Influence of the combination of calcium oxide and sodium carbonate on the hydration reactivity of alkali-activated slag binders

Abstract Calcium oxide (CaO) and sodium carbonate (Na2CO3) are potential activators for ground granulated blast furnace slag, but they result in slow strength development, especially at the early age. To overcome this issue and explore the application of CaO/Na2CO3 compound activators in alkali-activated slag (AAS) systems, this paper investigated the mechanical and hydration properties of CaO/Na2CO3-activated slag binders by measuring the compressive strength, pH value of pore solution and non-evaporable water content up to 28 days. The hydration products and microstructure of hardened pastes were analyzed by X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy techniques. It was found that the ratio of CaO to Na2CO3 played a vital role in the performance of the AAS binders. The combination of 2.5% CaO and 5% Na2CO3 significantly increased the quantities of hydration products and thus resulted in a much denser microstructure and higher compressive strength. This increment was attributed to the generation of sodium hydrate and low crystalline calcium carbonate by the reaction of CaO with Na2CO3, which was beneficial for the formation of hydration products. XRD and TGA results showed that the main hydration products were calcium silicate hydrate gels and hydrotalcite-like phases for all samples except the one with only Na2CO3. In addition, low crystalline calcium carbonate was detected in the samples with both CaO and Na2CO3.