Durability and microstructure characteristics of alkali activated coal bottom ash geopolymer cement

Accepted Manuscript Durability and microstructure characteristics of alkali activated coal bottom ash geopolymer cement İlker Bekir Topçu, Mehmet Uğur Toprak, Tayfun Uygunoğlu PII:

S0959-6526(14)00625-8

DOI:

10.1016/j.jclepro.2014.06.037

Reference:

JCLP 4432

To appear in:

Journal of Cleaner Production

Received Date: 14 December 2013 Revised Date:

24 May 2014

Accepted Date: 12 June 2014

Please cite this article as: Topçu İB, Toprak MU, Uygunoğlu T, Durability and microstructure characteristics of alkali activated coal bottom ash geopolymer cement, Journal of Cleaner Production (2014), doi: 10.1016/j.jclepro.2014.06.037. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Durability and microstructure characteristics of alkali activated coal bottom ash geopolymer cement

Eskişehir Osmangazi University, Civil Engineering Department, 26480, Eskişehir, TURKEY E-mail: [email protected] Phone: +90 222 2393750/3217 Fax: +90 222 2393613 2 Afyon Kocatepe University, Department of Civil Engineering, 03200, Afyonkarahisar, TURKEY, [email protected], Phone: +90 272 2281423 Fax: +90 272 2281422

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İlker Bekir TOPÇU1, Mehmet Uğur TOPRAK1, Tayfun UYGUNOĞLU2,*

Abstract: Many studies have focused on the production of mortar and concrete without cement.

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This is referred to as geopolymer mortar or concrete. This paper discusses the effect of alkali oxides (Na2O = 8, 12, 16 wt.% and SiO2 = 0, 4, 8, 12 wt.%) on compressive strength, microstructure and durability of circulating fluidized bed combustion coal bottom ash (CBA) geopolymer cements (GC). Durability and morphology tests were carried out through heating and

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freezing tests. The highest compressive strength (25.83 MPa) was achieved at Na2O wt.% = 12, SiO2 wt.% = 8. The optimum atomic ratios for a compact microstructure were obtained for Si/Al between 3.5-4 and Si/Na close to 0.5. Following the sintering, the main reaction products (N-A-

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S-H gel) became more amorphous at 800 oC, attaining Si/Al and Si/Na atomic ratios of 4.54 and 0.98. Sodium carbonate formation was observed at 800 °C. Also, the strength loss of GC was

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only 6.77 % after 30 freeze-thaw cycles. The results show that durable geopolymer concrete without cement can be produced by using waste bottom coal ash. Therefore, the production of geopolymer concrete has a high environmental impact, decreasing waste material in addition to global warming.

Keywords: Coal bottom ash; Geopolymer; SEM-EDX; Compressive strength; Alkali oxides. * Corresponding author: Tayfun Uygunoğlu, Civil Engineering Department, Faculty of Engineering, Afyon Kocatepe University, 03200 Afyonkarahisar, Turkey. Tel: +90272 2281423; Fax: +90 272 2281422; e-mail: [email protected]

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I. Introduction In the last five years, the total worldwide cement consumption was 3.2 Mt. Due to a world-wide increase in the demand for ordinary Portland cement (OPC), cement production could represent

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nearly 10% of total global warming CO2 emissions in the near future (Benhelal et al., 2013; Yank et al., 2013). The extended usage of mineral binders such as fly ash, sicila fume, granulated blast furnace slag etc. in the poduction of cement or concrete is motivated by a number of

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considerations related to the decrease of environmental pollution and CO2 emissions (Puertas and Fernandez-Jimenez, 2003; Palomo et al., 1999; Swanepoel and Strydom, 2002). Geopolymer

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cement which can be formed from silica-alumina oxides mixed with alkali hydroxides and alkali silicates is seen as a potential alternative binder to Portland Cement (Davidovits, 1999; Hardjito et al., 2004). Amorphous geopolymers are obtained at condensation temperatures ranging from 20 to 90oC, while crystalline ones are formed in autoclaves at 150–200oC (Davidovits, 1989; Yank et

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al., 2014). Geopolymers possess rapid strength gaining, are resistant to environmental effects, have low thermal conductivity, high volume stability, as well as high fire and chemical resistance (Zhaohui and Yunping, 2001; Murayama et al., 2002; Duxson et al., 2007). Properties of

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geopolymers mainly depend on nature and concentration of the activator (K or Na), curing

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temperature, humudity and pressure (Querol et al., 2002; Palomo et al., 2004; Xu and Van Deventer, 2000). Because the geopolymers harden slowly at room temperature, they are usually subjected to mild curing temperatures. Most studies on geopolymers have been conducted with curing conditions of about 95% relative humidity and temperatures ranging from 30 to 85 °C. Accordingly, curing times may vary from several hours to several days (Bednarik et al., 2000; Santa et al., 2013).

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As the largest volume of calcined industrial waste, fly ash (FA) has been widely studied as a source material for geopolymer synthesis (Chindaprasirt et al., 2008; Alvarez-Ayuso et al., 2007; Komnitas and Zaharaki, 2007). It has been estimated that the price of fly-ash based geopolymer

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concrete is about 10–30% cheaper than that of ordinary Portland cement (OPC) concrete (Habert et al., 2011). Also, mechanical characterization shows that geopolymer processed from fly ash has a compressive strength of 61.4 MPa and a Young's modulus of 2.9 GPa (ul Haq et al., 2014).

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Investigations have shown that class F fly ash (SiO2+Al2O3+Fe2O3≥70%) geopolymer can be used to manufacture structural elements such as concrete columns and railway sleepers (Bakharev,

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2006) and confirmed their applicability as structural members (Temuujin et al., 2011). Another material that is obtained using thermal power is coal bottom ash (CBA). Bottom ash is a coarse, granular, incombustible byproduct that is collected from the bottom of furnaces that burn coal for the generation of steam, the production of electric power, or both (Santa et al., 2013). CBA is

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coarser than fly ash, with grain sizes spanning from fine sand to fine gravel. Geopolymer cement made from fly ash or granulated blast furnace slag requires less of the sodium silicate solution than other pozzolans such as metakaolin and zeolite in order to be activated (Temuujin et al.,

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2009). There are many studies on the use of different types of fly ash in the production of

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geopolymer concrete.

Andini et al. (2008) investigated the usage of coal fly ash for the synthesis of geopolymers. Two different systems with silica/alumina ratios and stoichiometric for the formation of polysialatesiloxo (PSS, SiO2/Al2O3=4) and polysialatedisiloxo (PSDS, SiO2/Al2O3=6) were prepared by authors. The alkali metal hydroxide (NaOH or KOH) necessary to start polycondensation was added in the right amount as concentrated aqueous solution to each of the

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two systems. The systems were cured at four different temperatures (25, 40, 60, and 85oC) for several different time periods depending on the temperature. The results showed that the systems under investigation are suited for the manufacture of pre-formed building blocks at room

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temperature. Guo et al. (2010) investigated the geopolymers prepared from a class C (SiO2+Al2O3+Fe2O3≥50%) fly ash (CFA) and a mixed alkali activator of sodium hydroxide and sodium silicate solution. They obtained a high compressive strength when the modulus of the

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activator viz., molar ratio of SiO2/Na2O was 1.5, and the proper content of this activator as evaluated by the mass proportion of Na2O to CFA was 10%. Temuujin and van Riessen (2009)

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investigated the influence of raw and calcined fly ash on properties of geopolymer concretes. Preliminary calcination at 500 and 800oC causes decarbonation of the fly ash while it also leads to a decrease of the amorphous content of the fly ash from 60 to 57%. They found that geopolymer prepared using raw fly ash exhibited a compressive strength of 55.7 MPa, while the compressive

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strength of 500 and 800 ◦C calcined samples was reduced to 54 and 44.4 MPa. Temuujin et al. (2009) studied the influence of calcium compounds (CaO and Ca(OH)2) on the mechanical properties of fly ash based geopolymers. They cured the geopolymers at room temperature (20

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◦C) and 70 ◦C. Addition of calcium compounds as a fly ash substitute improved mechanical

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properties for the ambient temperature cured samples, while decreasing properties for the 70 ◦C cured samples.

The studies on fly ash based geopolymers are focused on their physical and mechanical properties. There are limited investigations on durability properties of geopolymers. On the other hand, due to the limitations of the standards, coal bottom ash (CBA) can not be used in the procuction of cement and concrete. Thus, the coal bottom ash that is stored in landfills causes

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environmental pollution. Furthermore, the transportation and storage of CBA is a cost. Activation of CBA is provided to obtain a binder material possessing similar properties as fly ash (Fernandez-Jimenez and Palomo, 2005; Hardjito et al., 2005; Barbosa et al., 2000). However,

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CBA has received less attention than other pozzolanic materials in the production of geopolymer cement. Therefore, the main objective of this study is to investigate the effects of alkali oxides on microstructure, atomic ratios (Si/Al and Si/Na) and compressive strength of goepolymer cement.

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Besides durability properties such as freeze-thaw (FT) and high temperature (HT), resistance of

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GC was investigated in this study.

2. Materials and methods

CBA from the Seyitömer Power Plant in Turkey was ground by a ball mill for 45 minutes to

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increase the surface area of the CBA from its original 930 cm2/g to 4100 cm2/g. The SEM micrograph of ground CBA is given in Fig.1. The chemical composition and physical properties of CBA are given in Table 1. As mentioned, the alkaline liquids are from soluble alkali metals

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that are usually sodium or potassium based. The most common alkaline liquid used in

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geopolymerization is a combination of sodium hydroxide (NaOH) or potassium hydroxide (KOH) and sodium silicate or potassium silicate. In the geopolymerization, as the H2O-to-Na2O molar ratio decreases, the compressive strength of geopolymer concrete increases. Furthermore, a higher ratio of sodium silicate solution-to-sodium hydroxide solution ratio by mass, results in a higher compressive strength of geopolymer concrete. Therefore, sodium silicate, (with content of Na2O=8.9%, SiO2=28.7% and water= 62.4%) and sodium hydroxide (98% purity) which were Merc commercial chemicals products, were used as the activation solutions (AS). The density of

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sodium silicate is 1.38 g/cm3. AS and water were mixed together one day prior of use for the series that are described in Table 2. On the day of casting, the geopolymer cement (GC), the AS and CBA were mixed together in a cement mixer for 1 minute at low speed. For all the samples,

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50 g water (including the water in the activating solutions) was used per 100 g of CBA, as it was found to be the optimum amount in our previous study (Temuujin et al., 2009). Following paste mixing, cubic specimens in the size of 40x40x40 mm were cast. GC specimens were cured in an

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electrical oven at a temperature of 80 oC and a relative humidity of 40% immediately after casting. After 20 h they were removed from the oven and stored at an ambient temperature (20 oC

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and 90% relative humidity). Optimum alkaline solution was determined by the values of the oneday compressive strength test carried out with a load-controlled pressure on geopolymer concretes (GC) according to standard EN 196-1.

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The main microstructural differences in GC16-0 (activated with 16% Na2O) and GC12-8 (activated with 12% Na2O and 8% SiO2) were examined through Si:Al and Si:Na atomic ratios obtained from SEM-EDX analyses. SEM-EDX examinations were carried out on the broken

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portions of the compressive strength test samples with a scanning electron microscopy (Zeiss 50

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Lastly, the effect of the SiO2/CBA ratio on mass and the compressive strength loss of GC16-0 and GC12-8 exposed to freezing-thawing (FT) periods and high temperature (HT) effects at 800 °C, were investigated on GC samples. TS 12390-9 (2012) is a durability test for concrete in

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which the specimens are exposed to cycles of freezing (1h at -20 °C in air) and thawing (2h at +20 °C in water) for 30 periods, and then their compressive strengths are measured. On the other hand, as a control series, compressive strength of unaffected specimens were measured for

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comparison. GC were subjected to 800 °C at a gradual incremental rate of approximately 6 °C/min from room temperature. When 800 °C was reached, specimens were maintained for an additional 3h. Then the furnace was shut down for cooling of the specimens to room temperature.

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After the HT test, mass loss and residual compressive strength were determined. All the reported

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results are the means of three samples.

3. Results and Discussion

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3.1. Effect of alkali oxides (SiO2, Na2O) on compressive strength of GC

Compressive strength is one of the most important parameters of concrete and is considered the

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characteristic material value for the classification of concrete. In the geopolymer synthesis,

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concentration of alkali activators significantly affects both the compressive strength and micro structure of geopolymers. The effect of alkali oxides (SiO2, Na2O) on compressive strength of GC is presented in Fig. 2. The test results shown in Fig. 2 demonstrate that the compressive strength of GC increases with the increase in the concentration of sodium oxide. The compressive strength of concrete specimens increases as sodium oxide concentration in the aqueous phase increases from 8 to 16 M; however, it decreases with a further increase in silica oxide concentration. It is determined that an increase in alkali concentration enhanced the geopolymerization process

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resulting in an increase in the compressive strength of SCGC. However, excess hydroxide ion concentration caused aluminosilicate gel precipitation at the very early stages, and subsequent geopolymerization was hindered, resulting in lower strength. The GC8 series has the lowest

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compressive strength when compared to other series (GC12 and GC16) due to the fact that it is stiff and difficult to place into the molds. Therefore we focused on the GC12 and GC 16 series. GC12-8 achieved the highest strength (25.83 MPa). Optimum strength values are obtained if the

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molar ratio of SiO2/Na2O varies from 1.0 to 1.4 (Swaddle, 2001; Fernandez-Jimenez and Palomo, 2005) while it was 0.69 (GC12-8) in this study. It was concluded that optimal alkali ratios

literature (Skvara et al., 2005).

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significantly change with the geopolymer raw materials and curing methods, as reported in the

The compressive strength of the material is enhanced by a higher SiO2/CBA ratio, which

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increases the number of strong Si-O-Si bonds in the final product. The amount of Na2O/CBA is assumed to play an important role in building up the the formation of crystalline structures of sodium aluminum silicate phases responsible for the geopolymerization reactions. The dissolution

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and hydrolysis of silica in alkaline water solution were significantly improved with an increase in

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the Na2O/CBA ratio, but excessive alkalinity adversely affected the strength of the final product due to the excess concentration of OH as demonstrated by Lee and Van Deventer (2002).

3.2. Microstructure

The SEM-EDX micrographs of GC16-0 (12.15 MPa) and GC12-8 (25.83 MPa) are presented in Fig. 3a and Fig. 3b. GC12-8 has notably denser microstructure than GC16-0. Atomic ratios (%)

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on specific points of the GC are given in Table 3. The Si/Na or K atomic ratio in an alkaline silicate solution affects the degree of polymerization of the dissolved species (Provis and Van Deventer, 2007; Davidovits, 1989). The Si/Al and Si/Na atomic ratios of GC16-0 were 3.27 and

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0.76 (EDX 1). The alkaline aluminosilicate systems called “geocements” is symbolized by N-AS-H. The formation process of these systems is a polycondensation. N-A-S-H gel in GC12-8 became more amorphous and more compact, attaining Si/Al and Si/Na atomic ratios of 3.99 and

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0.56 (EDX 2). A higher Si/Al atomic ratio and lower Si/Na atomic ratios in GC12-8 correspond to a more crystalline stage of the aluminosilicate gel. Increase in crystalline products increased the

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compressive strength of the geopolymer (Rattanasak and Chindaprasirt, 2009).

For N-A-S-H gel, some deposits on the surface and a considerable amount of unreacted sodium crystals around the CBA particle can be seen in Fig. 4a. This is mainly attributed to the

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insufficient dissolution of sodium crystals or the absence of sufficient amorf silica as illustrated in the literature (Lee and Van Deventer, 2002; Provis and Van Deventer, 2007; Rattanasak and Chindaprasirt, 2009; Slavik et al., 2008). GC12-8 exhibited quite a different microstructure. The

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gel structure formed during alkali activation and partially attacked CBA particles cemented to

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each other are seen in Fig. 4b. Thin sodium crystals in small quantities indicate that most of the sodium was consumed in geopolymerization reactions. The Si/Al and Si/Na ratios of GC16-0 were 3.91 and 0.51 (EDX 3) as shown in Fig. 5a. It can be seen that the atomic ratio of Si/Al increases and Si/Na decreases as the geopolimerization proceeds. It was observed in Fig. 5b that geopolymerization was completed before the N-A-S-H gel formed, indicating a moderate degree of reaction in the system, when the Si/Na atomic ratio is over 2 (Slavik et al., 2008). This arises from the high Si/Na atomic ratio (2.79) of the GC in EDX 4.

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3.3. Durability

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The compressive strength of GC decreased after the FT cycles. Fig. 6 showed that GC with a higher SiO2/CBA ratio has higher resistance to FT cycles. It was observed that GC12-8 possessed excellent FT resistance. Finally after 30 FT cycles, the compressive strength of GC had not

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decreased more than 20%, exhibiting correlation with the literature (Slavik et al., 2008). The correlation coefficients were high enough to show that there is a strong relationship between the

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S/CBA ratio-compressive strength loss of the GC exposed to FT cycles. The compressive strength loss of GC16-0 and GC12-8 decreases with an increase in the SiO2/CBA ratios. The SEM micrograph of GC12-8 after 30 FT cycles are given in Fig. 7. It was clearly seen that the structure of GC12-8 was destroyed and some cracking and fragmentation were observed. EDX 5 and EDX

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6 tests were made on microstructure of samples, and it resulted in lower Si/Al atomic ratios of 2.23, 1.41 and higher Si/Na atomic ratios of 2.23, 1.41 and 1.64, 1.56 with respect to N-A-S-H

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gel.

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The mass loss and compressive strength loss of GC12-8 and GC16-0 were determined after exposure to the HT effect (800 oC for 3 h). It can be noticed in Fig. 8 that, when the samples are exposed to 800 oC, the mass loss is about 15% in GC produced without sand, while it significantly decreases up to 7% for the specimens with a SiO2/CBA ratio of 3:1. This was attributed to the lower geopolymer content of this specimen. When exposed to 800 oC for 3h, GC12-8 and GC16-0 with a SiO2/CBA ratio of 0:1 showed compressive strength decreases of 27.25 and 29.14%, since these GC produced with a SiO2/CBA ratio of 3:1 showed deterioration

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of strength of up to 41.76 and 43.88% (Fig. 9). It was clearly seen that sand usage negatively affects the mechanical properties of geopolymers exposed to high temperature. This is due to the varying thermal expansion of geopolymer paste and the sand particles leading to the formation of

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cracks inside the specimen (Davidovits, 1999). Most of the geopolymer gel became more amorphous, attaining Si/Al and Si/Na atomic ratios of 4.54 and 0.98 (EDX 7) after the sintering reactions at 800 oC. Some part of the geopolymer gel showed quite a different microsructure

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attaining Si/Al and Si/Na atomic ratios of 4.20 and 1.93 (EDX8). Fig. 10d points to white crystals

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attributed to sodium carbonate crystals (EDX 9) in the geopolymer gel.

4. Conclusions

In this study, a number of durability properties and microstructure charactersitics of coal bottom

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ash based geopolymer concrete was investigated. Alkali oxides have been shown to significantly influence the microstructure and strength of the GC. Depending on the increase in the amount/content of Si ion in the N-S-S-H gel, the compressive strength of the GC12-8 series

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significantly increased. It was evidenced that a large quantity of sodium crystals appeared in

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GC16-0 due to excessive usage of NaOH. The optimum atomic ratios for a compact microstructure were obtained for Si/Al between 3.5-4 and Si/Na close to 0.5. The structure of the GC was destroyed and some cracking was observed following the FT cycles. GC with higher SiO2/CBA ratios showed higher FT resistance. N-A-S-H gel became more amorphous, attaining Si/Al and Si/Na atomic ratios of 4.54 and 0.98 following the sintering reactions at 800 oC. Furthermore, sodium carbonate formation at 800 °C was observed in GC16-0. With the increasing ratio of SiO2/CBA, mass loss at HT decreased while the strength loss at HT increased.

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This was attributed to the varying thermal expansion of the geopolymer paste and the sand particles leading to the formation of cracks inside the GC.

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Acknowledgement The authors are grateful to Anadolu University, Department of Materials Science and Engineering for the SEM investigations.

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Table 3. Atomic ratios (%) on specific points of N-A-S-H gel

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Table 4. Atomic ratios (%) on specific points of N-A-S-H gel after FT and HT effects.

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Table 1. Chemical composition and physical properties of CBA % weight of CBA 18.76 9.57 51.51 79.84 5.08 0.93 0.07 2.56 0.52 0.007 0.005 10.85

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Chemical composition Al2O3 Fe2O3 SiO2 S+A+F CaO MgO P2O5 K 2O Na2O SO3 Cl LOI Physical properties Specific gravity Blaine fineness (cm2/g)

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2.21 930

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Table 2. Samples composition of alkali oxides per 100 g of CBA Sample

a

Na2O, %

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SiO2, %

Ms

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GC8-0 8 0 0.00 GC8-4 8 4 0.52 GC8-8 8 8 1.03 GC8-12 8 12 1.55 GC12-0 12 0 0.00 GC12-4 12 4 0.34 GC12-8 12 8 0.69 GC12-12 12 12 1.03 GC16-0 16 0 0.00 GC16-4 16 4 0.26 GC16-8 16 8 0.52 GC16-12 16 12 0.78 a : Percentage with respect to CBA content (in mass). b : The sodium silicate modulus, Ms (Molar ratio of SiO2 to Na2O).

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Na/Al 4.30 7.12 7.67 1.09

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Table 3. Atomic ratios (%) on specific points of N-A-S-H gel. Al Si C Ca Fe Si/Al Si/Na EDX No Na 1 4.37 1.01 3.30 19.84 0.33 0.75 3.27 0.76 2 5.02 0.70 2.79 8.00 1.36 0.40 3.99 0.56 3 5.63 0.74 2.89 7.98 1.27 3.91 0.51 4 3.95 3.63 11.01 3.34 0.76 2.03 3.03 2.79

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ACCEPTED MANUSCRIPT Table 4. Atomic ratios (%) on specific points of N-A-S-H gel after FT and HT effects. Al 1.71 2.31 0.82 0.80 0.05

Si 3.82 3.25 3.73 3.36 0.82

C 15.21 6.52 7.62 7.42 25.46

Ca 0.56 0.62 1.78 1.50 0.07

Fe 1.21 -

Si/Al 2.23 1.41 4.54 4.20 16.4

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Si/Na 1.64 1.56 0.98 1.93 0.24

Na/Al 1.36 0.90 4.63 2.18 68.33

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Na 2.33 2.08 3.80 1.74 3.46

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EDX No 5 (FT) 6 (FT) 7 (HT) 8 (HT) 9 (HT)

ACCEPTED MANUSCRIPT Figure List Fig. 1. SEM micrographs of ground CBA (x1000). Fig. 2. Effect alkali oxides (SiO2, Na2O) on compressive strength of GC

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Fig. 3. SEM micrographs of (a) GC16-0 and (b) GC12-8 (x1000). Fig. 4. SEM micrographs of (a) GC16-0 (x6000) and (b) GC12-8 (x5000).

Fig. 5. SEM micrographs of (a) GC16-0 (x10000) and (b) GC12-8 (x27000).

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Fig. 6. Compressive strength loss- SiO2/CBA ratio relationship after 30 FT cycles. Fig. 7. SEM micrographs of GC12-8 after 30 FT cycles: (a) x500 and (b) x3000

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Fig. 8. Mass loss-SiO2/CBA ratio relationship after exposure to 800 oC

Fig. 9. Compressive strength loss-SiO2/CBA ratio after exposure to 800 oC

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Fig. 10. SEM micrographs of GC12-8after exposure to 800 oC

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Fig. 1. SEM micrographs of ground CBA (x1000).

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Silica-0% Silica-4% Silica-8% Silica-12%

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0 12

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Compressive strength, MPa

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Na2O, %

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Fig. 2. Effect alkali oxides (SiO2, Na2O) on compressive strength of GC

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a

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Fig. 3. SEM micrographs of (a) GC16-0 and (b) GC12-8 (x1000).

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b

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Fig. 4. SEM micrographs of (a) GC16-0 (x6000) and (b) GC12-8 (x5000).

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Fig. 5. SEM micrographs of (a) GC16-0 (x10000) and (b) GC12-8 (x27000).

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30 y = -3.538x + 26.22 R² = 0.9611

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Fig. 6. Compressive strength loss-SiO2/CBA ratio relationship after 30 FT cycles.

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b

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Fig. 7. SEM micrographs of GC12-8 after 30 FT cycles: (a) x500 and (b) x3000

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10 y = -2,417x + 15,925 R² = 0,9534 5 2 1/1

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y = -2,947x + 19,43 R² = 0,9974

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SiO2/CBA ratio

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Fig. 8. Mass loss-SiO2/CBA ratio relationship after exposure to 800 oC

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45 GC16-0

y = 4,581x + 25,515 R² = 0,9476

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30 y = 4,717x + 23,605 R² = 0,9667

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SiO2/CBA ratio

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Fig. 9. Compressive strength loss-SiO2/CBA ratio after exposure to 800 oC

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a

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Fig. 10. SEM micrographs of GC12-8after exposure to 800 oC

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Research Highlights

Effect of coal bottom ash, Na2O and SiO2 on properties of geopolymer was investigated.

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Low Si/Na ratio (0.5) correspond to a more crystalline stage of the N-A-S-H gel.

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The strength loss in geopolymer cement was 6.77 % after 30 freeze-thaw cycles.

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Sodium carbonate formations were observed at 800 °C.

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N-A-S-H gel became amorphous, attaining Si/Al and Si/Na of 5.04 and 1.06 at 800 oC.

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