Reactivity in Recycled Concrete Aggregate

Full scale tests with crushed concrete used in sub-bases in roads show that the material has capacity to reharden, very slowly but also after prolonged periods of time. The rehardening process in crushed concrete aggregate has been analysed with a thermogravimetric method, comparing the change in chemical composition in the aggregate when set with water, compacted and hardened. It has been shown that grinding of crushed mortar, even if reground to cement fineness, will not activate the unhydrated cores of cement observably. If the mortar is ground together with surface active additive, the material set with water gives compressive strength of 2.8 MPa in 15 days. This is likely due to increased bonding between the ground cores of sand and cement, allowing further crystallisation of ground material to occur closer and chemically better bonded to silicate surfaces.