Abstract
Partially replacing cement clinkers with activated clays or natural pozzolans is one of the most promising routes to decarbonize the cement industry and tackle the climate change crisis. The coupled substitution of cement with calcined clay and limestone also increases the replacement ratio and decreases the clinker factor. Therefore, the first results chapter of this PhD thesis (Chapter 4) investigated the possibility of adding soda ash, mainly consisting of sodium carbonate, in coupled substituted cement with amorphous pozzolans, which contain different Al2O3 content (metakaolin, rice husk ash, and obsidian).Besides the coupled substituted cement, with effective activation treatment, such as thermal and mechanochemical activation, natural clays and minerals can be turned into supplementary cementitious materials (SCMs) for producing low-carbon cement. Chapter 5 and Chapter 6 systematically investigated the impact of mechanochemical activation on the physicochemical and pozzolanic properties of 1:1(kaolinite), 2:1(muscovite, montmorillonite) clays, carbonate-bearing mixed clays, and obsidian minerals, including particle size distributions, morphologies, bulk and surface chemical structures. The results suggest that mechanochemical activation treatment is particularly efficient for improving the pozzolanic activity of 2:1 clays (i.e. muscovite and montmorillonite), which are difficult to effectively activate through thermal treatments and therefore not previously been extensively used as supplementary cementitious materials (SCMs). In addition to dehydroxylation and amorphisation, mechanochemical milling also leads to surface aluminium enrichment and reduction in binding energies of both Si and Al elements, all contributing to the enhanced pozzolanic reactivity. The outcome of this study represents a step-change in scientific knowledge and extends the frontiers of developing new SCMs from sustainable resources.
As for carbonate-bearing clays and obsidian, the results suggest that the mechanochemical activation of clay and marl results in delamination, dehydroxylation and amorphisation of the mineral components (including calcite), while for obsidian, the main effect was particle size reduction. Among all samples assessed, the mechanochemically activated obsidian exhibited the best performance as SCMs, achieved marginally higher strength performance at 20% cement replacement with compared with the CEM I cement mortar reference. Mechanochemical activation effectively improved the strength performance of the blended mortar cubes when compared with standard milled materials, which was primarily due to the enhanced activity of clay minerals (i.e., kaolin, montmorillonite, and muscovite) and improved specific surface areas. In comparison, the slightly higher strength performances was observed from the thermally treated clay and marl, mainly attributed to the formation of CaO (or portlandite after absorbing moisture from the air) at the calcination temperature of 800 °C (above decarbonation temperature). These results indicate that mechanochemical activation can effectively improve the pozzolanic reactivity of clay minerals that contain calcite up to 68% without directly emitting CO2 to the environment due to the carbonate-rich clay calcination, which is a significant advantage over the high calcination activation method.
| Date of Award | 4 Dec 2023 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | Turkish Ministry of Education |
| Supervisor | Andrew Heath (Supervisor), Juliana Calabria-Holley (Supervisor) & Xinyuan Ke (Supervisor) |