Microstructure and properties of geopolymers formed in the alkali activation process of fly ash

The paper presents the results of the study on the possibility of obtaining high-strength durable geopolymer concrete with fly ash as the basic component. As a result of the research conducted, it was found that the highest potential to obtain geopolymer concrete with high strength was shown for fine-grained, specially selected siliceous ashes from coal combustion. However, the geopolymer concrete obtained by alkaline activation of these ashes with the 8M NaOH solution was not resistant to freeze-thaw cycles. Replacement of 15% fly ash with calcined waste clay and the use of the mixture of NaOH solution and water glass as an activator substantially increased the durability of this concrete. This modification of the concrete composition changed the microstructure of the matrix in the hardened concrete, since the cancrinite was found in the study.