Mechanical–Chemical Activation of Cement-Ash Binders to Improve the Properties of Heat-Resistant Mortars

The article demonstrates the effectiveness of the mechanochemical activation of a cement-ash binder by increasing the specific surface area of the ash and introducing a sodium fluorosilicate additive (Na₂SiF₆). It has been experimentally proved that the introduction of a Na₂SiF₆ additive makes it possible to increase the degree of cement hydration, as well as the intensity of free CaO binding when heating the cement-ash binder in the range of 500 °C to 800 °C. Mechanochemical activation prevents a decrease in the strength of the preheated cement-ash binder. During cyclic heating and cooling of slag mortars based on the activated cement-ash binder, an improvement in the set of basic properties was observed: compressive strength, flexural strength, water absorption, dynamic modulus of elasticity, and conditional elongation. Experimental design was carried out to obtain experimental-statistical models of mortar properties based on composition, heating temperature, and number of heating-cooling cycles. These models made it possible to develop quantitative relationships for predicting mortar properties at elevated temperatures and to rank the factors in order of importance. The optimal values for the dosage of fly ash, sodium silicofluoride additive, and the binder's specific surface area were established. It was demonstrated that the activator has a positive effect on the thermal deformation of mortars.