Combined effect of rice husk ash and superabsorbent polymer on self-healing capability of mortar

This paper deals with combined effect of rice husk ash (RHA) and superabsorbent polymer (SAP) on self-healing capability of mortar with cracks. RHA is commonly used in concrete as supplementary cementitious materials (SCMs). RHA, due to its silica-rich composition, is characterized by long-term strength enhancement that results from the pozzolanic reaction. However, RHA consumes large amount of portlandite, which is one of the sources of Ca for self-healing of cracks. Therefore, using RHA may result in less self-healing performance than other types of SCM, such as fly ash. SAP is used as a water-entraining admixture that supplies moisture to damaged parts of concrete and is used for self-healing material. Therefore, the combination of RHA and SAP potentially enhances the self-healing performance compared to the system only with RHA. From this context, this study investigates the potential for combination of RHA and SAP as self-healing additives. High-Ca fly ash is also used for comparison. Six mix proportions of cement mortar were developed, with the mortar set to almost uniform flowability. Pre-cracked specimens were healed either by continuous water immersion or by exposure to wet-dry conditions. Self-healing performance was evaluated by the following: permeability recovery (i.e., permeability reduction), as monitored by temporal decreases in water discharge through a crack. As the results, the RHA-based system turned out to have lower self-healing performance in terms of crack closure than fly ash-based system. This is because portlandite is the main source of Ca-based healing products, and less portlandite was found to be available in the RHA-based systems than in the FA-based systems. On the other hand, the self-healing performance of RHA-based systems was effectively improved with combination with SAP.