On the robustness of one-part alkali-activated materials preparation

Alkali-activated materials (AAMs), and geopolymers as their subgroup, are recognized as one of the alternative low CO2 emission binders. Their conventional preparation involves solid aluminosilicate precursors and... [ view full abstract ]

Alkali-activated materials (AAMs), and geopolymers as their subgroup, are recognized as one of the alternative low CO₂ emission binders. Their conventional preparation involves solid aluminosilicate precursors and alkali-activator solution, such as sodium hydroxide and/or silicate. However, the activator solution introduces some practical issues due to high viscosity, corrosiveness, water content (increased logistics costs), and occupational hazards – especially for in situ casting. Consequently, there has been an increasing interest in the development of one-part or “just add water” AAMs. This procedure consists of mixing a solid aluminosilicate precursor with a solid alkali-activator and initiating the reaction by adding water. Research on development of one-part AAMs has gained pace in recent years. It has been demonstrated that it is possible to formulate mixtures with acceptable fresh and hardened properties. However, the fundamental understanding of reaction mechanisms and kinetics is still lacking compared to conventional AAMs. 

In the present work, one-part alkali-activated mortars prepared from ground granulated blast furnace slag and synthetic solid sodium metasilicate were evaluated in terms of the preparation robustness. The effects of selected basic parameters on the compressive strength development (as an indication of the reaction extent) were studied: mortar batch size, reactant mixing sequence, mixing intensity, mixing duration, and curing conditions, for instance. The mix design itself was optimized in earlier work and the compressive strength of the mortar reaches slightly over 100 MPa at 28 d age. The preparation of one-part AAM appeared to be sensitive to these aforementioned factors: compressive strength was decreased by several dozens of percents in worst cases. The robustness of the mix design and preparation of one-part AAMs has been documented scarcely before, and thus the presented results highlight the importance of correct working methods during mixing, casting, and curing.