Evidence for pore water composition controlling carbonate morphology in concrete and the further effect of gamma radiation

Radiation, heat and carbonation can all affect the performance of concrete structures. This study applies either a 1.5 MGy gamma radiation dose or thermal treatment at 60 °C, followed by an accelerated carbonation period. In comparison to a control sample, irradiation and heat treatment lead to an increase in the carbonation depth which correlates with an increase in crack depth induced by restrained drying-induced shrinkage. When compared to thermally treated samples, there is greater variability in the crack depth of irradiated samples which we ascribe to a radiation-induced change in the mechanical properties of the cement paste. X-ray diffraction is used to identify the calcium carbonate polymorphs that form as result of carbonation. The dominant phase in the control sample is vaterite whereas the dominant phase in both the irradiated and thermally treated samples is calcite. Gamma irradiation leads to a greater calcite:vaterite ratio than thermal treatment. The sequential application of gamma radiation/heat treatment followed by a carbonation period rules out radiolytically producedalcium peroxide octahydrate as the cause of the polymorph switch as it is too short-lived. A new mechanism is hypothesised, where elemental substitution into the calcium carbonate lattice results in deformation and so alters the morphology of the calcium carbonate, leading to the formation of a different polymorph.