Abstract
Moderately alkaline activators can be used to formulate cementitious binders with a high Supplemetary Cementitious Materials (SCMs) and a low portland cement content (hybrid alkaline cements). This study aimed to prepare hybrid alkaline cements containing large percentages of dehydroxylated bentonite (BT) and small Portland cement (OPC) fractions, with 5% Na2SO4 as a solid alkaline activator. The hydration kinetics of the pastes hydrated in water in the presence and absence of the solid activator were assessed by isothermal conduction calorimetry, whilst the reaction products were characterised with X-Ray Powder Diffraction (XRD) and Fourier-transform Infrared Spectroscopy (FTIR). The presence of the alkaline activator hastened OPC and BT/OPC hydration: more heat of hydration was released, favouring greater initial bentonite reactivity. The portlandite forming during cement hydration reacted readily with the Na2SO4, raising medium alkalinity and enhancing bentonite dissolution and with it reaction product precipitation (primarily (N,C)-A-S-H-like gels that co-exist with C-S-H- or C-A-S-H-like gels). The presence of sulfate ions favoured the formation of AFm-like phases. Preceding aspects accelerated the hydration reactions, with the formation of more reaction product and matrix densification. As a result, the 28 days Na2SO4 activated systems developed greater mechanical strength than the water-hydrated systems, with the 60% BT/40% OPC blends exhibiting higher compressive strength than the 100% OPC pastes.
Highlights
The addition of clays as natural pozzolans is standard practice in cement and concrete manufacture [1,2,3]
The portlandite forming during cement hydration reacted readily with the Na2 SO4, raising medium alkalinity and enhancing bentonite dissolution and with it reaction product precipitation (primarily (N,C)-A-S-H-like gels that co-exist with C-S-H- or C-A-S-H-like gels)
This study explored two key factors that contribute to the cementitious properties of such binders: (i) the hydration kinetics of the new cementitious systems in which the clay used is a dehydroxylated bentonite; and (ii) the type of reaction product generated in the presence of a solid activator such as Na2 SO4
Summary
The addition of clays as natural pozzolans is standard practice in cement and concrete manufacture [1,2,3]. The partial substitution of clays for cement purposes primarily to lower production costs by replacing a material (clinker) characterised by an energy-intensive manufacturing process. This practice reduces the industry’s environmental footprint by lowering CO2 and other greenhouse gas emissions and affords certain technological benefits. The use of mineral additions (pozzolans) in concrete reduces porosity, raises resistance to aggressive agents, lowers heat of hydration and thermal expansion and enhances fresh mix workability. To be viable cement additions, most clays must be activated by grinding and burning [1,2,3]
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