Effect of binary combination of waste gypsum and fly ash to produce building bricks

Abstract

South Africa's production of wet flue gas desulfurization (WFGD) gypsum and fly ash (FA) has increased significantly owing to the country's dependence on coal power plant for electricity. These wastes are mainly dumped in landfills, which contributes to environmental pollution. Recycling WFGD gypsum to generate value-added products may benefit the country economically and ecologically. Given the chemical compositions of WFGD gypsum and fly ash, it is feasible to mix them to create a binder material. On the time when the study was done, no evidence of study had been conducted on the binary combination of FA and WFGD gypsum produced locally to create building materials. This study proposes a novel approach to lowering environmental pollution by using two South African wastes, FA and WFGD gypsum, to produce sustainable building materials. The mixture of WFGD gypsum and FA is intended to produce binding phases comprising ettringite and calcium silicate hydrate (CSH). Ettringite can be formed by hydration of alumina (Al2O3) in fly ash with calcium sulfate (CaSO4) and calcium oxide (CaO) of WFGD gypsum, while CSH can be generated by hydration of silica (SiO2) in fly ash under the same conditions. FA was employed moderately to substitute WFGD gypsum in the production of FA: WFGD composite blocks. Composite mixes were produced by substituting 0–40 wt% of WFGD gypsum with FA. Composites bricks obtained were characterized using compression test and wet-dry cycles. Toxicity characteristic leaching procedure (TCLP) were done to determine perilous elements existing in the study sample and scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) for morphology analysis. X-ray diffraction (XRD) was utilized to establish the crystallinity of the material. The strength results showed that FA used in this study was poorly reactive. As the partial replacement of WFGD gypsum by FA increased from 0 to 40%, the strength of the composite material decreased. The pessimum effect of FA is mainly related to the low pH of the binary mixture and low pozzolanicity of FA used. Incorporation of 20 wt% FA and curing temperature of 40 °C seemed to be suitable for the preparation of WFGD:FA composite bricks. The composite bricks' highest UCS of 2 MPa exceeded the minimum strength for Class III mortar as per SANS 10145. Therefore, WFGD:FA composite bricks may be used as building materials.