d i n q uater-reducing admktures, SP's can be used in considerably larger dosages without such a significant delay of cernent hydration and setting. In facq they are capable of reducing the mater requirements of concretes by 15-30% (Kosmatka et al., 1995; Singh et ai., 1992), and can be used to produce flouable concrete or concretes of very low w/c ratio. Superplasticizers are either synthetic products, produced dLectiy from pure components; or obtained as industrial by-products from 0th- indusmes (Atcin et ai., 1994; Ramachandran, 1995). The most common agents that are used as SP's are m d y sul fona ted melamine- formaideh yde (ShIF) condensate salts and su1 fona ted nap hthalene- f o d d e h y d e (SNF) condensate salts, both of these agents are synthetic products that have been f o d a t e d from pure components et al., 1991). However, due to their comparaticely low costs, research c o n ~ u e s in the dedopment of a superplasticizer based on m&ed lignosulfonates, Gom which sugars have been completely rernored

(Ramachandran, 1995). Due to the benefits they provide in improving the hancikg, placing, compaction and xkg of cernent dong with other technicd economic advantages the use of superplasticizers are increasing in commercial use (Singh et al., 1992).

The e-xact mechanism with which superplasticizers act has been the centre of much debate, however, it is agreed that superplasticizing admixtures act by causing the cement agglommtes to disperse (Maihoua, 1981; Singh et al., 1992). This dispersion of the cernent agglomerates iiberates water that is typicdy bound and inaccessible for the fluidification of the concrete. The e f k of the superplasticizer is exhibited by large increases in slump for the respective w/c ratio. As long as suffiuent superplasticizer moledes are present in the solution thek influence wiu persist; as the polpers are con~ua i ly being entrapped in the hydration products the effects of the superplasackr dimliish (A-tcin et al., 1994). The increase in fluidity can be shoa lived, and may

ISSUES IN ADMlXtCIRE COMPATlBlLlTY

dissipate within as little as 30 to GO minutes from the time of the addition of the SP and the concrete rererts back to its original consistency (Malhotm., 1981). This can be even shorter at high tempefames. The newer generation superplasticizers that have been developed are less sensitive to these effects. This rapid loss of workability is terrned as 'slump loss'. The rate of slump loss can be ambuted to the type of superpiasticizer used, its dose rate, the chernical composition of the cemen& chernical and physical changes in the cement paste of Eresh concrete, and other extemai factors 6.e. iemperaueJ mi* the, etc.). This rapid dump loss c m be overcome mith an increased dose of the superplasticizer, although doing so may result in undesirable side effects such as excessive set retardauon, which may prove to be estremely costly. In

such a case it is said that the cernent and the superplasticizer are incompatible in terms of rheology (Jiang et al., 1998).

Although superplasackers are far more effective than ordinary \vater reducers are, they remain quite e-cpensive Li cornparison to them. For h i s reason the nvo are tvpically used in combination together (Kosrnatka et al., 1995). The superplastiQzer addition is typicaily delayeci und several minutes after the addition of the wxter reducer. This way the benefits of the WR can be achieved without the rapid consumption of the superplasticker in the early hydrauon reactions of the cernent.

2.4.3.1 How SUPERPLASTICIZERS W O R ~ rn CONCRETE

When &ed with water, portland cernent has a strong tendency to form flocs.

This tendency is the result of several types of interactions induding van der Waals in teractions berneen particles, electros tatic attraction, and snong in teractions involving water molecules and hydrates (xtcin et al., 1994). The flocculation of cement particles l a d s to the formation of an open nemork that cffectively traps part of the mising water, and renders it unavailabk for surface hydration of the cernent particles and for the fluidification of the mi\-. Hence, to achieve a workable mk larger volumes of water are used than is necessary for complete hydration of the cement Unfortunately when tqing to develop hgh-strength concrete the w/c ratio musc be reduced, which would mean a

ISSUES IN ADMIXTURE COMPATlBlLlTY

LITERATURE REVlEW 20 - - - - - - - - - - - - - - - - - -- - -- -

reduction in the workability of the concrete. In order CO produce a low w/c coaaete

with sufficient workability would require that the cernent partides are (A-tan, 1998) propedy deflocculated and (A-tcin et al., 1994) kept in a state of hi& dispersion.

This is where superplasticizers corne into effect. SuperplastiaZers are d a c e active agents, meaning that they act on the surfaces of cement partides and influence the

reacuons that d e place at the cernent-water surface interface. The exact method with which they work is of debate. FIoweoer, most researchers offer esplanations that incorporate one or more of the following mechanisms (A~cin et al., 1994):

1. Lnduced electros tatic repulsion berneen parrides; 3 Induced s teric hindrance preventing partide-to-particle con tas 3. Dispersion of cernent grains, reieasiag wiater trapped w i h

cernent Qocs;

4- Inhibition of the surface hydration reactions of the cernent

particies, lea- more -ter to fluidify the mts; and 5. Change in the morphology of the hydraaon products.

The f k t nvo mechanisms are the most s ~ c a n t in t e m s of fluidification of cement pas te and concrete, the others listed tend to be side effecrs of the hrs t nvo mechanisms.

When contacted aith uater, cernent partides acquire electrostatic charges of different magnitudes and charge. As a resdt of rhese electrostatic forces, cernent particles w4.i either be repeiled by others of like charge, or dl be attracted to particles eshibiting opposite surface charges. The combined influences of these forces \iu cause the coalescence of cernent particles and form stable floc structures, which is then Further stabilised by soluble electrolytes (e-g. allrzlis) (Joi icoe~ et al., 1994).

The most agreed to method of action for superpksticizers is that the dispersion of cernent partides is accomplished by the adsorption of the long chained polymeric molecules of the superplasacizer onto the surface of the cement grains (Andersen and Roy, 1988). Each SP molecule possesses a hydrophobie group that becomes adsorbed onto the surfaces of the cernent and hydrate particies; while an opposite hydrophilic

group becomes aiigned with the water side of cement-viater interface (Uchikawa et al, ISSUES IN ADMUCTURE COMPAflBlLlN

1792). The net resdt is that the adsorbed ~ ~ t u r e imparts a e h negati~~e elecmical charge on to the cernent pariicles. As a resulq an electrostauc repulsion is developed

benieen particles and causes the dispersion of the flocs that are n o d y f o m d , produchg the water-reducing effect (&tcin et al., 199+ Jolicoeur and Sirnard, 1998; Neville, 1997; Uchikawa et al., 1992).

O thers have pointed out that in addition to the electrostatic forces induced by the WR and SP admiunires, a series of stecic repuisive forces are also developed. The

physical interference (steric hindrance) of adsorbed m o l d e s onto the cement grains w i l l lead to additional s hon-range repulske forces (Jolicoeur and Simard, 1998; Jolicoeur et al., 1994; Uchikawa et al., 1997). Since the average molecular size of? an admixture is sereral hundred times the size of a -ter molecule the adsorbed molecules prevent

particle-partide contact (Guo, 1994). Stenc hindrance is a result of the osmotic pressure that is generated due to the tendency of admis~es to concenuate at the cernent g&s,

this increase in osmoac pressure reliet-es the local increase of density caused by orerlapping of the adsorption layers of admixture (Uchiliawa et al., 1997). The magnitude of these forces is closely related to the moiecular structure and size (molecuiar wveight) of the admixture. (Uchkawa et al., 1997).

In addition to the physical mechanisms that produce fluidifiing effects, superplasticizers also possess chernical properties that are capable of inhibiting surface hydration reactions of the cernent partides, leaoing more wxter to fluidify the SP molecules can bind ont0 h h l y reactive surface sites (cg. C3A or CAF) and reduce the rate of subsequent hydration reactions. Jolicoeur et al stated that "the eady hydration reaction behaklour shows that the SP effect is not merely that of a physical barrier to water and ion diffusion at the interface; it seems best described as a partial b l o c b g of speafic surface sites which play a key role in hydration reactions" (Jolicoeur et ai., 1994).

2.5 SLUMP ~ s S AND STIFFENING OF CONCRETE

It can be e\~ected that a4 concretes u d undergo a process of stiffeolig and

hardening brought about mainly by the hydraaon of cernent Rheological properties of

ISSUES IN ADMIXTURE COMPATiBILIlY

LITERATURE REVlEW 22

nonnal concrete are determined b y the amount of mkkg water, calcium sulphate, and the reactirity of the cernent, such chat slump loss problems are rare and accidental (Jiang et al., 1998)- This l o s of workability may be accelerated by the influences of evaporation of the mi~.i.g water and adsorption of =ter by the aggregates. The

reduction of the free water of the fresh concrete causes an increase in the inter-partide contact and bondng of the cernent particles, thus reducing the fluidity of the mix (Ravina and Soroka, 1994). Slurnp loss, as usuaUy evpehenced in practice posses no real problems because slurnp can cgpicallp be maineined long enough so that the concrete can be worked and manipulated to its desired h s h (Ratina and Soroka, 1994).

Even though slurnp loss is espected in aii concretes, the rate at wvhch slump is lost c m be drastically increased with the incorporaaon of chernical admixtures into concrete (Jiang et d, 1998; Meyer and Perenchio, 1979)). It is essencial that the Gesh concrete cemains workable so that the trauspomtion, placng, consolidation and tinishing can be done without excessive effort and in reasonable tirne. Concrete

suffenng Erom hadequate workability will be either re-tempered with water in the field, to irnprove workability, or wil( not be sufficiently consoiidated or poorly finished; neither

a desirable outcome (Hover, 1998). In low W/C ratio concrete where the use of a superplasticizer is a musG the

rheological properties become largeIy a hnction of the superplasticizer dosage and its interaction with other reactive components of the concrete. The influences of water

reducers and superplasticizers have ~ . p i c d y been as e-xpected. However, as w r e n t practices are reducng the \kater-cementitious mat* ratio to extremes, une-xpected

behaviouc occasionaliy arises in particuiar cernent-niperpiasticker cornbinations, despite the fact that both components have satisfied th& respective acceptance requirements. Such phenornena are usually deemed as cases of cernent-superplasacizer incompatibility (Atcin et al., 1994).

ISSUES IN ADMIXTURE COMPAflBlUTY

It shouldn't be assumed chat any indiscriminate combinaaon of admiunises and

cernent \dl behave satisfactory- An admixture rnay have a proren performance record when used separately; however, it mav produce compatibility problems a-hen used in combination with others (Neville, 1977). Furthermore, assurnptions should not be drawn on the performance of an admixture if another admixture that belongs to the

same genenc type has perfomed accepmbly. Admivtures of the same genePc type may still bebave differently in concrete due to variations in molecular weight, the different cations associateci with them, chah length, erc. The s a m e condusions caa be drawn regardmg the behaviour of cements of the same type; differences in mineral, alkali, sulp hate contents and heness can vary ciras tically (Ramachandran, 1995).

The reasoos for the seasitiviry of some cements to these issues are poorly

unders tood, but typicdy can be easily remedied by the replacement of either the cernent or of the adrm-tue. Howeier, an understanding musc be dwdoped so that the

occurrences of such events can be minimised, if not elimio;ited. The research p e r f o d in this field has evpanded drasticdy in the past few deades and many mechanisms of incompatibiiity have been discussed.

Since cernent consists of multiple compooents and phases, the chemistry involveci in i a hydration is lairv complicated and becomes Lrther agpvated by the addition of chernical admi~tures. The comple'rity of these interactions is compiicated by chernical admiutmes due to the great varkty of components, both organic and inorganic,

in their formulations (Heimuth et al., 1995). Hence, one shouid e-xpect that any esplmation on the causes that ma produce compaa i ty problems would involve many mechanisms, both p hpical and chemical. The cernent-superplas ticizer interaction c m be affected by nurnerous parameters of either the cemen& such as its chernical and phase composition, its heness and its content of sulphares and allialis; or of the superplasticizer itselc svch as its chemical nature (or molecular structure), its molecular weight, its degree of sulfonation and its counter ion (Jiang et al., 1998; Ramachandran,

ISSUES IN ADMlXTURE COMPATBlLlTY

1995). Unfortunately, knowing the chemical compositions of the cernent or c h a n i d admixtures c m dratv no dehnite condusions, ody generai trends have been discovered.

There have been reported fieid cases where concretes have experienced an unexplained rapid slurnp loss soon after mi+ (Hersey, 1975; Meyer and Perenchio, 1979; Dodson and Hayden, 1989). These occurrences usu* arise in the presence of chemical admixtures in a Iimited aumber of cernents, even though when used alone these cements exhibit no tendency to produce abnomal se* (Hersey, 1975). The factors respousible for the slurnp loss of admixture enhanced concrete, although complicated, may be attnbuted to an amay of influences induding initial slurnp value, type and arnount of SP used, type and arnount of cernent, time of addition of SP, humidity, temperature, miuing, the presence of other ad&inires in the miu, and the accelerated formation of e ttringite (Ramachandran, 198 1 ; Ramachandran, 1995). Extensive research has been conducted in this field ooer the years, and the key principies are:

1. The C A content of the cernent; 2. The effect of the sulphate content and its form in cernent;

3. Aikalis and their influence 4. Average molecular weight of the superpiasticizer used;

5. Delayed addition of the superplasticizer; 6. Temperame of the concrete;

7. The fmeness of the cernent and the presence of excessive &es; and 8. reduced paste and water content; and 9. Superplas ticizer degree of sulfonation and nature of the counter-ion,

Each of these factors =dl be discussed in the foiiowing sections.

ce men^ being the most reacuve cornponent in concrete, h a , received much attention in terms of research to determine the effects of its composition on the rheological behaviour of fiesh conccete. The foiioaing sections desabe some of the

ISSUES IN ADMIXTURE COMPAfl6lLlN

relex-ant issues, as determined by previous works, regardkg the composition of the cernent and th& influences on slump and slump loss in superplasticized concrete.

Comprising only a s m d percemage of a portland cernent's composiaoo, the C A present can sgni i can* affect setting and eady hydraoa. For this reason much of the

previous work regarding the influences of amer reducers and superplasticizers has been devoted to studying theif role in C d hydraaon (Ramachandran, 1995). It has long been accepted that due to their high reactivicy Mth water and rapid hydraton, relative to the other phases of the cemeoh the aluminate phases and th&. hydratioa producn are key to the early hydration processes and slump loss. The behavour of hydrating cements and structural formation during the &t tsvo to three hours is governed by reactions of the aluminate phases, partidady tncalcium aluminate. The processes of se+ and early

sue+ development, on the other hand, is mostly developed by the hydration of the silicate phases, particdady C jS aolicwur and S k d , 1998).

C d sen-es as a base for the development of a loosely coagulateci structure. In tirne, it protides a rapidly det-eloping crystalhe stmcnrre of ettrkigite, which et-entually permeates the entire volume of the systern. This structure may be destroyed, and usudy is destroyed through the mechanicd action of mixing. This results in the fomiation of small free crystals of etmngite, which determines the water requirements related to workability. The comparatively slow silicate reactioas that determine the strength of

cernent paste in hardened mortars and concrete develop against the background of the aluminate reactions (Khaiil and Ward, 1980).

In portland cemen t the dumina containing phases, especially the ticalcium aluminate, are the phases that react rapidly euough to gire rise to the undesirable quickset (Lerch, 1946). As oon as contact is initiated benk-een the cernent and miter, the rate at which heat is evoloed inaeases rapidly and reaches a s h q peak. This peak has been a&buted to the rapid reaction of C d compounds. In comparson with silicates,

ISSUES IN ADMIXTURE COMPATIB1LilY

LITERATURE REVIEW 26

aluminates are g e n d y tugh in reactiviy and react very rapidly owing to the high solubility ofboth the unhydrated and hydrated states (Khalil and Ward, 1980).

Invesgators have ofien related the problem of rapid slump loss to the C d content of superpksticized concrete due to its high reactivity, g i k b i g it the greatest potential influence upon slump loss. The results of these investigations are cootroversd

because CJA reactions are highly dependent upon the amount of gypsurn added to the cernent to control its se* (Khalil and Wrd, 1980). However, its influence cannot be complerelr dismissed. Research bas indicated that the tricaluum alumliare phase, and to a lesser degree te+ncalcium durninoferrite, and their hydration pro duc^ exhibit preferenual adsorption of water reducers and superplastickrs, thereby rend- them inactive and allowing the main cementing compounds (CS and CzS) to control the reaction (Blank et al., 1963). Due to its rapid adsorption of dispersants, the C d phase may act as a sink for water-redung and superplastiaMg adxriistures, rend- them una~dable for fluidifcatiou of the mis (Ramachandran, 1995). Accordlig to Malhotra, cernents having moderate to high C d contents ( C d > 9.0h) when used in superplasucized concrete -hibit increased slurnp loss, @Ialliotra, 1981), while the opposite eeffet c m be witnessed in concretes made with cements h a ~ g a low C A content ( C a < 5.0/o) (LMalhotra, 1981).

It is important to realise that the duminous compourtds of portland cernent hydrates very rapidly, and if dowed to proceed unchecked w d form a structure built of s tout crysds of calcium hydroaluminates, resulting in a fias h set (Guo, 1994; Helmuth et al., 1995). Thus it is elident that the rate at which C3A hydrates musc be regukted. For this purpose some fomi of calcium sulphate (CaSOr) is inter-ground with cernent clinker to moderate this reaction. The sulphate ions that go into solution control the reaction

rate by reactlig with the tricalcium to f o m miseci aluminate sulphate products namely

ettPngite and monosulfoaluminate (Dodson and Farkas, 1964; Feldman and Ramachandran, 1966; Guo, 1994; Jolicoeur and Sirnard, 1998; Lerch, 1946; Meyer and

ISSUES IN ADMIXTURE COMPATlBlLllY

LITERATURE REVfEW 27

Perenchio, 1979)). Due ro th& mtical role in the h y d d o n processes of cemenk any change to the suiphates and their duences on cement due to the use of chernical admixtures may be expected to be sigdcant-

To produce normal set portland cement paste, mortar or concrete, Cas04 must be sufficiendy soluble in the uater/cement aqueous phase to provide the calcium and sulphate ions necessary for the formation of an etuhgite on the cernent parlicies-

This protecuve film reduces the rate at w-hich M e r C d can enter solution. In cernent, SOS also serves a second purpose, in addition to retardlig the hydration of the aluminates; it is also responsible for acceleracing the hydration of the silicates, if present in the proper amounts (Haque et al., 1987; Khalil and Ward, 1978; Khalil and Ward, 1 980).

In a paste made Gom a commercial cernent an optimum concentration of Cas04 is soon estabiished in the iiquid medium so that the hydroaluminates produced are

immediately m e d Lito calcium sulfoalumliate or etmngite by reacclig with Cas04 and Lune. The etmiigite, when deposited on the surfaces of cernent proiides an

effective barsier to W e r reactions. This hLn of ettngite, which snrrdily retards the hydration, lasts as long as the Cas04 concentration of the iiquid medium remains at a certain htgh level. M e r this, hydration resumes its inpetus and a normal se- begins (Guo, 1994). For the initiai reactions that are necessq to form these dense coatings the solution must become rapidly saturated u,ith respect to both calcium hydroside and gypsum. Should this saturation not be achia-ed, or is delayed, there is considerable risk of early stiffening, or in extrerne cases, flash se5 associated with rapid reaction of the aluminates and heat liberation (Helmuth et al., 1995). In the absence of an effective retarder, such as h e l y ground gypsum, the release of siiica and dumina into soluaon

lads to the rapid precipimtion of alumina-silica gel that causes both early stiffening and retards hydration of C3S and strength de\-elopment (Helrnuth et al., 1995).

At this point it becomes necessary to distinguish the aferences between 'flash' and 'fdse' set. Flash set resuits Gom rapid hydration of C d in the absence of calcium sulphate and is often distinguished by its considerable heat evolution and the resultbg stiffening of fksh set which cannot be easily dispelled by mechanicd disturbance

ISSUES IN ADMIXTURE COMPATlBlLlTV

(Dodson and Hayden, 1989)- Talse' or 'plaster' set rnay develop if the gypsurn in the cernent has been dehydratecl to hemihydrate during intergkdmg with a chker that is too hor If the cement contains more hemihydrate than is required for immediate

combination with the aluminate phases, it aiu rehydrate and produce a rigid skdeton of gypsum when contacted with w-ater (Heknuth et al., 1995; lMeyer and Perenchio, 1979). If 'alse' set occurs afier miviag is completed, a brie re-mixing of the concrete aithout req* any additionai amer can d p elLniaare it

The rate at whch C a hydrates is slowed as long as gypsm is present The process of C d hydration may e v e n d y deplete the sulphate concentration in the aqueous solution. Shouid ali the gypsum be consurned before ali the C d has been reacted to orm calcium sulfoaluminate a rapid reactioo ad occur mith the remairing C d (Lerch, 1946; Meyer and Perenchio, 1979). Thus, the length of rime that C d persists is a hnction of the sulphate content of the cernent (Meyer and Perenchio, 1979).

The required gypsum content ro produce a propedy rerarded cernent is af6ected b - the composition and fineness of the cemen& dosage and type of admixture used, as weil as other iduences such as increased temperatures (Haque et al., 1987; Khalil and Ward, 1980; Lerch, 1946). Due to its effects on strength developmenb shrinkage, and n v e h g , there esists an optimum range of SOS contents (Haque et al., 1987; Ward et al., 1980), howvver, due to its impact on durability a masimum iimit has been instituted by certain organisations induding the ASIM. Due to its key role in the early hydration reactions of tricalcium aluminate it is obvious that should there be an alteration to the So l content some fom of abnormal setting may occur (Guo, 1994).

2.6.1.2.1 INFXUENCE OF WATER REDUCERS & SUPERPLAST~CIZER ON THE ROLE OF SIJLPHATES

As already noted, if the action of Cas04 in a paste is disturbed, abnomial setting take place. The incorporation o f a water-reducing or superplasticking admknue can

cause the disturbance necessary to initiate such a case of abnonnal setring- Even though

a cernent may behave normally nithout the addition of a chernical admixture, it rnay

LITERATURE REVlON 29

behare as if under-suiphated when one is presmt (Khalil and Ward, 1978). Si.rnddy, the S03 content of cernent oui have a pronounced influence on the slump loss of superplasacized concrete- Researchers have conduded that the S03 content of concretes should be optimised for use with such chemid admi~nrre (Kaque et al., 1987; Khalil and Ward, 1980; Ward et al., 1980). Optimisation of the S03 content can result in improved retention of the increased workability bene& obtained by the use of the admixture (Khaiii and Ward, 1980). HoWever, optimisation of the sulphate coatent of a cooaete is not viable. It would be exrremely dimcult for a manufacturer to opcimise the S O 3 coatent of his cernent for a particular admixture, since this wodd have to be doue at the time of production.

Cernent sery low in C d eidier does not require or requices v q iittle gypsum to provide set-control and to optimise the strength potentiai of the cemenc. In the presence of admistures, howeser, the gypsum requirements are rnodified to the =tent that \vithout the additionai gypsum such cements can e-&bit excessive retardation (Ward et ai., 1980). The presence of calaum Lignosulfonate (0 increases the optimum gypsum content for a given cernent-

As suggested by Tagnit-Hamou et. al. and Tuthill e t al. problems of cement/fluidiser incompatibility cari result from inadquate calcium sulphate in the

cernent of ION- w/c ratio pasres with superplasacizer or water reducers (Jiang et ai., 1999; Tuthill et al., 1961). The presence of chernical admisues can exert an influence on the availability of sulphate ions raising the required SO, content for nomial hardening, and may lead to the loss of concrete fluidity and belated harde* of the concrete (Aitcin et al., 1994). A pronounced flse setting of cernent pas te and, hmce, a rapid slump loss of concrete is usuaiiy brought about by the use of a superplasacizer or water reducer, which is probably amibuteci to delayed retardmg effect of CaSOr while the solution status of the latter is distubeci (Guo, 1994).

The strongest arguments against adjusMg the S o i content of cements commerciaily are (Xtcin, 1998) the as yet uannwered question of potential durability of concretes containiog greater than normal arnounts of SO3 and (A-tcin et al., 1994) the problem of quaiity control (Ward et ai., 1980).

ISSUES IN ADMUCTURE COMPATlBlUTY

LIT ERATURE REVlEW 30

Having gone through the effects of sulphate and its concenmtion in ponland

cemen& it is important to note tbat the form of sulphate used has a s w c a n t impact on

its ability to regulate the setting of superplasticized concretes. Key to the issue of sulphate's ability to produce cernent that cari perfomi as desired is that it must be readily available in the solution- Thus the rate at wfiich the SO4> ions c m dissolve into solution musc be chemicaily baianced with the chernical r e a c t i ~ q of the C a (A-tcin et al., 1994). Hence any changes to the solubilitg rate of the sdphates c m s ~ c a n d y influence the early setting properties of the concrete.

~Uthough gypsum (CaSO.&HrO) is more suitable for this task than other sulphate orms, the use of naturai anhydrite (CaSO4) is growing in popularity due to the reduced cost associateci Mth its use. N a d anhydrite, even when used in large amounts \dl perforrn normally in portiand cernent. Howerer, cases of early stiffening have been detected in concretes where mniral afihydrite (used to replace one-third or more of the gypsum) m s employed when water-reducing and superplasticizing admixtures u-ere utilised (Docison, 199; Dodson and Hayden, 1989; Helmuth et ai., 1995).

The problem of rapid set and impaired uater-reduction that results d e n anhy&te is used in superplasticized concrete is a consequence of its rate of solurion. The rate at which anhydrire goes into solution is slower (a few hours) when compared to dcin that of gypsum or calcium dpha t e hemihydrate; diis inabiliq to rapidly solubilize produces a ccsulphate-sraned" system in the concrete, in which the superplasticizer acts as if it has been added to an under-sulphated qstem (Dodson and Hayden, 1989). Dodson and Hayden also detected that in the presence of a calcium lignosulfonate WR that the rare of solution of the naturai anhydete can be further reduced, and the amount of soluble sulphate becornes insuffiuent to keep up with the demand of the C d (Dodson and Hayden, 1989).

Other problems regarding the sulphate content of the cernent are a result of the rnanufacniring processes. The recent use of hgh-sulphur fbels, air-pollution control sys tems, and recyding of cement kiln dust has led to significmtly higher dliker sulphate

ISSUES IN ADMIXTURE COMPATlBlUTY

LITERATURE REVIEW 31

contents (Heimuth et al., 1995). An excess of sulphate in the dinlrer will cause the alkalis present to be in the form of highly soluble aikali sulphates, which in this fomi can accelerate the hydration of both alite and C A or the escess sulphate miIl form as insoluble Gactions in the silicate or aluminate phases, or as anhydrite (Helmuth et al., 1395). Unfominateli; it has becorne more diffidt to propedy regulate the tirne of set by M e r addition of sulphates due to the presence of strict indusq limitations with a masirnum sulphate content. Also, indusrrial by-products are being used in some cases as a source of calcium sulphate, as a partial replacement for gypsum. The incorporation of these m a t d s may lead to further i-aciability in cernent performance.

2.6.1.3 FINENESS OF THE CEMENT AND EXCESSIVE FINES

A bnef ovemiew of the effects of the cernent fkeness is necessary. It is well

known that a cement of high heness \dl hydrate at a higher rate than a cernent of the same composition aith a lower specific surface. The inaeased surface area associated with a hgh fineness increases dramatically the amount of cernent physically contacted with water and afi-ailable for immediate hpdration. Aitcin et al ha\-e demonstrated that cernent particles hydrate at diEferent speeds according to the heness (Xtcn et al., 1987). As a result, the superpiascizer dosage wwdI have to be increased with cements of

increased 6neness to cornpensate for the rapid hydration of the fine cernent grains (Mtcin et al., 1994).

Accorduig to th& work, superpiasticizers are most effective in retarding the

hydration of the medium size hction of the cernent (4-30~). The superplasticizer is

unable to affect the hydration of the h e s t hction (c4 pm) due to th& high reactivity and the fact that the S03 and alkal is tend to concentrate in the fine fraction of the

cernent; and the coarse fraction of the cernent (30-72p) is not influenced b y the presence of a superpiasticizer since it is not very active in the hydraaon process et al., 1987).

The presence of ultra fine materiai, indudlig cernent and 6ne aggregate particdate inaeases the superpiasticizer demand of a conaete. "The h e part of a

ISSUES IN ADMIXTURE COMPAflBlClTV

conaete concentrates all of the swface area of the grains likely to adsorb molecules of superplasticizer" (De Larrard et al., 1997). Concretes possessing an excess amount of fine materials require a more substantial coating of aggregate surtace areas br the cernent paste. This additional surface a r a can hold some of the superplasticizer solution and prevent it kom fluidi-ing the miu (Dhir and Yap, 1983).

n e hneness and C d content of cernent govern the initiai flow characteristics of cemenL while floa- loss is dictateci mosdy by the ionic s~ength of the pore solutions (Bonen and S a r h , 1995). Thc rheological behaviour usvailp eahibited by high-allrali cements is considerably poorer compared to other cements containing low-er leveis of alkalis. The increased Bow- loss of hgh ionic strength pore soluaon cm be attributed to a greater electrostatic attraction (Bonen and S a r k , 1995). Increases to the concentration of alkalis n a d y alter the pH of a conaete and conoibute to a change in the ionic strength of the pore solution and thus influences the rheology of the cement paste

(Jolicoeur et al., 1991). The presence of alkdis in portland cernent clinker promotes the dissolution of Cu\, thus increasing its rate of reaction and accelerates early hydration. Alkali-rich solutions, provded that the ggpsum content is low, encourages the formation

of aluminate hydrate and produce a quick set in concrete(Jaweed and Skalny, 1978). The liquid phase of a cernent paste is ss f icant ly affected by the presence of

alkalis. When alkaii-containing cement is mtxed aith water, the allrali metai ions readily go into the tiquid phase of the hydrating system and influence the rate of hydration and the rnorphology of the hydration products vaw-ed and Skalny, 1978). The presence of alkalis in cernent causes a decrease in the initial ettxingite formation, but has the effect of

accelerating the C3S and C2S hydration (Jawed and Skalny, 1978). Increases to the a W content have the effect of dramatically increasing the

solubility of the sdphate ion (Dodson and Hayden, 1989). For cements of the same C d content, those high in alkalis react with gypsum more rapidly and require larger additions of gypsum than those low in a k a h (Lerch, 1946). The presence of alkalis in the cements

ISSUES IN ADMIXTURE COMPAliBltlTY

- - - - - - -- - -

causes a higher coasumption rate ofgppsum (Odler and Wonnemann, 1983). It appears that at least part of the &dis of the cernent are present in the aluminate phases, and that aluminate phases containiog allialis react uith =ter more rapidly than do similar phases which are W - k e e or of Io=-er alliali content- Thus the cements of higher alkali content require a lacger addiaon of gypsum for proper retardation than do similar cements of

l o w r alkali content (Lerch, 1946). Thus the possibility of flash set is naeased if insuffiaent levels of SOS are present in htgh alliali cements.

When the aUralis are present in sulphate Fonns ( N a 2 S 0 ~ or GSO4) the adsorption of superplasticizer on C d and CAF is inhibited, thus p 6 m q inaeased adsorption on C S and CzS, resulting in reduced viscosity of cernent paste (Nana et al., 1989). Unfortunately, increased adsorption of the superplasticizer onto the siiicate phases d result in retarded set- of the cernent However, the presence of eucessire allrali sulphates compresses the electric double layer, providing an increase in ~scosi ty of cernent paste. Hence, with regards to the fluidity of the cernent paste, cbere ests an optimum alliali sulphate levei of cernent paste containing SP (Nawa et al., 1989)). It must be noted that the presence of aikali sulphates does not alter the progess of C S and C d hydration. When alkalis are present in sulphate forms (NazSO4 or =O&), the tirne of set is accelerated due to the formation of syngenite that may result from reactions with

the gypsum (Oder and Womemann, 1983). The formation of a ngid syngenite structure not only Ieads to quick set- but also decreases the sulphate content in the liqud phase of the kydrating cernent to the estent that it cannot adequatelv retard the hydration of C d ; this in turn dso leads to e d y stiffening (Jawed and S U y , 1978).

There is some evidence that cements contaliing Na20 require larger amounts of gypsum than do similar cements containhg an quivalent quantitg of &O (Lerch, 1946; Jawed and Skalny, 1978). However, when relatively large amounts of S 0 3 are present in clinker a substanaal hct ion of the total allialis passes into the solution Mthin a few minutes (Jawed and Skalny, 1978).

In addition to th& influence on the hydration of 0rd.ina.q portland canenk alkalis also influence the pedormance of admktures. With increased concentrations of

ISSUES IN ADMUCTURE COMPA'IIBILIW

LITERATU RE REVlEW 34

alkatis the adsorption of admixtures onto the cement phases are deaeased considerably

(Ramachandran, 1995). The ratio of d p h u r to total alliali detemiines the quantity of ahdi sulp hates in a

ciinker. When a dinker contains relatidy large amounts of SO3, a substantial fiaction of the alkalis goes into solution within a fav minutes. In ION- S03 d