Zeolite Reinforced Autoclave Aerated Concrete (AAC)

Michael W. Grutzeck, Maria DiCola

The Pennsylvania State University, University Park, PA 16802

Abstract

Asbestos reinforced concrete is one of the best cementitious composites ever developed.

Due to the hazardous nature of asbestos fibers however alternates have long been under study.

The technology is straight forward. By introducing a fiber that is in equilibrium with the matrix,

the fiber will coexist with and perhaps even bond to the matrix. This behavior will provide a

degree of toughening of the matrix via mechanisms such as pull out, crack bridging and crack

branching. Unfortunately, unlike asbestos, some fibers are not in equilibrium with the matrix and

they will react and partially/totally disintegrate during curing.

Autoclaved aerated concrete (AAC) is light weight due to its cellular character, but it is

also a rather brittle material that is prone to chipping and breakage. It is a mix of Portland

cement, lime and either Class F fly ash or quartz flour. Its cellular nature is provided by the

reaction of flaked aluminum powder with the caustic mixture soon after mixing and molding.

AAC is steam cured in an autoclave at 180°C for 8-12 hours. In an attempt to toughen AAC it was

proposed to grow a zeolitic phase in situ and contemporaneously with the developing

tobermorite matrix during autoclaving.

It was known that certain zeolites could be synthesized and were stable at 180°C and it

was also known that zeolites could be made from Class F fly ash mixed with NaOH solution. It

was hypothesized that a tougher AAC could be produced by adding NaOH to the mix in order to

encourage the formation of zeolites. This was found to be true, but the foaming process in the

presence of NaOH became daunting. Runs were not reproducible. As an alternative, prereacted

zeolitized fly ash was gradually added to a fly ash AAC mix as a substitute for untreated fly ash

and returns. Samples were mixed and foamed in a conventional fashion. Runs were now

reproducible-rises tended to fill the molds completely and as a result final densities were nearly

the same.

Results reported here show that 40 wt% pretreated Class F fly ash that was partially

zeolitized could be substituted for 40 wt% Class F fly ash/returns in a conventional fly ash AAC

mix and that in doing so the AAC could be toughened by ~ 5%. Although this is not a very large

increase it does suggest that the inclusion of a second phase (zeolite A) that is in equilibrium

with the tobermorite that forms in the matrix can toughen AAC. If it were possible to foam a

mixture containing only NaOH and Class F fly ash in a reproducible fashion these zeolite

containing mixtures may provide both a tougher and a “greener” alternative to conventional AAC.

1. Introduction

Zeolites are a class of minerals known for their wide range of commercial uses. They

come in a variety of shapes and sizes, but micometer sized crystals are the norm. Although

occasional hand sized specimens of zeolites are found, to date it has been impossible to grow

such crystals in the laboratory. Perhaps this limitation on size is due to the process by which

zeolites are made to nucleate and grow in the laboratory; time scales are often compressed.

Synthetic zeolites are normally micrometer sized [1].

Zeolites have a relatively open three-dimensional framework structure. The framework is

based upon interconnected [SiO4]4-

and [AlO4]5-

tetrahedra arranged in such a fashion that the

linked tetrahedra define a limited number of regular polyhedra. The tetrahedra are linked

together at their corners through a common oxygen ion. Because of the charge deficiency caused

by the presence of Al in tetrahedral positions zeolites must contain counterions. These ions are

solvated and are incorporated in the voids and channels that occur in the structure along with

additional water molecules. Imagine for a moment a structure made up of tetrahedrally

coordinated (AlSi)O4 ions each linked to one another in three dimensions by a common oxygen

atom (if you will, by the “bars” in the playground structures we called “monkey bars”) and one can

begin to visualize the internal structure of a zeolite [2].

Zeolites can be synthesized from a range of aluminosilicate materials mixed with caustic

solutions (normally Na and K based). Many zeolites are synthesized in an autoclave at elevated

temperatures. The similarity of zeolite synthesis to autoclaved aerated concrete (AAC)

production is explored here. The fact that both AAC and certain zeolites can be synthesized

under saturated steam pressure in a steam heated autoclave at 180°C led us to believe that in situ

growth of zeolites could possibly be used to increase the fracture toughness of AAC. In fact it

has been demonstrated that zeolites can occur in equilibrium with crystalline calcium silicate

hydrate found in AAC, i.e. 1.1 nm tobermorite [3].

The objective of the current study was to determine whether or not an AAC sample

consisting of tobermorite and zeolite would be tougher than a conventional fly ash based AAC

sample. The chemistry seemed correct and it was well known that small amounts of a second

phase dispersed in a given matrix will toughen the matrix via several mechanisms including

crack bridging and pull out.

2. Background

The process of making an AAC sample is more art than a science. There are strict

protocols that must be followed time after time in order to achieve a semblance of product

consistency. Conventional AAC is made from either Class F fly ash or quartz flour [4-6]. Both

starting materials are fine grained and chemically reactive when mixed with Portland cement,

lime, anhydrite and Al powder [4-6]. Typical recipes are given in Table 1 [7]. Mixes can be

made with or without using returns. The industry uses returns and returns tend to make a better

block, but some scientists and engineers use return free mixtures for laboratory sized samples

because they are easier to formulate. Al powder is added at the last minute of mixing, just prior

to molding. Once added the mix is stirred for another 30 seconds or so and then dumped into a

mold. The mixture is very liquid and literally sloshes around in the mold. Once in the mold two

things start to happen. The Portland cement and lime react with the water. The cement dissolves

and a small amount of hydrate is formed. The lime slakes and produces Ca(OH)2 which also has

the benefit of removing some of the water from the mixture and thickening it slightly. The pH of

the solution rises to ~12 and the Al powder which up until this point has done very little will

begin to react with the now alkaline solution and begin to produce hydrogen gas bubbles. Soon

things are fizzing away and the sample rises to about twice its size. This is the aeration process

that produces the cellular character of AAC. Once the “cake” is finished rising and hardens to the

touch (~1 hour) the sample is cut to size and autoclaved at 180°C for 8-12 hours.

Given the limitations imposed upon us by the existing AAC technology it was our

intention to work with the existing mixing and curing process as much as possible so as to make

new discoveries easily adaptable by an existing AAC facility. Because zeolites are

aluminosilicates our first decision was to work exclusively with Class F fly ash based AAC and

Table 1. Formulation of a Typical Quartz AAC and a Fly Ash AAC (weight %)

Ingredient Quartz AAC (wt%) Fly Ash AAC (wt%)

with returns* w/o returns with returns w/o returns

Quartz Flour 25.4 34.7 -- --

Dry Class F Fly Ash -- -- 41.1 41.7

Dry Returns 10.0 -- 9.0 --

Portland Cement 14.0 14.7 10.2 19.7

Quick lime 9.0 9.4 3.5 4.8

Al flake 0.05 0.04 0.04 0.07

Anhydrite/gypsum 1.8 (G) 1.8 (G) 1.7 (A) 1.7 (A)

Water 39.8 39.4 34.5 32.1 *Returns are the recycled unautoclaved trimmings that the AAC industry uses as one of their raw materials.

begin to add NaOH to the basic mix to see if it was possible to grow zeolites in situ within the

normal tobermorite containing matrix. It was known that it was possible to mix Class F fly ash

with NaOH to produce zeolites [8-23]. It was also known that it was possible to blend Portland

cement with fly ash and NaOH and cure it at 185°C to achieve a phase assemblage containing

tobermorite and zeolite [3,24], but because of the caustic nature of the NaOH, it was in fact

impossible to foam the material in any fashion that was close to reproducible. The NaOH caused

the mix to foam in the mixer-which is not good. After trying oxidized Al powder, coarser grained

powders, chemical foams and the like this effort was abandoned. However, during the one year

spent on this task and some 150 trial mixtures information was gained on the phase chemistry

and behavior of these materials as a function of bulk density.

Using this information it was then decided to make a series of five samples using

prereacted fly ash, fly ash that had been reacted with ~2M NaOH solution in a 5 gallon

continually stirred bucket at 90°C for 3 days. The resulting partially converted slurry had a pH

near 10 and this seemed to indicate that the reaction had gone as far as it could. The slurry was

air dried at 38°C and then used as a partial to complete substitute for Class F ash and “returns” used

in the AAC making process. See Figure 1 later. Returns are the trimmings from the green cake

that is removed from the cake as it is trimmed and cut to size before autoclaving. Now it was

possible to keep conditions relatively unchanged – we did not add NaOH during mixing and thus

the Al powder reacted in a normal fashion. This allowed us to produce a Control Sample

consisting of 100 % fly ash and returns and companion samples containing 25 %, 50%, 75% and

100% zeolite substituted for the fly ash and returns. See recipes given in Table 2. It was found

that the zeolites were often consumed by the reaction of lime with the added zeolite to produce

tobermorite, but that on occasion they survived and continued to co-exist with tobermorite that

formed. These data are presented here. There is a suggestion that small quantities of zeolites (up

to 40 wt% substitution for fly ash and returns) add strength and toughness to a conventional

AAC block. This is a possible means of addressing toughness issues. Durability of the composite

block is under study. Zeolites could well lend freeze thaw resistance to the block because the

zeolites have large amounts of internal space that might be able to accommodate expansion.

3. Experimental Methods

Samples were mixed in a large laboratory sized Warring blender capable of holding 6

liters of liquid. Class F fly from Allegheny Energy’s Ft. Martin power plant was used as the major

ingredient. It was used as received or in a partially zeolitized state. The fly ash/zeolite was mixed

with vertical kiln lime from Mississippi Lime, Type I Portland cement from Medusa now known

as Cemex, anhydrite from U.S. Gypsum and Al powder from MD-Both Industries. Returns were

prepared from the same mixtures made without returns. After the mix had expanded and become

hard at 38°C, it was pulverized and dried. This became the source of returns. Returns are used by

the industry to incorporate green cake trimmings and waste. It has been found that including

returns in the AAC mix produces a better performing block. Once again see Table 2 for the

recipes used to make the 5 mixtures that were studied.

Table 2. Recipes used to make a mix that would expand and fill the mold to slightly overflowing. Weights in

grams. Figures in parentheses are wt% of total mix.

sample water Class F

fly ash

returns zeolite Portland

cement

anhydrite Miss.

lime

Al-

powder

control 2350

(34)

2800

(41)

613

(9)

0 695

(10)

116

(2)

238

(3)

3.3

(0.05)

25 zeolite 2415

(35)

2100

(30)

460

(7)

853

(12)

695

(10)

116

(2)

238

(3)

3.3

(0.05)

50 zeolite 2481

(36)

1400

(20)

307

(4)

1707

(25)

695

(10)

116

(2)

238

(3)

3.3

(0.05)

75 zeolite 2546

(36)

700

(10)

153

(2)

2560

(37)

695

(10)

116

(2)

238

(3)

3.3

(0.05)

100 zeolite 2612

(37)

0 0 3413

(48)

695

(10)

116

(2)

238

(3)

3.3

(0.05)

The ingredients were combined as follows. The liquid (hot tap water) was placed in the

blender and the returns were added to it and blended for the few minutes or so needed to weigh

out the fly ash/zeolites. These were then added to the blender and blended for 5 minutes or so

needed to finish weighing the other ingredients. Speed was adjusted with a rheostat to achieve a

vortex in the mixer. The Portland cement was added next and allowed to blend for 3 minutes.

The anhydrite was added next followed closely by the lime. The lime is a slow slaking vertical

kiln lime that slakes in 3-6 minutes. It was added and blended for 1 minute followed by the Al

powder dispersed in a little water. The entire mix was blended for 15 seconds and then poured

into oil coated 4x4x25 inch bar molds kept in a 38°C walk in curing chamber. The bars were

covered with a plastic tent and allowed to rise/cure overnight. They were then demolded and

autoclave cured at 180°C for 12 hours. After curing, the bars were cut to final size on a bandsaw,

notched at their midpoints to ~1/4 inch deep and tested for their toughness using a 3-point bend

test. The broken arms of the bars were cut into 4-inch cubes and tested for compressive strength.

Smaller samples were dried at 70°C to constant weight and their densities were calculated. SEM

and X-ray analyses were also carried out using a Hatachi S 3000 H SEM and a Sintag X-ray Cukα

diffractometer running at 2-4° per minute.

4. Results

A summary of dry density, compressive strength, bending MOR and fracture toughness

data for the 5 mixtures studied is given in Table 3. Bending MOR and fracture toughness were

measured using 4 x 4 x 27 inch bars of AAC that were notched (~ 0.5 inches) at their mid points

using a band saw. The method is similar to that used by Whittman and Gheorghita [25] to

measure fracture toughness. The bar was loaded as in a three point bend test which had the effect

of opening the preexisting crack. Toughness was calculated by measuring the area under the

stress-strain curve. An SEM image of the zeolitic fly ash is given in Figure 1, Figures 2-6

represent the microstructures of the zeolite substituted AAC samples, and Figure 7 provides

insight into how well the added zeolites survive the autoclaving process. The peaks present in the

X-ray pattern represent the phases in the AAC samples. Zeolite and tobermorite peaks tend to

grow in intensity as more zeolite is substituted for fly ash/returns. This suggests that crystallinity

of the sample is increasing, which is in fact also reflected in the accompanying SEM images.

Table 3. Dry density, compressive strength, MOR and fracture toughness of zeolite containing AAC samples.

sample Dry density

(Kg/m3)

Compressive

strength (MPa)

Bending MOR

apparent (MPa)

Fracture toughness

(KN/m3/2

)

Control 638 ± 13 4.85 ± 0.70 σ 0.527 ± 0.001 σ 0.1047 ± 0.0006 σ

25 zeolite 608 ± 19 4.23 ± 0.53 σ 0.613 ± 0.024 σ 0.1194 ± 0.0037 σ

50 zeolite 596 ± 14 3.35 ± 0.22 σ 0.590 ± 0.012 σ 0.1164 ± 0.0019 σ

75 zeolite 603 ± 5 2.74 ± 0.12 σ 0.424 ± 0.030 σ 0.0809 ± 0.0043 σ

100 zeolite 601 ± 11 1.97 ±0.09 σ 0.323 ± 0.007 σ 0.0652 ± 0.0004 σ

Figure 1. This is a micrograph of our prereacted fly

ash showing the extent of the zeolite overgrowths that

were produced during continual mixing of the fly ash

in a 2 M NaOH solution at 90°C for 3 days. It is

interesting to speculate as to the reactivity of these

grains with Portland cement/lime. A priori, it would

seem that the very fine grained zeolite crystals that

make up the overgrowth around unaltered glass cores

would be more reactive than the glass. Zeolites are a

proven pozzolan. For this reason it is expected that the

Na-Al-Si in the zeolite would combine with the lime in

the mix to produce tobermorite. This would expel Na

and Al which would in turn form hydrogarnet and a

rather caustic solution. The caustic could then react

with the glassy cores to produce more zeolites.

Figure 2. Control sample made without zeolite additive. Matrix is dense with some porosity. Open spaces are

being bridged by needle shaped crystals. The crystal development in the bubble is not that striking. The

particles that are present are covered with a film of extremely small fibrous crystals. The overall impression

is one of a “gel-like” appearance with crystal growth occurring on the surface of the gel..

Microstructure

The microstructure of the suite of samples is not as telling as one might expect. The basic

macro structure of AAC (hand specimen) consists of mm sized gas bubbles dispersed in a matrix

of some kind. In Figure 2-6 we reproduce a set of 5 photomicrographs that are as representative

as possible taken at equivalent original magnifications. What we show for each figure is a low

magnification overview of the appearance of a broken surface of the matrix material between

bubbles (1,000X on the left), and a much higher magnification of the crystal development in a

bubble (5,000X on the right). It has been observed that the inside of bubbles allow crystals to

grow into empty space and develop their crystalline structure to the fullest extent. Figure 2

represents the Control sample and Figure 6 represents the equivalent sample made with 100%

zeolite. The other figures represent the samples in between these end members.

Figure 3. Control sample with 25 wt% zeolite substituted for flay ash. The matrix looks dense with little if

any open spaces. There are spherical shapes present but few broken spheres (fly ash remnants). It looks as if

spheres are pulled out (plucked from the matrix whole) rather than breaking during the sample preparation

process. The crystal development within a bubble is very striking. There is a sense that the rounded shapes

are now covered with these intricate “balls of yarn” looking crystals.

Figure 4. Control with 50 wt% zeolite substitution for fly ash. The morphology of the 50:50 sample is also

dense but perhaps there is a bit more porosity. There is little spherical character left. Particles seem to be

more angular and well meshed together. There are some small needle shaped crystals present here and there

bridging gaps but this is not as common as in other samples.The bubble morphology once again has a gel-like

appearance, much like Figure 2. Crystals seem to grow out of the gel layer and link the particles together.

X-ray Diffraction Patterns

X-ray diffraction patterns are provided as a means of gauging how the zeolite

substitutions fare when mixed with Portland cement, AAC returns, lime and anhydrite. The

series represents a complete substitution of 3400 grams of a blend of 82 % Class F fly ash and 18

% returns by weight by our pretreated zeolite containing fly ash in four 25% steps. The trace at

the bottom of Figure 7 represents the control sample that contains no zeolites at all. The

Figure 5. 75 wt% Zeolite substitution for fly ash has the affect of reintroducing a spherical feel to the

morphology. Rounded particles seem to abound, some are covered fine crystals whereas others are smooth.

The latter situation suggesting that a covering was pulled away during sample preparation. The sapmle

appears to have porosity associated with the more spherical particles, they do not seem to be lined together as

well as other samples. The morphology development in the bubble indicates that two phases are present: the

ball of yarn crystals and some very smooth rounded particles. These could be glass remnants during

dissolution of a larger fly ash particle or the nucleus of growing zeolite crystals.

Figure 6. 100 wt% Substitution of zeolite for fly ash causes the sample to develop a noticeable “fuzzy”

appearance. The sample matrix consists of lots of spheres of fly ash covered with very fine crystals. Some of

the fly ash spheres have broken during sample preparation exposing scoratious glass cores. Particles seem to

have open pore spaces around them. The structure in the bubble again shows two phases. In this case the

fibers are straighter and better at filling spaces between the grains. Also present is the second phase

consisting of smooth crystalline shapes.

uppermost trace contains all zeolite and no flyash or returns. One is able to see a gradual change

in crystallinity with the addition of our Na-zeolites. As might be expected, the peaks for Na-P1

(N) increase as we add more Na-P1 to the system. Unexpectedly however the crystallinity of the

tobermorite (T) also increases. Mullite (M) and quartz (Q) are present in the fly ash. These tend

to remain reasonably constant. Some calcium carbonate (CC) and katoite (K) are also present in

all of the samples. Katoite and tobermorite are normally found in conventional fly ash based

AAC. The association of zeolite and tobermorite in the Na-zeolite containing AAC is a new

finding. There is a definite change in phases that are stable with one another occurring as we

move from one end member to the other.

Figure 7. X-ray diffraction patterns for zeolite substituted AAC samples. T=tobermorite, N=Na-P1,

K=katoite, M=mullite, Q= quartz, CC=calcium carbonate

5. Discussion

As is common when reporting AAC data, the data are often plotted as a function of dry

density. This serves the purpose of taking differences in density into account. For it is well

known that less dense AAC samples are not as strong as denser ones. In the current case however

formulation design tended to produce samples having approximately the same densities. See

Figure 8 – blue plot. The measured dry densities decrease slightly as one begins to substitute

zeolitic fly ash for raw fly ash+returns but then it levels off at ~ 0.6 g/cc for the next 4 mixtures.

It is a small drop, but one that might be anticipated in as much as rise volumes were kept

relatively constant and the fact that zeolite densities are lower than fly ash (for example window

glass = 2.5, zeolite = 2.0). Unlike density however there is a near linear decrease in strength with

increasing percentage of zeolite added (pink plot), values drop from ~ 5 MPa to 2 MPa. This was

unexpected. The SEM microstructures of these samples suggest that the more zeolite rich

samples were more crystalline, i.e. they contained a greater proportion of tobermorite and zeolite

Na-P1 than the control sample. Why a greater amount of crystallinity should reduce strength

remains puzzling. More gratifying was the fact that ~40 percent substitution of zeolite in the mix

resulted in a marked increase in bending strength and fracture toughness (light blue and yellow,

respectively). The SEM images suggest that the relatively blocky zeolite crystals might be acting

as barriers to cracking-perhaps causing crack bridging to occur.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 0.2 0.4 0.6 0.8 1 1.2

PERCENT ZEOLITE SUBSTITUTED IN AAC

DE

NS

ITY

(g

/cc),

CO

MP

RE

SS

IVE

ST

RE

NG

TH

(M

Pa x

10-1

), M

OR

(MP

a),

FR

AC

TU

RE

TO

UG

HN

ES

S

(kN

/m3/2

)

Figure 8. Plot of density ( g/cc blue), compressive strength (MPa /10 pink), MOR (MPa light blue) and

fracture toughness ( kN/m3/2

yellow) as a function of the percentage of zeolitic fly ash used in preparing the

mixture.

6. Conclusions

It is concluded that zeolites can be added to a Class F fly ash AAC mixture as a partial

replacement for raw fly ash/returns. Toughness and bending MOR rise slightly up to and reach a

maximum value at ~ 40 wt% zeolite additions. Beyond this crystallinity increases but toughness

and MOR fall off. Thus it is concluded that a small substitution of zeolite will increase

performance, but the increase is rather small considering the effort needed to achieve it. If

however one looks to other possible uses for such a material one can begin to envision how a

zeolite panel might be able to interact with the environment if placed inside an office building.

Such a panel could possibly adsorb moisture, organics and perhaps even deactivate bacteria and

viruses. These composites could be one of the so called “smart building materials.” As a new

building material a zeolite containing AAC could well provide a different level of protection than

conventional lime-silica building materials.

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