Bimodal Pore Size Evolution of Metakaolin-Based Geopolymers at the Initial Stage of Curing: A Small-Angle Neutron Scattering Study

The nanoscale pore size distribution of metakaolin-based geopolymers (MKGPs) during the first day of curing with different alkaline activators (sodium and potassium silicate solutions and their mixture) and at different temperatures (25 and 40 °C) was evaluated using small-angle neutron scattering (SANS), followed by the application of the polydisperse spherical (PDSP) model. The model outputs were then deconvoluted by using a bimodal log-normal distribution fitting. Small pores, with an assumed modal value of 2.5 nm, formed immediately after mixing and gradually decreased their pore volumes over time. In contrast, larger pores, with modal values larger than 5 nm, formed after induction periods, suggesting that their development corresponds to the pores in aluminosilicate gel interstices formed by gel clustering. The nanoscale pore size distribution evolution varied with curing temperature and the type of activators used. The initial pore formation in MKGPs informs their final nanoscale porosity distribution, which in turn governs the diffusive properties of the material, as evaluated by through-diffusion experiments of HDO with the same MKGP specimens.