The effect of matrix composition and calcium content on the sulfate durability of metakaolin and metakaolin/slag alkali-activated mortars

The work in hand presents the results of an experimental research on the effect of different precursors (binders) used in alkali-activated materials (AMM) and its composition (i.e. SiO2/Al2O3 molar ratio) on their sulfate durability. A reference matrix is formed from the activation of metakaolin (MK); this matrix was modified by the partial replacement of MK with either 20 wt% silica fume (SF) or 20 and 40 wt% blast furnace slag (BFS), so that the SiO2/Al2O3 molar ratio of calcium-free and calcium-rich AAM changed from 3.0 to 3.9. The properties assessed prior to the durability testing were: density (pycnometry), compressive strength, capillary sorption and oxygen permeability. The sulfate durability was investigated by exposing the matrices to a magnesium sulfate solution for 30, 90 and 180 days of attack, after which the residual compressive strength was determined. The reductions in strength after each period of testing were correlated with variations in the pH of the sulfate solutions and with geometry changes (expansion) measured in cylinders exposed to the durability tests. X-Ray diffraction was used to determine the minerals formed onto the surface of the samples after magnesium sulfate attack. The results show that the MK-based AAM present a higher resistance to magnesium sulfate attack. Furthermore, the partial replacement of MK with BFS is responsible for reductions in the mechanical properties after attack to sulfate. This is associated with the formation of ettringite and gypsum in the presence of calcium from BFS, besides the loss of alkalinity from the migration of alkali (Na+) to the solution.