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

•Low grade metakaolin is effective for green binders of low demand of alkalis and high strength.•The strength was studied as a function of the chemical composition and temperature.•Higher aggregate:binder ratio reduced the strength, but values >30MPa were obtained.•Consumptions of 280kg/m3 of low grade metakaolin resulted in 30MPa at 28days.•Curing at 75°C for 24h resulted in more than 85% of the 28day strength at 20°C. A low grade mineral composed of 50% kaolinite and 50% quartz was activated with solutions of Na2O⋅rSiO2⋅xH2O. 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 30MPa at 28days. The curing of mortars at 75° for 24h favored a rapid development of strength at 1day, but without a significant increase afterwards at 20°C. The microstructures of samples cured at 20°C were dense, while those exposed at 75°C showed porosity, as well as unreacted particles of quartz, limestone aggregates and metakaolin in a matrix of geopolymeric gel.