Understanding the Reaction, Structural Evolution, and Physical Property Development of Low-carbon Alkali-activated Slag Cements

This study explores the use of the less commonly used low-carbon activators, sodium carbonate and sulphate, for the activation of blast furnace slag with the aim of furthering the use of alkali-activated binders as a sustainable alternative to Portland cement. It was found that there is delay in the onset of the main reaction by >48 hours for the sodium carbonate-activated slag binder due to an initial preferential reaction occurring in the first 2 hours after mixing, as observed in situ, preventing the calcium availability and alkalinity evolution of the system. However, once the main reaction does take place, a C-(N)-A-S-H structure forms as well as small amounts of N-A-S-H, not too dissimilar to a sodium silicate-activated binder. The inclusion of CaO in 1 and 3 wt.% replacements of slag in the system improved the initial rate of reaction, however, the specific mechanism is unchanged. Conversely, the sulphate-activated slag had a >100 hour delay in the onset of reaction, which itself saw limited development of key binding phases, due to the near-neutral nature of the activator. Overall, this study determined that the performance of an alkali-activate slag binder is a direct outcome of reaction pathway, controlled by activator chemistry.