Repair of ordinary concrete using alkali activated slag/fly ash: High temperature resistance and micro structure evolution of adhesive interface

In this paper, the effect of alkali activated slag-fly ash (AASFA) geopolymer as repairing materials on the high temperature resistance of the new-to-old concrete (NTOC) adhesive interface was studied. The deterioration of splitting tensile strength and shear strength with increasing temperature was investigated when different ratios of slag-fly ash were used as repair materials. The mechanism of AASFA improving the high temperature resistance of the adhesive interface was analyzed from the micro morphology, pore structure, and hydration products. The results showed that GPC50 as repairing materials, the mechanical properties and high temperature resistance of adhesive interface were the best. For OOPC-GPC50, the development of the cracks was inhibited at each temperature, especially at 100 °C and 200 °C. AASFA continued to hydrate under high temperature environment, and the filling effect induced by hydration products inhabited the deterioration of pore structure, which reduces the harmful pores formation around adhesive interface. The regenerated gel could provide additional adhesiveness, thus improving the strength of NTOC. Under high temperature environments, Ca(OH)2 reacted with AASFA at the adhesive interface of OOPC-GPC50. As the temperature increased, the reaction continued to develop, and more Ca(OH)2 was consumed in further.