In situ X-ray pair distribution function analysis of geopolymer gel nanostructure formation kinetics

With the ever-increasing environmentally-driven demand for technologically advanced structural materials, geopolymer cement is fast becoming a viable alternative to traditional cements due to its proven engineering characteristics and the reduction in CO2 emitted during manufacturing (as much as 80% less CO2 emitted in manufacture, compared to ordinary Portland cement). Nevertheless, much remains unknown regarding the kinetics of reaction responsible for nanostructural evolution during the geopolymerisation process. Here, in situ X-ray total scattering measurements and pair distribution function (PDF) analysis are used to quantify the extent of reaction as a function of time for alkali-activated metakaolin/slag geopolymer binders, including the impact of various activators (alkali hydroxide/silicate) on the kinetics of the geopolymerisation reaction. Quantifying the reaction process in situ from X-ray PDF data collected during the initial ten hours can provide an estimate of the total reaction extent, but when combined with data obtained at longer times (128 days here) enables more accurate determination of the overall rate of reaction. To further assess the initial stages of the geopolymerisation reaction process, a pseudo-single step first order rate equation is fitted to the extent of reaction data, which reveals important mechanistic information regarding the role of free silica in the activators in the evolution of the binder systems. Hence, it is shown that in situ X-ray PDF analysis is an ideal experimental local structure tool to probe the reaction kinetics of complex reacting systems involving transitions between disordered/amorphous phases, of which geopolymerisation is an important example.