Formulation, use and durability of concrete with low clinker cements

Concrete production accounts for half of all the materials yearly manufactured. For this reason, and despite its low embodied energy and low CO2 emission compared with other construction materials, concrete industry contributes to up to 8% of the man made CO2 emissions and 3% of the energy globally consumed. This is mainly related to the production of clinker, main component of cement. For this reason, over the past decades the construction industry is under a high pressure to improve concrete sustainability, being this in agreement with the European targets to reduce by 2050 around 80-90% the greenhouse gas emissions compared to 1990 levels. Clinker replacement by supplementary cementitious materials (SCMs) is one of the most effective approaches to reduce the enviromental impact of cement [1]. Current levels of clinker substitution are around 20-30% but our research is targeted to reach levels of substitution up to 50%. However, the use of low clinker cements is limited on practice by (a) local availability of SCMs, (b) limited early strength due to their lower early reactivity and (c) the lack of knowledge about their long-term properties. In particular, there are uncertainties concerning their carbonation rate leading to reinforcement corrosion and the reliable performance tests to be applied based on the underlying mechanism. This work is part of an extensive research involved in a project funded by the Swiss National Science Foundation entitled “Concrete Solutions”, whose main objective is to decrease the embodied energy and CO2 footprint of concrete. In our work package we specifically aim at elaborating a new generation of low clinker cements that can realistically be produced in Switzerland, considering the locally available raw materials. On the one hand, we facilitate the use of cement with low clinker content, on the other hand we secure the durability of reinforced concrete made with high cement substitution. Moreover, we assess the environmental impact of this low energy and CO2 concrete. In a first stage of this study, we increase the early mechanical properties of low clinker concrete by using accelerators. Moreover, high fluidity at low water/cement ratios is ensured by the use of superplasticizers. The main challenge of this approach is to identify comb-shaped superplasticizers whose fluidizing properties are not penalized by the presence of the accelerators . In terms of durability, preliminary studies [3] have proved that the accelerated carbonation tests do not propertly describe the carbonation rate in blended cements. For this reason, we investigate the durability of these low clinker concrete under natural carbonation conditions. Furthermore, reliable performance tests and predictive models for carbonation are being developed. Finally, we examine the protective nature of the low clinker cement for steel. In particular, we investigate the kinetics of corrosion on carbonated samples once it has initiated, as the corrosion propagation stage can strongly influence the total service life of concrete.