Use of municipal solid waste incinerator (MSWI) bottom ash in high calcium fly ash geopolymer matrix

Highlights

• Municipal solid waste incinerator bottom ash (WA) and fly ash geopolymer was investigated.

• Compressive strength and microstructure of geopolymer matrix were tested.

• The results were compared to cement matrix containing WA.

• WA could increase the denseness and homogeneity of geopolymer and cement matrix.

Abstract

In order to reduce municipal solid waste incinerator (MSWI) ash, the effect of replacement level of high calcium fly ash by MSWI bottom ash on fly ash based geopolymer properties were studied. The results were compared to cement matrix containing MSWI bottom ash. It showed that the 20% MSWI bottom ash replacement level produced the highest compressive strength at 7 d of 52.8 MPa. The MSWI bottom ash could replace fly ash at high dosage level of 40% and the compressive strength was still higher than that of the control geopolymer mortar. The porosity, pore size distribution, and SEM indicated that the use of MSWI bottom ash could increase the denseness and homogeneity of geopolymer and cement matrix. In addition, the increase of MSWI bottom ash level significantly increased the crystalline phases of calcite and quartz as shown in the XRD pattern and C-O stretching in the FTIR images of both matrices.

Introduction

Geopolymer is an alternative cement binder synthesized by mixing aluminosilicate material and high alkali solutions (Davidovits, 1999). It utilizes by-products such as metakaolin (Pangdaeng et al., 2015), fly ash or bottom ash (Pavithra et al., 2016) as aluminosilicate source to react with high alkali solutions such as sodium hydroxide, potassium hydroxide, sodium silicate or potassium silicate. Fly ash geopolymers were found to have high early and later age strengths (Swanepoel and Strydom, 2002), excellent resistance to sulfate and acid attack (Sata et al., 2012), and well sealant to store CO₂ in the underground (Zhuang et al., 2016). Chindaprasirt et al. (2007) showed that high calcium fly ash could react with sodium hydroxide solution and sodium silicate solution to produce high strength geopolymers. A number of researchers indicated that supplementary materials such as metakaolin, ground granulated blast-furnace slag (GGBFS), iron ore tailing (IOT), ordinary Portland cement (OPC), and silica fume could be used to improve properties of fly ash geopolymer. Duan et al. (2016a) indicated that partial replacement of fly ash at levels ranging from 0 to 20% by metakaolin improves the mechanical properties, optimizes the microstructure and reduces the level of damage from sulfate attack. A denser microstructure with higher surface Vickers-hardness of metakaolin containing geopolymer paste than those of geopolymer paste without metakaolin can be obtained. Deb et al. (2014) reported that the compressive strength of fly ash geopolymer concrete increased with the increase of GGBFS content. Strength development of those blended geopolymer concrete cured at ambient temperature was similar to that of water-cured OPC concrete. The optimum amount of GGBFS in fly ash geopolymer also reflected a dense microstructure and reduced porosity for fly ash-slag geopolymer composites (Khan et al., 2016). Okoye et al. (2016) reported that addition of silica fume improved the compressive, tensile and flexural strengths of the fly ash geopolymer concretes and those strength also increased as the silica fume content increased. In another study (Duan et al., 2016b), replacing of fly ash with 20% iron ore tailing (IOT) leads to a reduction of the porosity and microcrackings which results in much denser microstructure of fly ash geopolymer paste blended. Aliabdo et al. (2016) showed that the adding of OPC improved compressive strength, splitting tensile strength, and modulus of elasticity of fly ash based geopolymer concrete except workability. Moreover, many researchers also indicated that industrial materials or residual wastes have extensively been studied as raw materials for geopolymer synthesis. Amin et al. (2016) found that sugar industry waste and locally available china clay can be utilized as raw material for synthesis of geopolymers. Antunes Boca Santa et al. (2013) studied the use of bottom ash from coal and calcined paper sludge from paper sludge as raw materials for geopolymer synthesis and showed that the best results were obtained with a solution of 15 M NaOH and sodium silicate and a mixture of 2:1 bottom ash and calcined paper sludge. Topçu et al. (2014) showed that durable geopolymer concrete without cement can be produced by using waste bottom coal ash. Haq et al. (2014) reported that bottom coal ash geopolymer showed higher thermal conductivity and lower compressive strength than that fly coal ash geopolymer.

Municipal solid waste incinerator (MSWI) ash is a residual waste from the disposal waste management by the combustion of municipal solid waste in combustor facilities (Hassan, 2005). The quantity of residual waste or MSWI ash was also increased with the increase of the disposal waste management by the combustion. These residual waste has 2 components such as MSWI fly ash and MSWI bottom ash (Hassan, 2005). Ninety percent of the total amount is bottom ash and ten percent is fly ash (Lin and Lin, 2006). The utilization of these ashes in cement and concrete industry reduces the environmental problems (Sikalidis et al., 2002), waste-management cost (Zhang et al., 2010), the amount of solid waste (Kurama et al., 2009), greenhouse gas emissions associated with Portland clinker production (Aydın et al., 2007), conserves existing natural resources and decreasing waste volume stored in landfills (Müller and Rübner, 2006). Previous study indicated that the MSWI fly and bottom ashes could be used as a blended cement material up to 10–30% by weight of binder. Goh et al. (2003) conducted a series of studies on the use of MSWI fly ash as a blended cement material. The compressive strength of blended cement mortar containing 10% of MSWI fly ash indicates that in higher than the control cement mortar. The 7 d strength activity index of 123.6% achieved by the MSWI fly ash, which is higher than the requirement of 75%, suggests its contribution toward the strength development of the blended cements. Lin et al. (2004) further studied paste specimens in which 10–40% of OPC was replaced by the MSWI fly ash. The compressive strength at a later stage of blended cement paste samples containing 10% and 20% of MSWI fly ash, varied from 95 to 110%. Moreover, the wet ground MSWI bottom ashes can be advantageously used as a partial replacement in the production of cement concrete with improved properties of strength, elastic modulus, and water penetration (Bertolini et al., 2004). These reports indicate that MSWI ash can be used as supplementary cementing material. For geopolymer system, Jin et al. (2016) studied the effect of the geopolymerization of MSWI fly ash and the stability of the metakaolin-MSWI fly ash based geopolymer as a desirable candidate for structural materials in the environment and reported that the metakaolin-MSWI fly ash based geopolymer presented excellent stability in the acid and alkaline environments, and also exhibited highly similar characteristics in the metakaolin-based geopolymers microstructures. However, there is still a lack of study on the use of these ashes in high calcium fly ash geopolymer system.

This work aimed to study the possibility of MSWI bottom ash used as a replacement material for fly ash geopolymer matrix. The MSWI bottom was used in combination with high calcium fly ash to produce geopolymer matrix. The effect of the replacement levels of fly ash by MSWI bottom ash on compressive strength, porosity, pore size distribution, scanning electron micrographs (SEM), x-ray diffraction (XRD), and fourier transform infrared spectroscopy (FTIR) of geopolymer matrix were presented and compared to Portland cement mortar containing MSWI bottom. This study would lay a groundwork for the future utilization of MSWI bottom ash in the geopolymer concrete product and lead to reduce cement consumption and waste materials.