Permeability of sodium silicate-activated metakaolin: Impact of reduced activator concentration and alkali earth oxide/hydroxide

Chemical degradation of cement-based materials is strongly influenced by the permeability and associated pore structure of the paste. In this work, the permeability of sodium silicate-activated metakaolin with a silicate modulus of 1.5 has been quantified using the beam bending technique and compared with pore size data obtained using nitrogen adsorption-desorption (NAD) and mercury intrusion porosimetry (MIP). Pastes with lower CO₂ emissions have been evaluated, including the effects of reduced alkali concentrations (i.e., 2.5 versus 5.0 m Na₂O activator) combined with addition of alkali earth oxide/hydroxides. While it is found that all pastes with reduced alkali concentrations exhibit larger average breakthrough pore sizes and higher permeability than the control system, both calcium hydroxide and magnesium oxide are effective at decreasing the average breakthrough pore size and permeability. On a molar basis calcium hydroxide is more effective than magnesium oxide at reducing permeability; however, more magnesium oxide can be added overall without a significant loss of workability, making it a more viable approach. Insights from X-ray diffraction (XRD) reveal that the observed physical effects can be attributed to the ability of calcium hydroxide and magnesium oxide to form gel hydrate phases with silicates from the activator and promote metakaolin dissolution.