TRANSFORMATION OF WOOD ASH WASTE INTO CONSTRUCTION MATERIALS of woo… · Further classification of ashes is done by type of collection from a boiler: 1) Bottom ash, 2) Relatively

TRANSFORMATION OF WOOD ASH WASTE INTO CONSTRUCTION MATERIALS

Neven Ukrainczyk1, Eduardus A.B. Koenders1, Nina Stirmer2 1 Technische Universität Darmstadt, Institute of Construction and Building Materials, Franziska-Braun-Straße 3, 64287 Darmstadt, Germany, email: [email protected], [email protected] 2 University of Zagreb, Faculty of Civil Engineering, Department of Materials, Fra Andrije Kacica Miosica 26, 10 000 Zagreb, Croatia, email: [email protected]

SUMMARY: This work investigates partial replacement of cement with fly ash waste generated by burning forest residues and waste wood from the timber industry. The ash has minerals complementary to Portland cement with a relatively high amount of free CaO and MgO that might exert significant expansion. The ash exhibits hydraulic and pozzolanic activity that initially increases and subsequently decreases the amount of portlandite in the hydrated material. With ash addition the rate of hydration, strength and workability are decreased. Optimum dosage showed 15 % of ash, where it replaces 5 % of cement and 3.33 % of sand, which can still produce a structural grade cementitious material with acceptable workability and mechanical properties.

PRETVORBA PEPELA IZ DRVNOG OTPADA U GRAĐEVNI MATERIJAL

SAŽETAK: U radu je istražena djelomična zamjena cementa otpadnim letećim pepelom dobivenim izgaranjem šumskih ostataka i otpadnog drva iz drvne industrije. Pepeo sadržava minerale komplementarne portlandskom cementu s relativno velikim udjelom slobodnoga CaO i MgO koji bi mogli prouzročiti znatno širenje. Pepeo pokazuje hidrauličku i pucolansku aktivnost koje u početku povećavaju, a potom smanjuju količinu portlantida u hidratiziranom materijalu. Dodatkom pepela smanjuju se brzina hidratacije, čvrstoća i obradivost. Pokazalo se da je optimalno doziranje 15 % pepela koji zamjenjuje 5 % cementa i 3,33 % pijeska čime se može proizvesti cementni materijal konstrukcijske kvalitete uz prihvatljivu obradivost i mehanička svojstva.

1. INTRODUCTION

Woody and agricultural biomass are among the highest biomass potentials for energy production as a sustainable fuel. This will lead to the production of a foresee amount of 15.5 million tons of biomass ash in the EU-28 [1], which will double its current amount. Presently, most ashes in Europe are landfilled, causing financial and material losses as well as an environmental burden. In general, biomass ash composition and properties are highly variable depending on: 1) type of base-biomass feed stock, 2) geographical location, 3) combustion technology (e.g. fixed bed, or pulverized fuel boilers). Further classification of ashes is done by type of collection from a boiler: 1) Bottom ash, 2) Relatively coarse fly ash, and 3) Fine fly ash. A possible application for biomass ash is as a cement and/or sand replacement in cementitious materials [2-4]. However, some standards (e.g. EN 450-1) prohibit use of biomass ash in concrete. This results in rising costs for the biomass ash waste management that forces power plant owners to search new opportunities to recycle ashes.

2. EXPERIMENTAL

Materials used are: 1) Commercial cement CEM II/A-M(S-V) 42.5N. Blended cement was chosen for possible synergy with biomass ash, namely an activation of the pozzolans in the cement by alkalies in ash. 2) Fly ash produced by 1MWe co-generation plant Lika Eko-Energo d.o.o., Udbina, Croatia: with a fixed bad (moving grate) furnace fuelled by forest residues and waste wood from timber industry in Croatia. TGA was done with NETZSCH STA409, 10 K/min, 50mg Pt crucible with N2 flow of 30 cm3/min. The hydration was stopped by grinding the sample with addition of acetone in agate mortar (exposure to CO2 was minimized). Scanning electron micrographs (SEM) were obtained using a SEM-TESCAN VEGA TS5236LS scanning microscope. Samples were placed over a graphite strip and coated with gold. Specific surface area of cement and fly ash was measured by a BET method using Micromeritics ASAP 2000. Mortar mixtures M, M10F10, M15F15, M20F20, M5F15 are designated by M (reference: 450g cement and 1350g sand, w/b=0.5), followed by the number representing the percentage of mass ratio between wood ash and cement. Paste mixtures are prepared with w/b=0.5, where P is a reference (45g cement), P10 has 10 % cement replacement with wood ash, P15, P20, P30, and WA (only woody ash). By Le Chatelier tests (cylinders H=D=30mm), the paste expansion value is obtained as a difference (d2-d1) between distance of the Le Chatelier needle tips before (d1) and after boiling (d2).

Proceedings of the 1st International Conference on Construction Materials for Sustainable Future, Zadar, Croatia, 19 - 21 April 2017

101

Table 1: Experimental plan for mortars (for pastes see text).

Sample name

Stand. Sand, g

Cement, g Wood ash (WA), g

WA/binder, %

Sand replace-ment, %

Cement replace-ment, %

Workability, mm

M 1350 450 0 0 0 0 155±5

M10 1317 438 45 10 2.5 2.7

M15 1301 432 67.5 15 3.7 4.1

M20 1284 426 90 20 4.9 6.4

3. RESULTS AND DISSCUSION

3.1. WOOD ASH CHARACTERISATION

Chemical composition of wood ash obtained by XRF (on pressed tablets, mass. %) was: 15% SiO2, 55.5% CaO, 10.7% K2O, 2.59% Al2O3, 2.66% MgO, 3.98% Fe2O3, 1.4% SO3, 0.64% Na2O, 0.51% TiO2, 0.9% P2O5, 0.037% Cr2O3, 0.045 ZnO, 0.63% MnO, 0.127% BaO, 0.073% SrO, 0.032% CuO, 0.053% SnO2, 0.056% Rb2O, 0.1% ZrO2, 0.01% Y2O3. It shows a relative high level of CaO, MgO and K2O. Total alkali oxides may be considered acceptable in amounts up to 2% in cement and up to 5% in fly ash (EN 450-1). Alkali content in woody ash was around 10% which contributes with 1.5% for 15% replacement of the cement. However, higher additions of ash were investigated here as well, because the alkalies in biomass ash are were expected to activate the pozzolans in the blended cement, and subsequently combine with pozzolanic calcium-silicate hydrates in the long term. Besides alkalies, the ash also didn’t meet the following EN 450-1 requirements: reactive CaO less than 10%, reactive SiO2 greater than 25%, MgO less than 4%, the sum of (SiO2 + Al2O3 + Fe2O3) greater than 70%. Moreover, the ashes might require attention on the amounts of Cr, Cd and Zn. Chloride content of the ash was below 0.1% limit: 0.037% for water soluble, and 0.054% for acid soluble. The amount of unburned carbon (EN15104) was 1.87%.

Analysis of wood ash by X-ray diffraction (Figure 1) determined the main mineral phases of the sample as being: lime (free CaO, 4.5%), MgO (2%), larnite (β-C2S), calcium carbonate (CaCO3), quartz (SiO2), Brownmillerite (C4AF) and calcium aluminosilicate (C2AS). Excessive amounts of free CaO and MgO (above 1%) must be avoided as this may cause expansion, cracking and strength loss of the hydrated material.

10 20 30 40 50 60

0

200

400

600

800

1000

Re

lative

in

ten

sity

Wood Ash (WA)

CaO

-C2S

MgO

CaCO3

SiO2

C4AF

C2AS

2(CuK)

Figure 12: left: XRD analysis of fly ash; right: SEM micrographs showing the morphological diversity of woody ash particles

SEM-SE micrographs (Fig. 1 right) shows the morphological diversity of woody coarse fly ash, ranging from spherically fused to irregularly shaped and porous particles. The specific surface area obtained by BET for cement (1.66 m2 g-1) is higher than for woody fly ash (0.58 m2 g-1). This is in agreement with results obtained from the particle size distribution (Fig 1) where 50 vol. % of the fly ash particles were bigger than 146 µm, while for cement this was 24.5 µm with a maximal particle size of 40 µm.


102

3.2. EFFECT OF WOOD ASH ON CEMENT HYDRATION

The analysis of calorimetric results (Fig. 2 left) shows that the end of the induction period occurred after 2, 5, 6 and 7 hours respectively for samples with an increased content of woody ash. The hydration of pure cement achieved the highest and narrowest first heat maximum in the shortest time, i.e. the reaction quickly moved into the induction period. Contrarily to this, woody ash had an elongated first hydration maximum, and had not a visibly expressed induction period, but the reactions of dissolution and precipitation overlapped. With addition of woody ash, two initial maximums were expressed. The first, earliest one (at 0.1 hrs) was reduced, while the second (at 1 hrs) was increased (Fig. 2 left). The addition of ash showed a retardation of setting time, a lower main maximum of the reaction rate reached at later times (11, 15, 16 and 17h), while demonstrating the extensive retardation of cement hydration by ash. Due to the retardation of the hydration reactions the final heat evolved after 45 h significantly decreased with (10, 20 and 30%) ash addition: by -6%, -13% and -25% of the reference cement, respectively. The hydration of ash alone exhibited a significant hydration heat development, attributed to hydration reactions of CaO and MgO, as well as larnite and aluminate phases.

Results of volume stability (soundness) tests (Fig. 2 right) show that addition of more than 15% of ash results in unacceptable expansions (the limit value is 10mm) which increases very rapidly with further ash dosage. It is interesting that the 10% addition shows lower expansion than plain cement paste, as confirmed by three mixing repetitions of the test, with a total of 9 replicates. For hydration of plain ash the expansion of 70 mm was observed already after 24 h of curing, even without boiling. The effect of this detrimental expansion could be minimized/avoided by washing (pre-hydration and carbonation) and mechanical (grinding) pre-treatments.

0 10 20 30 400.0

0.1

0.2

0.3

0.4

0 1 2 3 40.0

0.2

0.4

0.6

0.8

Hydration time, h

P

P10

P20

P30

FA

T, K

Hydration time, h

P

P10

P20

P30

FA

T

, K

0 10 20 30

0.1

1

10

100L

e C

hatile

r e

xpa

nsio

n, d

2-d

1,

mm

cement replacement = WA/(WA + cem), %

WA as recieved

grinded: <80m

standard limit

Figure 13: left: isothermal calorimetry; inset: of the first 4 hours. right: Le Chatiler expansion

XRD results (Figure 3) shows the effect of ash addition. Beside clinker phases, also the following hydration products were identified: Ca(OH)2, ettringite, AFm (anionic clay) phases, namely C4AHx and Monosulphate aluminate. With elapse of hydration time there was a relative increase in diffraction peaks of the hydration products, predominantly Ca(OH)2, CaCO3 and calcium-silicate hydrate (CSH), as well as ettringite and AFm phases. Unfortunately, overlap of CaCO3 and amorphous CSH diffraction lines (at around 29.3 degrees two theta) made their separation and semi-quantitative analysis impossible. Hydration of ash alone showed the development of C4AHx, CaCO3 and/or CSH. The main effect of ash on hydration visible by semi-quantitative XRD analysis was in production of the C4AHx phase, which increased with ash addition and with hydration time. This demonstrates that ash had a reactive form of aluminate phases, namely C4AF and C2AS (Fig 1 left), which contributed to a pozzolanic reaction that reduced the content of the most soluble Ca(OH)2. Moreover, very interestingly was the hydration of ash alone, showing formation of a very stable, desirable and durable hydration product stratlingite (C2ASH8), whose origin may be attributed to a combined hydration of C4AF and C2AS [5].

The thermogravimetric analysis (Fig. 4) of investigated binders shows a gradual loss of mass during temperature increase. The mass loss is the consequence of the release of water from the hydration products until approx. 400 °C and decarbonization after approx. 600°C. The breakdown of Ca(OH)2 occurs at around 430 °C. The loss of mass until 400°C is the result of the breaking down of CSH gel, AFm phases and ettringite. Based on the loss of mass (water) at 430 °C and stoichiometry of Ca(OH)2 decomposition, the amount of Ca(OH)2 per mass of powder has been calculated by employing a tangential approach [6] and is shown in Fig 5 left. In the case of pure cement hydration, the Ca(OH)2 content reaches its maximum on the 7th day. In wood ash hydration, the maximum Ca(OH)2 content is achieved at day 3, which points to pozzolanic activity. By adding the ash, the reduction of Ca(OH)2 content occurs after 28 days of hydration. This can be explained with the pozzolanic activity of the biomass fly ash and/or the activation of pozzolanic ingredients in the cement.


103

Specimens mixed with a constant water-to-cement ratio showed a decrease in workability by increasing ash content (samples with 10, 15 and 20 % cement replacement). This can be attributed to a coarser size distribution of ash than cement only. With increasing cement replacement level the strength reduced. However, with a 15% dosage of ash, which replaced only 5% of cement, but 3.33% of the sand, still a good structural grade mortar (or concrete) with acceptable mechanical properties was produced. The results (Fig 5 right) also show that the compressive strength of all mixtures reduced once cured in a heated water bath at 100 °C.

10 20 30 40 50

Re

lative in

tesity E

trin

git

2(CuK)

Etr

ingit

Ca(OH)2

CaC

O3+

CS

H

AF

m

SiO

2

C3S

/C2S

Ca(OH)2

PC30

PC20

PC 1d

WA 1d

PC10

Hydration time = 1Day

5 10 15 20 25 30 35 40 45 50 55

Re

lative In

ten

sity

Ca(OH)2

Ca(O

H) 2

+ C

3S

/C2S

Etr

ingit

2(CuK)

Etr

ingit

Ca(OH)2

CaC

O3+

CS

H

AF

m

SiO

2

C3S

/C2S

PC30

PC20

PC 3d

WA 3d

PC10

Hydration time = 3Days

5 10 15 20 25 30 35 40 45 50 55

Re

lative

in

ten

sity

Ca(O

H) 2

Ca(O

H) 2

+ C

3S

/C2S

Etr

ingit

2(CuK)

Etr

ingit

Ca(OH)2

CaC

O3+

CS

H

AF

m

SiO

2 C3S

/C2S

Hydration time = 7 Days

PC30

PC20

PC 7d

WA 7d

PC10

5 10 15 20 25 30 35 40 45 50 55

Hydration time = 28Days

C2A

SH

8

PC30

PC20

Re

lative

In

ten

sity

Ca(O

H) 2

Ca(O

H) 2

+ C

3S

/C2S

Etr

ingit

2(CuK)

Etr

ingit

Ca(OH)2

CaC

O3+

CS

H

AF

m

SiO

2 C3S

/C2S

PC 28d

WA 28d

PC10

Figure 14: X-ray powder diffraction of pastes hydrated 1, 3, 7 and 28 days: effect of woody ash addition


104

200 400 600 800 1000

80

85

90

95

100

Ma

ss lo

ss, %

Temperature, °C

1D PC

1D 10FA

1D 20FA

1D 30FA

1D WA

200 400 600 800 1000

75

80

85

90

95

100

Ma

ss lo

ss, %

Temperature, °C

3D PC

3D 10FA

3D 20FA

3D 30 FA

3D WA

200 400 600 800 1000

70

75

80

85

90

95

100

Ma

ss lo

ss, %

Temperature, °C

7D PC

7D 10FA

7D 20FA

7D 30FA

7D WA

200 400 600 800 1000

70

75

80

85

90

95

100

Mass loss, %

Temperature, °C

28D PC

28D 10FA

28D 20FA

28D 30 FA

28D WA

Figure 15: TG analysis of pastes hydrated 1, 3, 7 and 28 days: effect of woody ash addition


105

0 5 10 15 20 250

2

4

6

8

10

12

14

16

Ca(O

H) 2

, %

(g/1

00g p

ow

der)

Hydration time, h

P

P10

P20

P30

WA

Heate

d in w

ate

r bath

at 100

oC

0 20 40 60 80 100 120 140

0

10

20

30

40

50

60

Hydration time, days

Com

pre

ssiv

e s

trength

, M

Pa

M

M10

M15

M20

Heate

d in w

ate

r bath

at 100

oC

Figure 16: Development of Ca(OH)2 during hydration quantified by TG analysis

Results show that wood ash is broadening the particle size distribution (PSD) of cement as it comprises particles smaller than 1 μm and larger than 100 μm. An extended De Larrard’s model [7] was implemented in Matlab and used for calculating packing density of a mixture of polydisperse constituent materials: cement, wood ash and sand. The aim of particle packing modelling was optimization of mixture of cement, wood ash and sand to obtain perfect fractions of these components to achieve the best packing density. In mortar mixture optimisation, fine fraction of ash was taken as a partial replacement of cement (3 - 6.5%), while ash coarse fraction was partially replacing sand (2.5 – 5%). Particle packing density and PSD of each constituent material was measured and used to calculate the mixture packing densities (Fig 6). The calibration of the packing model with the experimental results provided that the maximum packing density is achieved with 70% sand, 25% cement and 5% biomass fly ash.

0 20 40 60 80 100

0,48

0,50

0,52

0,54

0,56

0,58

0,60

0,62

Pa

ckin

g d

en

sity, vo

l. fra

ctio

n o

f p

art

icle

s

Cem. replacement, WA/(cem + WA), vol. %,

Model

Measured


106

Figure 17: Modelling of particle packing density of mixtures

4. CONCLUSIONS

Chemical requirements for woody ash use in concrete by EN 450-1 (year 2012) were not met due to coarse particle size distribution, an insufficient amount of pozzolanic oxides (SiO2, Al2O3 and Fe2O3) and an excessive amount of alkalies, reactive CaO and MgO. However, the ash was broadening the particle size distribution of cement as it comprised particles smaller than 1 µm and larger than 100 µm. This showed the potential of woody ash to improve the packing density of blends where both sand and cement were partially replaced by ash. Moreover, presence of clinker minerals showed potential as cement replacement material. Addition of 20% ash resulted in unacceptable expansions which increased rapidly with further ash dosage. This expansion was due to a delayed hydration of free and dead burned CaO and MgO.

Plain ash hydration produced a maximal Ca(OH)2 quantity at 3 days and decreased with further hydration demonstrating the pozzolanic activity of the ash. With increasing ash addition more Ca(OH)2 was produced initially than for plain cement due to hydraulic properties of the ash with a relatively high content of reactive CaO, but at 28 days, inversely, there was less Ca(OH)2 due to activated pozzolanic reaction.

Hydration of ash alone showed a development of C4AHx, stratlingite (C2ASH8), CaCO3 and/or calcium-slilicates hydrates. The main effect of ash on hydration, visible by semi-quantitative XRD analysis, was in production of C4AHx phase, which increased with ash addition.

With increase of cement replacement level, hydration kinetics, workability, compressive and flexural strength significantly reduced. However, the optimum dosage of 15% woody ash, where it replaces 5% of cement, but 3.33% of the sand, still produced structural grade mortar (or concrete) with acceptable workability and mechanical properties. Thus, potential reuse of ash could reduce landfilling and at the same time improve the sustainability perspective of cement production, reducing energy needs for cement production, cutting back in CO2 emissions, and preserving natural resources (i.e. limestone) with no concern for depletion of biomass ash supplies.

ACKNOWLEDGMENTS

Authors acknowledge the support of HRZZ project ‘’Transformation of wood biomass Ash into REselient Construction Composites - TAREC2’’. Credit is given also to Lika Eko-Energo d.o.o., Udbina, Croatia (Mr. Zeljko Lovrak) for supply of the woody ash samples.

REFERENCES [1] Carrasco-Hurtado, B; et al., Addition of bottom ash from biomass in calcium silicate masonry units for use

as construction material with thermal insulating properties, Constr. Build. Mater. 52 (2014) 155. [2] Berra M., Mangialardi T., Paolini A. E., Reuse of woody biomass fly ash in cement-based materials, Constr.

Build. Mater. 76 (2015) 286. [3] Cheah B. C.., Ramli M., The implementation of wood waste ash as a partial cement replacement material

in the production of structural grade concrete and mortar: An overview, Resour. Conserv. Recycling 55 (2011) 669.


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[4] Ukrainczyk, N., Vrbos, N., Koenders, E. A. B.: Reuse of Woody Biomass Ash Waste in Cementitious Materials, Chemical and Biochemical Engineering Quarterly 30 (2015) 137-148.

[5] Gosselin C., Gallucci E., Scrivener K., Influence of self heating and Li2SO4 addition on the microstructural development of calcium aluminate cement, Cem. Concr. Res. 40 (2010) 1555.

[6] Marsh B. K. and Day R. L., Pozzolanic and cementitious reactions of fly ash in blended cement pastes, Cem. Concr. Res. 18 (1988) 301-310.

[7] Fennis, S. A. A. M., Walraven, J. C., UIJL, J. A. den, Compaction-interaction packing model: regarding the effectof fillers in concrete mixture design, Materials and Structures, 46 (2013) 463 - 478.


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TRANSFORMATION OF WOOD ASH WASTE INTO CONSTRUCTION MATERIALS of woo… · Further classification of ashes is done by type of collection from a boiler: 1) Bottom ash, 2) Relatively