A study was conducted to determine the quantity of water bound by hydration, the products formed during hydration, the microstructures, and the mechanical properties as a function of the relative content of reactive magnesium oxide (magnesia, MgO), pulverized fuel ash (pfa), portland cement (PC), and water used to prepare the mixes. It was confirmed that hydration of MgO leads to the formation of brucite and that PC forms its normal hydration products, even when both materials are present together. It was found that calculated changes in porosity and consumption of water during hydration based on the proposed hydration reactions agree well with the measurements. It was also found that the stiffness and strength improve directly proportional to the PC content. It is proposed that the lower strength of mixes with high reactive MgO contents is due to a combination of their high water demand and the difference in morphology between brucite and C–S–H gel.

Excessive drying shrinkage is one of the major issues of concern for longevity and reduced strength performance of concrete structures. It can cause the formation of cracks in the concrete. This research aims to improve the autogenous self-healing capacity of traditional Portland cement (PC) systems, adding expansive minerals such as reactive magnesium oxide (MgO) in terms of drying shrinkage crack healing. Two different reactive grades (high ‘N50’and moderately high ‘92–200’) of MgO were added with PC. Cracks were induced in the samples with restraining end prisms through natural drying shrinkage over 28 days after casting. Samples were then cured under water for 28 and 56 days, and self-healing capacity was investigated in terms of mechanical strength recovery, crack sealing efficiency and improvement in durability. Finally, microstructures of the healing materials were investigated using FT-IR, XRD, and SEM-EDX. Overall N50 mixes show higher expansion and drying shrinkage compared to 92–200 mixes. Autogenous self-healing performance of the MgO containing samples were much higher compared to control (only PC) mixes. Cracks up to 500 μ m were sealed in most MgO containing samples after 28 days. In the microstructural investigations, highly expansive Mg-rich hydro-carbonate bridges were found along with traditional calcium-based, self-healing compounds (calcite, portlandite, calcium silicate hydrates and ettringite).