Crack repair by activation of the pozzolanic or slag reaction

Nowadays, industrial by-products like fly-ash (FA) and blast-furnace slag (BFS) are commonly used as cement replacing material. In combination with ordinary Portland cement (OPC), the BFS reaction is activated by the presence of alkali hydroxides in the pore solution and the Ca(OH)2 released during the OPC hydration. However, former research has shown that the slag hydration degree significantly decreases in pastes containing very high amounts of slag (> 80%). Also for FA-OPC mixes with a high FA/OPC ratio (> 50%), the activation level of the pozzolanic reaction is strongly reduced. While FA and BFS concrete seem to be inferior with regard to the early age microstructure and strength development, their self-healing capability can be much higher, precisely because of the low hydration degree of the slag and fly-ash particles. Upon cracking, the unreacted particles can be activated again in order to close the crack and to regain water impermeability and strength. In this study, autogenous as well as autonomous healing of mixes containing BFS or FA was investigated. First of all, calorimetric measurements were performed in order to evaluate the renewed hydration reactions of crushed pastes (age ~ 1 - 2 months), which were brought into contact with water to simulate autogenous healing, or with different activators (NaOH, Na-silicate,…) to simulate autonomous healing by alkali activation. Secondly, cracks were created in mortar bars with different binder compositions at an age of 28 days. The specimens were then stored under water or in alkaline solutions and the extent to which cracks close was monitored by optical microscopy. The results clearly showed the self-healing capacities of BFS and FA mixes. Based on this study, the most appropriate activators for BFS and FA were chosen