The Effects of Phosphate Compounds on the Microstructure and Mechanical Properties of Fly Ash Geopolymer Mortars

Coal-fired power plants are a main source of energy in Poland. In the rapidly growing demand for the reduction of CO₂ emission in the energy industry, the use of biomass for energy purposes has increased significantly. The combustion of biomass results in the generation of fly ash, with higher levels of CaO, K₂O, P₂O₅, in contrast to the fly ash derived from the combustion of coal. The aim of this study was to examine the influence of phosphate compounds on fly ash-based geopolymer mortars. Geopolymers were made by mixing two types of fly ash-one from the combustion of wood biomass and the second from the combustion of coal in a heat and power station. Basic activators (NaOH and Na₂SiO₃) were used for the alkali activation. The maximum level of tetraphosphorus decaoxide addition was established at 5% of the total mass of the aluminosilicate precursors mass. The results showed that the phosphate oxide concentration within the specimens demonstrated a positive correlation with flexural and compressive strength across all temporal intervals (7, 28, and 56 days). The porosity, however, for samples with a 5% addition of P₄O₁₀, increased more than twofold in comparison to reference samples (from 4.26% to 9.98%).