Synthesis and integration of cement-based capsules modified with sodium silicate for developing self-healing cements

Encapsulated healing agents comprise an emerging technology that aim to extend the service life of critical infrastructure and reduce the environmental impact of the construction industry. This paper focuses on the development and characterization of cement-based macro-capsules, made by the pan-coating technique, consisted of a reactive OPC core and a durable shell produced by the reaction of OPC with sodium silicate (SS). Two types of capsules in the range of 2-5 mm were produced in order to evaluate the effect of SS concentration on the microstructure and mechanical performance of the shell. Both types of capsules were integrated in cement mortar mixtures in different fractions (5-20%) by replacing equal sand volume. The fresh properties of the mixtures, the mechanical performance of the hardened specimens and the self-healing efficiency by means of water sorptivity were assessed and compared to the reference mixture of plain cement mortar. The results show that the addition of sodium silicate improves the crushing load of capsules under compression loading, yielding up to 75% survivability during the mixing process. The integration of capsules did not affect the rheology of the fresh mortar mixtures, while it became clear that the shell properties play a significant role on the mechanical performance of the composite mixtures. Healing assessment revealed that the addition of 20% capsules can reduce the sorptivity coefficient up to 80% after a 28 days healing period.