Optimization study of waste glass powder-based alkali activated materials incorporating metakaolin: Activation and curing conditions

Abstract

Abstract The glass represents a significant part of the total solid wastes available each year around the world. The recycling of glass wastes is problematic in urban areas of most countries since waste glass ends up in landfills constituting thus an environmental issue and a public health problem. On the other hand, the finely ground glass has been demonstrated to possess an excellent pozzolanic activity and was successfully used to replace a part of the cement in the concrete with good yields. Also, the recycling of the glass in alkali-activated materials arouses great interest in recent years. This work is based on an optimization study of a recycled glass powder-based alkali-activated mortars incorporating metakaolin. Firstly, the influence of parameters such as w/b ratio, the activation temperature, the duration of the thermal activation, and the curing conditions (temperature and humidity) on the mechanical properties of specimens, was studied. Decreasing w/b allowed to improve the strength of materials with an optimum value of w/b equaled 0.42. The compressive strength increased with the temperature of activation up to 60\xa0°C. Over this temperature, the strength significantly dropped. The enhancement in duration of thermal activation also induced a strength improvement. Thermal activation for 3 days was found to provide the best strength; however, 2 days were favored for energy saving. Three curing conditions were investigated including normal curing (20\xa0°C/100% HR) and two special curing conditions (20\xa0°C/50% RH) and (40\xa0°C/50% RH). Samples cured at (20\xa0°C/50% RH) provided better mechanical strength. This curing condition was thus defined as the optimum for the studied system. Then, the effect of both metakaolin and NaOH concentration on the mechanical properties, microstructure and thermal behavior of activated materials using the defined conditions, was studied. Results revealed that increasing NaOH concentration favored the dissolution reaction with respect to the polymerization reaction delaying the overall geopolymerization process. The addition of metakaolin, in contrast, promoted the polymerization process inducing the densification of the matrix with a progressive increase in the strength. The optimum conditions defined for the studied system were: w/b\xa0=\xa00.42; 92\xa0wt% GP/8\xa0wt% MK activated with 5\xa0mol/L NaOH at 60\xa0°C for 2 days and curing condition of 20\xa0°C/50% RH. Three type of gels have been detected in the studied materials: Na silicate gel, C–S–H and N-A-S-H type gels. The increase in NaOH concentration favored the formation of Na silicate gel along with carbonation for the highest concentration of 10\xa0mol/L. While NASH type gel predominated with increasing MK content. In both cases, the content of CSH appeared constant.