Geopolymer mortars based on a low grade metakaolin: Effects of the chemical composition, temperature and aggregate:binder ratio

A low grade mineral composed of 50% kaolinite and 50% quartz was activated with solutions of Na2O center dot rSiO(2)center dot xH(2)O. Pastes and mortars with aggregate:binder ratios of 3:1, 5:1 and 7:1 were investigated varying the molar ratios SiO2/Al2O (2.7-3.3), Na2O/Al2O3 (0.7-1.0) and H2O/Na2O (11-13). The results showed that the compressive strength was reduced by increasing the aggregate:binder ratio, nonetheless, mortars with a ratio 7:1 achieved 30 MPa at 28 days. The curing of mortars at 75 for 24 h favored a rapid development of strength at 1 day, but without a significant increase afterwards at 20 degrees C. The microstructures of samples cured at 20 degrees C were dense, while those exposed at 75 degrees C showed porosity, as well as unreacted particles of quartz, limestone aggregates and metakaolin in a matrix of geopolymeric gel. (C) 2013 Elsevier Ltd. All rights reserved.