Influence of Calcium Oxide Additive on the Properties and Structure of Building Geopolymers Based on Fly Ash from CHPP-22

The physicomechanical properties and microstructure of geopolymer building materials, based on low-calcium fly ash from CHPP-22, were investigated as functions of the type of activator, temperature-and-humidity conditions of curing, and an additional source of calcium [CaO] added in the amounts 10, 15, and 20 wt.%. It was found that the most effective type of activator for the studied fly ash is a sodium silicate solution with silicate modulus SiO2 /Na2O = 1 and the preferred curing regime is steam treatment at 90°C. The introduction of 15% CaO into the geopolymer mixture made it possible to increase the compressive strength by more than 2.5-fold (to 38 MPa) with open porosity decreasing from 24 to 7% and water absorption from 15 to 3%. However, a subsequent increase in the weight content of CaO effects degradation of the properties due to significantly shorter setting times of the raw mixture.