Synthesis of geopolymer from South Africa CFA for construction application

Abstract

In compliance with sustainable development goal nine, new construction materials’ impact becomes increasingly important in the 21st century. This is because of the anthropogenic activities related to cement production and concrete casting, which have led to high levels of CO2 emissions. Coal fly ash (CFA ) from coal combustion during energy production is a waste product that is costly to dispose of in an open environment and is also associated with air and land pollution. The use of waste CFA as raw material for the synthesis of geopolymer has been reported as a route to convert this waste into value-added products, simultaneously remediating its environmental impact as well as eliminating additional greenhouse gas emissions associated with the production of cement and concrete. As a result of improved formulations, good mechanical and durability characteristics have been achieved that meet concrete standards; however, the 24-hour oven heating curing regime may not favour energy savings, which in turn affects the cost of the product, CO2 emissions, and other applications. As a raw material, CFA was physiochemically examined and characterised to understand its properties in the mix design process. Formulations were designed based on the effect of fine and coarse aggregates on CFA-based geopolymer paste properties and then cured at different regimes: room curing with plastic cover, room curing without plastic cover, along with the reference oven heating method. These formulation’s properties were investigated using concrete standards such as fresh properties, mechanical strength, durability and thermal profiles based on the optimised formulations. The best formulations at different curing conditions were characterised using XRF, SEM, XRD, and FTIR analytical techniques to understand the material’s composition, morphology, and mineral phases and identify organic or inorganic bonds in the materials. Radon and gamma measurements were also performed to determine whether CFA-based geopolymer formulations are carcinogenic. It was essential to quantify greenhouse emissions for the optimised formulations as well as understand all the fundamental chemistry and the link from material composition to its behaviour and the cost associated with the most suitable curing regime. The XRF, XRD and particle size analysis of CFA indicate its properties that comply with ASTM 618, using low calcium CFA based on silica, aluminium and calcium oxide content, with high particle size. CFA and products synthesised using CFA-based geopolymer formulations exhibited high quantities of crystalline structures with quartz and mullite minerals. The effect of sodium hydroxide concentration and variation of formulations at the fresh state impacted the initial and final setting time, and consistency resulted. Better setting properties using 12 molarity (M) of 170- and 230 minutes with GPP-M2A formulation on initial and final setting time with a consistency of 7 mm penetration was achieved compared to other NaOH concentrations tested. Adding fine and coarse aggregates to GPP-M2A to create GPC-M2C as an optimised formulation led to a density increase of 1748.6 to 2182.2 kg/m3, lowering compressive strength from 29.7 to 26.7 MPa. Considering flexural, tensile, and modulus of elasticity tests, these formulations with fine aggregates showed greater ductility and increased yield than those with no fine and coarse aggregates. Based on curing regimes, the given conditions of oven curing resulted in early strength similar to that standard of concrete of being 25 MPa at three days of curing, while this strength is achieved more slowly after 21 and 28 days of ageing, respectively, using room curing, with and without a plastic cover respectively. After three months of ageing, products of room curing with plastic covered had similar compressive strength to oven curing. CFA-based GPC cured room curing without plastic-covered resulted in heavy efflorescence, while high early shrinkage was observed from that cured at oven condition of 60 oC. All formulations resulted in good durability properties. A panel of about 60 mm thick cured under plastic-covered conditions achieved an excellent thermal fire rating of 1 hour. An oven curing condition at 60 oC for 24 hours is unfavourable for industrialisation due to its CO2 emissions factor of 0.684 kg CO2 eq and yearly utility costs of R1 361 933.35 compared to 0.094 kg CO2 eq and R33 146.79 associated with room curing, which is also lower than the CO2 emissions factor of 0.132 kg CO2 eq associated with conventional Ordinary Portland Cement (OPC) concrete. CFA-based geopolymer formulations may be recommended for open infrastructure, but the average total radon and gamma activities must be controlled for housing. CFA showed 684 Bq/kg of average total activity in this study, which is above the world average of 420 Bq/Kg, while the synthesised formulations produced 408 to 459 Bq/kg on average. The GPC-M2C formulation cured at room temperature under a plastic covering condition, being considered the optimised formulation, resulted in comparable standard properties to that of normal OPC concrete. The results of this study indicate that synthesised CFA-based geopolymer formulations that cure at room temperature with plastic cover are recommended as being an environmentally friendly, lower cost and feasible process with suitable early strength, as most strength decisions are reached after 28 days of ageing.