Abstract
Calcium carbonate cements have been synthesized by mixing amorphous calcium carbonate and vaterite powders with water to form a cement paste and study how mechanical strength is created during the setting reaction. In-situ X-ray diffraction (XRD) was used to monitor the transformation of amorphous calcium carbonate (ACC) and vaterite phases into calcite and a rotational rheometer was used to monitor the strength evolution. There are two characteristic timescales of the strengthening of the cement paste. The short timescale of the order 1 h is controlled by smoothening of the vaterite grains, allowing closer and therefore adhesive contacts between the grains. The long timescale of the order 10–50 h is controlled by the phase transformation of vaterite into calcite. This transformation is, unlike in previous studies using stirred reactors, found to be mainly controlled by diffusion in the liquid phase. The evolution of shear strength with solid volume fraction is best explained by a fractal model of the paste structure.
Highlights
Due to the carbon footprint of ordinary Portland cement (OPC), there has been an increased interest in alternative hydraulic binders [1,2,3,4]
We present the X-ray diffraction (XRD) measurements corresponding to the recrystallization of calcium carbonate polymorphs from amorphous calcium carbonate (ACC) and vaterite to the final calcite cement
At high volume fractions, corresponding to our study, the weakest link is between flocs and the yield stress is expected to depend on the volume fraction as a power law: τY ∼ mfp where m depends on the interparticle forces and the particle size and the exponent p depends on the d+x fractal dimension, df, of the flocs
Summary
Due to the carbon footprint of ordinary Portland cement (OPC), there has been an increased interest in alternative hydraulic binders [1,2,3,4]. Inspired by the outstanding mechanical properties of limestone, which is mainly composed of calcium carbonate (CaCO3 ), alternative production paths to form calcium carbonate binders have been investigated [1,5,6], the most famous being the Calera carbonation process [7] Another path to prepare pure CaCO3 cements is of special interest as a model system: Combes and co-workers [5,6] mixed water with two metastable CaCO3 phases. The mechanical properties still remain poorly understood despite their chemical similarities with limestone These properties develop during the process of preparing the initial powders, mixing the paste, setting reactions at constant liquid fraction and during drying. We focus on the properties of a calcium carbonate paste while it is transforming from ACC and vaterite
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