Mechanical properties of geopolymer mortars activated with laboratory-made sodium silicate

For the production of alkali-activated binders, the use of commercially available activators, particularly sodium silicate (SS), has a few disadvantages, such as high costs, high embodied energy and adverse environmental impacts associated with the synthesis of the activators. For example, commercial SS is commonly produced by the reaction of sodium carbonate and silicon dioxide at a very high temperature of 1200–1400 °C, which leads to high emission of CO2. The present study explores the feasibility of using silica fume as a potential precursor for the production of SS, which might be used to replace commercial SS as an alkaline activator. A laboratory method to produce cost-effective and eco-friendly SS has been developed by the authors recently. Mechanical properties of fly ash-based geopolymer mortars activated by a mixture of the laboratory-made SS and sodium hydroxide (SH) are presented in this paper. The main experimental parameters include the mortar age (3, 7, 28, and 56 days), SS/SH ratio (1.0, 2.0, and 2.5), type of SS (laboratory-made and commercial), and whether the samples were rested before heat curing or not. Mortar cubes and prisms were tested to determine the compressive and flexural strengths, respectively. The test results show that the resting before heat curing had a negative impact on the strengths of mixes with SS/SH of 2.0 and 2.5. The directly heat-cured specimens of mixes activated with laboratory-made SS achieved 28-day compressive strengths of 31.19, 40.85, and 42.56 MPa when the SS/SH ratios are 1.0, 2.0, and 2.5, respectively. In general, the obtained strengths of samples using laboratory-made SS are comparable to those of samples using commercial SS. This research demonstrates that it is feasible to use silica fume to produce cost-effective and green SS in making geopolymer