The importance of early strength in structural applications: Obsidian‐based geopolymer mortars and silica fume substitution study

Geopolymer mortars, which cause less CO2 emissions than concrete and its raw material cement, are an innovative, environmentally friendly and sustainable building material. Geopolymers are formed by activating silica and alumina materials with alkaline activators. In this study, a geopolymer mortar containing obsidian (OB), also known as volcanic glass, which is free, and silica fume (SF), which is the waste material of the silicon and ferrosilicon industry, was investigated. The behavior of OB‐based geopolymer mortars under different curing times ranging from 2 to 120 h and different thermal treatment temperatures such as 90, 150, and 200°C were examined. The effectiveness of OB and SF on the physical features, compressive strength (CS), and micro‐structural of the geopolymers were characterized. Results demonstrated that the peak CSs were acquired in 96 h at 90 and 150°C heat treatment temperatures, while the highest CSs were acquired in 72 h for specimens subjected to 200°C heat treatment. Reduces in CSs were detected when the curing time exceeded the ideal limit. OB‐based geopolymer was found to be stable with SF and there was a linear relationship between SF substitution ratio and CS. The density of the mortar pieces decreased with increasing thermal curing hours. Devolving on the thermal temperature and curing time, the microstructures became more compact and microvoids and cracks decreased. It was observed that SF substitution reduced the total pore size of the geopolymers and allowed the pore structure size to be reduced. The results obtained from the study are expected to encourage the utilize of industrial wastes and new binders in the manufacture of geopolymers.