Blended Alkali-Activated Cements Based on Blast-Furnace Slag and Calcined Clays: Statistical Modeling and Effect of Amount and Chemistry of Reactive Phase

Abstract

In this study, binary ground granulated blast-furnace slag (GGBFS)/calcined clay cements activated by 4 M NaOH and cured at ambient temperature were investigated to determine the correlation of the mechanical properties and phase-formation behavior of the mixed cements with respect to the percentage, amount, and chemistry of the reactive phases of the calcined clays. A calcined kaolinite/montmorillonite calcined clay containing 60% reactive phase was found to be suitable as a secondary precursor in blended cements. The optimal formulation of the alkali-activated GGBFS/calcined clay was 70/30, having a 28-day compressive strength of 30 MPa and corresponding to Ca/Si=0.75, Na/Si=0.16, Na/Al=0.54, and Si/Al=3.3. X-ray diffraction, thermal, and Fourier-transform infrared analyses revealed that the reaction products of the blended cements were a C−N−(A)−S−H gel, calcite, and zeolites. Experimental–statistical modeling showed that the reactive-phase parts in the calcined clays and the Na/Si oxide ratio were statistically the most responsible for the strength of the mixed alkali-activated cements.