Carbonation of 20-year-old blended cement pastes

This PhD study aims to understand the carbonation mechanism of slag-rich waste forms and consequently benefits assessing the long term efficiency of immobilising waste materials with cementitious disposals. Three phases of experiments had been managed to achieve the aim. The first was an examination of three 20-year-old alkali-activated neat slag pastes. This section supplied the information about the immobilisation mechanism of alkaline metal cations within a cement system. The second was a characterisation of four 20-year-old water- activated slag-bearing pastes with curing at 40°C for the initial five years. This part concerned the effects of aging and heating treatment in cement hydration. The third was the accelerated and gentle testing of the water-activated pastes. The carbonation mechanism was mainly drawn by the micro-structural and nano- structural features of carbonated pastes and comparison to those of hydrated pastes. The obtained experimental data for the alkali-activated pastes confirmed the outstanding suitability of slag for fixing metal cations. It was because the formed C- S-H gel had been verified to be entirely tobermorite-based and possess a high substitution degree of bridging Si tetrahedra by Al tetrahedra. Meanwhile, the main fixation mechanism has been proved to be the incorporation of metal cations into the structure of the C-S-H through balancing the charges caused by the substitution. Thus, the significant incorporation of Ae+ within the structure of the C-S-H could be advantageous of immobilising alkaline cations through charge-balancing. The collapse of the C-S-H structure during carbonation has been observed due to not only decalcification, but also dealumination. Thus, according to the above findings, the carbonation is very likely to negatively affect the fixation of waste materials within cement systems. It was because, along with the extraction of the bridging Al tetrahedra, the charge-balancing cations were not needed and possibly would eventually be released. A conceptual model of carbonation occurring in hardened cement pastes has been proposed at the end of the chapter conclusion. More findings about the efficiency of different activators on hydration and the effects of curing temperature and aging on hydration have been discussed in details in the following chapters.