Formation of Microstructure in Low-clinker Binders Rich in Limestone

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Owing to the energy-intensive production of Portland cement clinker, the development of low-clinker binders is of high ecological and economic importance for the future production of environment friendly concrete. Limestone is a suitable binder component which is already in the quarries of the cement producers, requires relatively little energy for grinding and is available in virtually unlimited quantities. However, the production of suitable binders based on clinker with above 50% limestone exceeds the current limits of technology and can only be achieved through knowledge of the mechanisms that control the development of the solid structure from the densely packed fresh paste. These include the interactions between particle surfaces and dissolved ions and superplasticizer molecules which determine the dispersion of the particles, nucleation and growth processes and, in particular, the spatial distribution of hydration products between the limestone particles and thus the evolution of phases and strength.In the experiments, the materials (CEM I, calcite flour) are separated into a series of particle size fractions. The calcite fractions are then combined in different proportions to obtain a highly packed particle mixture. Next, various calcite size fractions are replaced volumetrically by equivalent CEM I fractions to produce different fresh pastes (CEM I particle sizes, w/c ratio). Here, the water content of the fresh pastes corresponds to the space between the packed particles. The type (nanostructure) and dosage of the superplasticizer are varied.Besides the flowability (Minislump) of the fresh pastes, the early hydration is studied in detail: zeta potential (electro-acoustic), chemical composition (including TOC) of the pore solution (extraction under pressure), heat of hydration (heat-flow calorimetry) and the formation of the hydration products (XRD analysis). Strengths and pore size distributions (MIP) are determined after longer hydration times.The effect of other minerals as a substrate for C-S-H nucleation in densely packed binder systems is also investigated. Pastes are produced with densely packed quartz or aragonite analogously to calcite.It is expected that the evolution of the microstructure also depends on the aluminium content of the binder because this affects the chemical reactivity of limestone. To investigate this, the CEM I particles are partly replaced by fine fly ash.The experimental results provide new insights into relationships between the granulometric composition of the binder (components, surfaces, particle sizes, packing, interparticle separation), the pore solution (ions, superplasticizer molecules), the adsorption of ions and superplasticizer molecules and their effect on flowability and microstructure (phases, porosity and strength).