Development of a weakly alkaline alkali-activated mortar: use of sodium carbonate/sodium sulfate–magnesia for activating fly ash–slag mixture

The evolution of properties of a one-part fly ash-slag-based geopolymer mortar activated with sodium carbonate (Na₂CO₃, henceforth referred to as SC), sodium sulfate (Na₂SO₄ (SS)) or a combination of SC and SS along with magnesium oxide (MgO) were systematically explored. The pH and fresh behaviours of slurry, the hardened properties of mortar, micro-properties and cost/environmental impacts of one-part geopolymer mortars activated using the weakly alkaline activators were analysed. The results demonstrate the inclusion of SS decreased the strengths to some extent. Appropriate amounts of SC and SS in combination regulated the gel network formation process as well as the accumulations of calcite, ettringite and hydrotalcite, which was more beneficial in terms of hardness than the use of a single activator (SC or SS). The composite activator led substantial pore refinement, but decreased in sphericity and compactness. The compressive strength of the activated mortar reached a maximum of 45.5 MPa at 28 days and nearly all the mixes met the bond strength requirements for structural repairs in the relevant Chinese standard (Repairing mortar (JC/T 2381-2016)). Based on costs and the embodied carbon dioxide emissions, the use of the composite activator led to a more environmentally friendly product.