Fly Ash Geopolymer Mortar: Impact of the Substitution of River Sand by Copper Slag as a Fine Aggregate on Its Thermal Resistance Properties

Abstract Geopolymer materials are increasingly finding application in various settings because of their excellent mechanical properties and their ability to reduce the level of CO2 emission in the atmosphere, thus contributing to a sustainable environment. Mortars containing an alkali-activated fly ash binder in which river sand is used as an aggregate are found to display poor adhesive strength and incompatibility at elevated temperatures, necessitating researchers to look for alternate aggregates to be used in geopolymer mortars. In this study, we have substituted copper slag as an aggregate [C] in place of river sand [R] in a fly ash geopolymer mortar and investigated its effect on the thermal resistance of the mortar. We report the results of our investigations in this paper. Our experimental results show that the mortar using copper slag as an aggregate (C-type mortar) exhibited good thermal stability in the temperature range of 200–1000oC without any spalling. Thermal strain was low, and the mortar was found to have a thermal expansion of just 0.002% and a thermal conductivity value of 0.6 W/m K at 1000⁰C. Further, the C-type mortar exhibited a compressive strength of 33 MPa on exposure to 1000oC and negligible mass loss even after several thermal cycles, facilitating the recycling of industrial waste such as copper slag in thermal applications. In contrast, mortars using river sand as an aggregate (R-type mortars) exhibited a higher mass loss on thermal treatment. Thermal-induced changes were studied using field emission scanning electron spectroscopy (FESEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The network rich in silicon atoms in the crystalline phase was found to contain aluminium as nepheline along with an amorphous geopolymeric gel in the heated samples. There were no differences in the binding energy of Si, Al, O and Na atoms of heated and unheated samples, which confirmed the retention of the desired geopolymeric microstructural network even at elevated temperatures.