Calcium oxide (CaO)-based expansive cement, also known as “non-explosive demolition agents”, has been widely used to fracture massive rocks and concrete structures. When water is added to expansive cement, calcium oxide reacts with water, and changes into calcium hydroxide expanding 1.96 times larger in volume with generating heat. In previous researches, although expansive pressure due to the volume increase has been only focused, effect of thermal stress induced by the heat has not been carefully discussed. In this meaning, the fracture mechanism of calcium oxide-based expansive cement has not been clarified.
To clarify the fracture mechanism, the authors made an experiment to fracture a mortar specimen of 300 mm cubic by setting a calcium oxide-based expansive cement in a 48 mm diameter hole bored in the center of the specimen. AE (acoustic emission) sources clustered on a middle part of the shortest line between the borehole wall and one of the lateral surfaces, just before macroscopic cracks generated. This suggests that macroscopic cracks originated between the borehole wall and the lateral surface rather than the vicinity of the hole. From the elastic theory, although the expansive pressure causes tensile stress in tangential direction of the hole, the stress is the maximum at the hole-wall and decrease in proportion to square of the distance from the hole. On the other hand, thermal stress due to heating of the hole is tensile away from a certain distance from the hole. Thus, it could be concluded that thermal stress has large effects in the fracture caused by the calcium oxide-based expansive cement, at least, in a rock or concrete block without preexisting crack.
