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 >30 MPa were obtained.
• Consumptions of 280 kg/m3 of low grade metakaolin resulted in 30 MPa at 28 days.
• Curing at 75 °C for 24 h resulted in more than 85% of the 28 day 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 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 °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.