Search

Various strategies are applied in order to reduce the green-house gas-emissions, such as optimizing the process of clinker production, the use of alternative fuel and the partial substitution of the clinker in composite cements by the so-called supplementary cementitious materials (SCM). Currently, the most suitable SCMs from the available materials are semi-hydraulic or pozzolanic materials such as granulated blast-furnace slags (herein "slag") and fly ashes from coal combustion process [1]. Application of, limestone, is limited because of its little contribution on the performance [2]. Ternary systems (herein "composite cement") simultaneously containing more than one SCM besides Portland cement clinker contribute to the reduction of CO2 emissions due to further decrease of the clinker content. Recent research work on quaternary systems revealed that use of slag and fly ash in combination with limestone may lead to the further increase of the cement clinker replacement ratio [3] [4]. The main processes taking place during the hydration of Portland cements (OPC) are well understood. In contrast, the hydration of composite cements are more complicated. As both, part of the Portland cement and the added SCMs react simultaneously. This leads to their interaction and thus influencing the reactivity of each other. HeidelbergCement launched the research program aiming at the understanding of the parameters that allow maximizing the clinker replacement by slag and limestone while keeping adequate cement performances. Understanding the interactions between the reacting clinker, slag and limestone [4] [5] allowed the development and the optimization of production methods of the composite cements at a clinker replacement of up to 50%. These efforts have led to the establishment of the know-how allowing the optimization of the cement composition and properties. During the Endurcrete project (EU funded project Horizon 2020), the methods of optimization were used to produce composites cement with low environmental footprint for improved durability. This contribution presents the optimization of the multicomponent cement for this purpose maximizing the cement performance evolution while minimizing its environmental impact.

Various strategies are applied in order to reduce the green-house gas-emissions, such as optimizing the process of clinker production, the use of alternative fuel and the partial substitution of the clinker in composite cements by the so-called supplementary cementitious materials (SCM). Currently, the most suitable SCMs from the available materials are semi-hydraulic or pozzolanic materials such as granulated blast-furnace slags (herein "slag") and fly ashes from coal combustion process [1]. Application of, limestone, is limited because of its little contribution on the performance [2]. Ternary systems (herein "composite cement") simultaneously containing more than one SCM besides Portland cement clinker contribute to the reduction of CO2 emissions due to further decrease of the clinker content. Recent research work on quaternary systems revealed that use of slag and fly ash in combination with limestone may lead to the further increase of the cement clinker replacement ratio [3] [4]. The main processes taking place during the hydration of Portland cements (OPC) are well understood. In contrast, the hydration of composite cements are more complicated. As both, part of the Portland cement and the added SCMs react simultaneously. This leads to their interaction and thus influencing the reactivity of each other. HeidelbergCement launched the research program aiming at the understanding of the parameters that allow maximizing the clinker replacement by slag and limestone while keeping adequate cement performances. Understanding the interactions between the reacting clinker, slag and limestone [4] [5] allowed the development and the optimization of production methods of the composite cements at a clinker replacement of up to 50%. These efforts have led to the establishment of the know-how allowing the optimization of the cement composition and properties. During the Endurcrete project (EU funded project Horizon 2020), the methods of optimization were used to produce composites cement with low environmental footprint for improved durability. This contribution presents the optimization of the multicomponent cement for this purpose maximizing the cement performance evolution while minimizing its environmental impact.

A correction to this paper has been published: https://doi.org/10.1617/s11527-020-01558-w

The production of cement is associated with significant CO2 emissions. The effective manner to cope with this challenge is the production of composite cements characterized by high Portland cement clinker replacement ratio. This contribution reports on the composition optimization of the ternary cement, with the purpose to maximize the cement performance evolution while minimizing its environmental impact. For this purpose, based on the understanding of the cement hydration and performance evolution, a modelling tool was developed allowing predictions of the compressive strength depending on the cement composition and curing time. The approach used accounted for the specific interactions among the Portland cement clinker, granulated blast-furnace slag and limestone and for the specific microstructure features in the investigate systems. The agreement between the predictions and performance evolution measured had confirmed the accuracy of the models developed as well as of the general concept of the composite cement optimization. Consequently, the effective global warming potential of the composite cements was calculated. This revealed that composite cements characterized by approximatively 50 wt.-% of Portland cement clinker, 40 wt.-% of granulated blast-furnace slag and 10 wt.-% of limestone are characterized by an appreciable performance and by the appreciably low environmental footprint.

The phase assemblage of binary Portland cements with 45 wt% replacement by calcareous fly ash and slag has been investigated by experiment, mass balance and thermodynamic simulations. The input for these calculations was based on clinker reaction measured by XRD-Rietveld refinement, on SCM reaction measured by SEM-EDS full element mapping and on C-S-H composition by SEM-EDS point analyses. A sensitivity analysis estimates the uncertainty of determination of phase volume as at best +/- 2 cm(3)/100 cm(3), and thus of rather semi-quantitative character. The differences between experiment and calculations regarding AFt/AFm, C-S-H and portlandite were assessed. Gel/space ratios computed using the volumes of all hydrates showed a clear correlation to compressive strength, which was unaffected by the mix composition. This suggests that the type of hydrates formed has little or no influence on the compressive strength and that the key factor is the space filling.

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. Digital full text not available