Carbonation mechanism of alkali-activated fly ash and slag materials: In view of long-term performance predictions

As the building sector is expanding, a growing interest in technologies that can reduce the CO2 emission from concrete production has led to partial replacement of cement with by-products from various industrial processes. Besides the partial replacement of cement, development of alkali activation technology ensures full replacement of cement in concrete. Although alkali activated materials (AAMs) are one of the most sustainable alternatives to cement-based concrete, structural application of AAMs is still not viable, as their long-term performance is not sufficiently studied. For instance, no recommendations are yet given to the scientific and engineering communities as a general approach for testing carbonation of AAMs. Furthermore, there is a limited number of case studies of long-term performance of AAMs in the past to assist the predictive models of their service life.
The long-term performance (carbonation resistance) of AAMs is mainly dependent on the microstructure features of the binder (e.g. phase assemblages and pore structure), which can be modified using different constituents and materials mixture designs. Therefore, the aim of this thesis was the development of a conceptual carbonation mechanism that can be applied to analyse carbonation resistance of any alkali activated concrete mixture. For this reason, the carbonation mechanism was studied at different length scales, from paste to concrete level, while the effects of carbonation on the chemical, physical and mechanical properties were captured. The relationship between carbonation rate, pore solution chemistry and microstructure was investigated. An advanced microstructure characterization of fly ash (FA) and ground granulated blast furnace slag (GGBFS) was performed using PARC software. The combined effect of GGBFS content, curing (sealed/unsealed) and exposure conditions (natural indoor/outdoor and accelerated carbonation) on the carbonation resistance of pastes was considered. Based on the parameter studies (GGBFS content, curing, exposure conditions), recommendations for design of alkali activated concrete for engineering practice are given in view of carbonation resistance.