Artificial pozzolana from silica gel waste–clay–limestone composite

This study investigated the possibility of using artificial materials obtained during combustion of silica gel–clay–limestone composites as a pozzolanic additive for Portland cement. The granules of the composites investigated were formed from dried components that were later wetted. The granules thus obtained were burned at 700°C for 1 h. In the samples, 5, 15 and 25% (by weight) of the Portland cement was replaced with the additive. The specimens were then hardened for 28 days under normal conditions. It was estimated that combustion of silica gel–clay–limestone composite at 700°C produced a new artificial pozzolana (SGCL), the pozzolanic activity of which is 260 mgCaO/g. During the combustion process, the fluorine from silica gel was not released into the atmosphere, but combined to form a stable CaF₂ compound. It was established that 5 and 15 wt% SGCL pozzolana additive increased the compressive strength of the samples that were cured for 7 and 28 days under normal conditions. This effect is related to the on-going pozzolanic reaction and the formation of stratlingite (C₂ASH₈) and hydrogarnet (C₃AS_xH_6–2x). It was estimated that CaF₂, which was formed during combustion of the SGCL composite, remained in a stable form during the curing process of the samples. This was because all investigated samples of cement with different amounts of additive released a negligible quantity of fluorine (<1·5 ppm) into the liquid phase.