MIX DESIGN APPROACH FOR HIGH TEMPERATURE RESPONSE FOR OPC

Abstract

Substitution a part of the clinker with fly-ash (further, FA) and application of alkali metal compounds are considered as the main factors to enhance the heat resistance of portland cement. This modification allows hydrates of the Na₂O-CaO-SiO₂-Al₂O₃-H₂O hybrid system to be synthesized without undergoing structural destruction at high temperatures. The possibility of increasing the heat resistance of portland cements by using an aluminosilicate admixture of the fly ash type with the simultaneous use of sodium water glass has been proved. Methods of physico-chemical analysis revealed the peculiarities of the microstructure of alkali-activated FA-containing portland cements, such as the absence of portlandite Са(ОН)₂ in hydration products as well as the synthesis of C-A-S-H phases of katoit and zeolite-like N-C-A-S-H phases of hydronepheline Na₂O·Al₂O₃·2SiO₂·2H₂O and gismondine CaO·Al₂O₃·2SiO₂·4H₂O. Thus, heat resistance of such cement, in contrast to portland cement CEM I, was caused by the smooth course of structure formation processes at high temperatures with increased fragmentation by prevention of rehydration of СаО, formation of the high-temperature phase of gehlenite 2CaO·Al₂O₃·SiO₂ by sintering of C-A-S-H phases, and recrystallization of zeolite-like phases into nepheline Na₂O·Al₂O₃·2SiO₂ and anorthite CaO·Al₂O₃·2SiO₂ without structural destruction. These processes resulted in increasing the residual strength of the mortar based on proposed alkali-activated portland cement up to 41.6…90.6 % compared to 5…35 % of analogues based on CEM I. The optimized composition region of alkali-activated fly ash portland cement based on CEM IV/A 32.5N was determined: fly ash – 28...30 wt. %, density of water glass – 1150...1160 kg/m³. The proposed cement was characterized by residual strength ³ 70 % while increasing of compressive strength after 28 days of hardening ³ 50 MPa.