Compressive strength and microstructure of ordinary cured and autoclaved cement-based composites with mechanically activated kaolins

Abstract

The effects of two different mechanically activated kaolins, AKV (61% kaolinite, 14% quartz and 16% mica) and AKG (51.6% kaolinite and 40.6% quartz) on the compressive strength of cement composites and microstructure of pastes were investigated. Composite mixtures, in which 10, 20, 30, 40 and 50% of ordinary Portland cement (OPC) was replaced by AKV or AKG, were prepared with w/b of 0.5, and exposed to different curing conditions (ordinary curing for 28 days and autoclaving). Factors affecting microstructure were investigated on pastes by X-ray diffraction (XRD), Differential thermal analysis/thermal gravimetry (DTA/TG) analyses. Mercury intrusion porosimetry (MIP) and Scanning electron microscopy with Energy-dispersive spectroscopy (SEM-EDS). AKG composites exhibited higher compressive strengths under both curing conditions. Positive autoclaving effects on strengths were predominantly pronounced at the higher cement replacement levels. Comparison of the autoclaved and ordinary cured paste microstructure, revealed more intensive pozzolanic reaction during autoclaving conditions (CH content near zero) and higher total porosity. The negative effect of hydrogarnet on the strength was compensated by the formation of the crystalline tobermorite. Obtained results revealed that mechanically activated kaolin, with high content of quartz, could be a promising pozzolanic addition, even at high cement replacement levels (30-50%), especially when autoclaving curing conditions were applied.