The rheological properties of fresh cement paste are highly influenced by a large number of parameters, among which the most important factors are the applied shear stress, and the shear history, the age of the sample and the temperature. The effects of these parameters on the yield stress (designated as structural limit stress in this work), the viscosity and the structural recovery rate (i.e., the change in dynamic viscosity with time at rest) were studied. In parallel, the changes in ion composition of the carrier liquid, mineral phase content and granulometry were investigated. The results reveal that all investigated rheological parameters exhibit an approximated bi-linear trend with respect to the degree of hydration, with a period of quasi-constant properties until a degree of hydration of approximately 0.07, followed by a non-linear increase. This increase could be attributed to the formation of calcium hydroxide (CH) and calcium-silicate-hydrate (C-S-H) via calorimetry results. With regard to the effect of the shear history of the sample on the rheological properties, the structural limit stress showed a minor dependency on the shear history immediately after the end of shearing, which, however, vanished within the first minute at rest. The same is true for the structural recovery rate. The presented results give detailed insights into the influences of hydration and shear on the rheological properties—especially the thixotropy—of fresh cement pastes.

A thorough characterization of base materials is the prerequisite for further research. In this paper, the characterization data of the reference materials (CEM I 42.5 R, limestone powder, calcined clay and a mixture of these three components) used in the second funding phase of the priority program 2005 of the German Research Foundation (DFG SPP 2005) are presented under the aspects of chemical and mineralogical composition as well as physical and chemical properties. The data were collected based on tests performed by up to eleven research groups involved in this cooperative program.

Calcined clays (CCs) as supplementary cementitious materials (SCMs) can be a promising option to reduce clinker content and CO₂ emissions in eco-friendly concretes. Although CCs as components of composite cements in combination with Ordinary Portland Cement (OPC) and limestone powder (LSP) have attracted industry interest, their use as concrete additives is limited. This study investigates the effects of the addition of CCs on the fresh and hardened properties of industry-standard ready-mixed concretes. Four concrete mix designs, each with three superplasticizer dosages, were tested, resulting in 12 variations. The CCs used, which are typical of 2:1 bentonite clays with low metakaolin content, reflect the clays available in Germany. The results showed that CCs significantly influenced the workability, which could be controlled with a high superplasticizer dosage. Increased CC contents reduced bleeding tendencies, which was beneficial for certain structural applications. Early age strength decreased with CCs, but the 28-day strength exceeded that of pure OPC concretes up to 30 wt% CCs. Resistance to CO₂-induced carbonation decreased with higher levels of CCs but was comparable up to 15 wt%. Freeze–thaw damage decreased, and chloride migration resistance improved due to a denser microstructure. The global warming potential (GWP) of the concretes tested is in line with that reported in the literature for concretes made from highly blended cements, suggesting that CCs can improve the sustainability of concrete production.

The workability and the de-airing behaviour of fresh concrete are strongly influenced by the cement paste contained therein. The rheological properties of cement pastes and concretes in general are a function of the mix composition, hydration processes, shear history and external influences (such as temperature, vibration etc.). Microscopic interparticle interactions control the macroscopic bulk rheology. In the paper at hand, the results of a 6-year project are described, in which the interactions between particles, between particles and the carrier liquid, as well as superplasticizers and variations in particle inventory and the carrier fluid composition, were studied. The work in the first step yielded in a surface complexation model for a cement paste model system, which allows to predict the particles’ zeta potential and other surface parameters as a function of the ion brine composition. Further, an in-depth analysis of the mechanisms of thixotropy in cementitious suspensions was carried out and a corresponding model was developed. This model is based on the results of combined rheological and tomographical measurements. In doing so, a prediction of the rheological properties of cement suspensions subjected to a complex shear history during the entire lifetime of fresh cement suspension is now possible. The prediction of the rheological properties becomes even more challenging when Ordinary Portland Cement (OPC) is replaced by supplementary cementitious materials (SCMs) like calcined clays, where the clay composition and its calcination conditions significantly influence the flow behaviour and the entire hydration process. The analysis of calcination conditions for varying clay mineralogy and the influence of SCMs on the hydration processes of OPC ensure an extended application of the findings for low-clinker concrete. Concerning the practical application of the results, the effect of the rheological properties of the paste on the de-airing behaviour of concrete was investigated and modelled. A proper de-airing process of fresh concrete can be achieved by tuning the predicted rheological properties of cement paste.