Dolomite calcined clay composite cement - hydration and durability

Abstract

Considerable amounts of CO2 are emitted during the production of cement. One way to reduce these CO2 emissions is to replace some of the cement clinker with supplementary cementitious materials (SCMs). A cement containing SCMs is called a composite cement. The use of composite cements has considerably increased over the last few decades and is predicted to continue to do so as the global demand for concrete increases in the future. This will lead to a high demand for SCMs, which in the case of some of the traditionally used SCMs could exceed supply. There is therefore a need to identify potential new SCMs. The PhD project investigated the possibility of using dolomite in combination with calcined clays as SCMs in Portland composite cements. In this project, metakaolin was used as a model material for industrially available calcined clays. The main objectives of the PhD project were to understand the hydration of Portland composite cements containing dolomite and metakaolin, the resulting phase assemblage, and its stability during carbonation, leaching and chloride exposure. Special focus was put on the hydrate hydrotalcite that was shown to form in composite cements containing dolomite. The compressive strength, phase assemblage, and microstructure of samples, in which various amounts of Portland cement clinker were replaced with either dolomite or with a combination of dolomite and metakaolin, were investigated. In the first part of the project, the hydration study, two experimental approaches were applied. In the first one, dolomite or limestone were used to replace 0-20%wt of a Portland metakaolin cement. The Portland metakaolin cement represented a mix of Portland cement clinker and metakaolin in the ratio 6:1. It was shown that approximately 10%wt of a Portland metakaolin cement could be replaced with either dolomite or limestone without impairing its compressive strength after curing at temperatures of 20 °C and 38 °C for 28 days and 90 days. The phase assemblages in samples containing limestone or dolomite were generally quite similar, though dolomite was shown to be slightly less reactive than limestone, especially at lower curing temperatures. This lower reactivity resulted in less carbonates delivered from dolomite to form carbonate AFm phases. In the second experimental approach, various combinations of dolomite and metakaolin were used to replace 40%wt of a Portland cement clinker. After curing for 360 days, especially at elevated temperatures (38 °C and 60 °C), the dolomite in samples with little or no metakaolin content had reacted significantly. The dolomite reaction was shown to consume portlandite and resulted in the formation ofhydrotalcite (Mg6Al2(OH)18·3(H2O)) and calcite: 6CaMg(CO3)2 + 2Al(OH)3 + 6Ca(OH)2 + 3H2O • Mg6Al2(OH)18·3(H2O) + 12CaCO3 This reaction was shown to depend on the availability of portlandite in the system. In samples with high metakaolin content, the reactive metakaolin consumed the portlandite before the less reactive dolomite started to react, which limited the dolomite reaction. The pore space and the availability of aluminium were found to play minor roles in limiting the dolomite reaction. The durability part of the PhD project investigated the stability of the hydrotalcite formed in composite cements containing dolomite. Samples with little or no metakaolin content were selected because they formed the greatest amount of hydrotalcite in the hydration study. It was shown that hydrotalcite can withstand high degrees of leaching and carbonation. After chloride exposure, the formation of a chloride-containing hydrotalcite was observed, which increased the chloride binding of the cement paste samples investigated. With mass balance calculations, it was shown that the chloride-containing hydrotalcite in samples containing dolomite can contribute to the chloride binding of the cement paste to an extent comparable to Friedel's salt in samples containing limestone. It could be concluded that dolomite performs in a similar way to limestone in cementitious systems with regard to compressive strength and phase stability. The phase assemblages in composite cements containing dolomite and limestone are similar. The reaction of dolomite in systems containing little or no metakaolin was shown to result in the additional formation of hydrotalcite, a stable reaction product in the aggressive environments tested, which is able to take up a significant amount of chloride.