The microstructure and transport properties (water absorption, porosity, and electrical conductivity) of magnesium (alumino-)silicate binder systems prepared from MgO with silica fume (MS), metakaolin (MK), or a combination of both (MS + MK), were investigated. The results revealed that the pure MS system, despite the complete formation of M-S-H, exhibited rapid water absorption and began disintegrating after brief contact with water. This was likely due microcracking induced by drying, raising concerns about the viability of such a binder. In contrast, the incorporation of metakaolin significantly reduced water absorption, porosity and electrical conductivity. This improvement was likely due to a more stable microstructure resulting from the formation of M-A-S-H and hydrotalcite-like phases, the continuous presence of brucite-like phases and unreacted MgO, and the plate-like structure of metakaolin particles, all of which interlocked to enhance the microstructural integrity. These findings underscore the importance of optimising precursor blends in magnesium silicate binders to enhance their durability and performance.