Mechanisms of Phase Transformation and Creating Mechanical Strength in a Sustainable Calcium Carbonate Cement

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.

Keywords: calcium carbonate; cement; colloidal suspension; hardening; phase transformation.

Figure 1

Scanning electron microscopy (SEM) micrographs of synthesized calcium carbonate powders in the form of (A) Amorphous Calcium Carbonate (ACC) phase and (B) vaterite phase with one calcite grain. (C) Calcite cement after recrystallization of ACC:V 1:3 mixture. (D) X-ray diffraction (XRD) patterns of lyophilized ACC and vaterite powders.

Figure 2

Mass fraction evolution of calcite, X_C (increasing with time), vaterite, X_V (circles), and ACC, X_ACC (always below 0.1), with time for four different CaCO₃ cement compositions.

Figure 3

Mass evolution with normalized time for four different CaCO₃ cement compositions. The solid line shows the predicted evolution from models with diffusion, vaterite dissolution and calcite growth as the rate-limiting steps.

Figure 4

Time evolution of storage modulus, G′, for three different CaCO₃ cement compositions (ACC:V 1:1, 1:2 and 1:3) measured at constant frequency, f = 10 Hz, and deformation, γ = 0.003%.

Figure 5

Time evolution of the normalized storage modulus, G_N′, for three different CaCO₃ cement compositions (ACC:V 1:1, 1:2 and 1:3) measured from the time evolution of the elastic modulus. The average characteristic reaction times, τ_i, are also included.

Figure 6

Vaterite at initial stage of phase transformation. Inset: Magnification of a vaterite grain demonstrating the surface roughness consisting of ~100 nm spheres on the 2–5 µm large vaterite grains.

Figure 7

Rheological properties of 1:2 ACC:V suspension of 36% volume fraction. The shear modulus G′ (blue triangle down) and the shear stress τ (black circle) are plotted as a function of the shear strain γ measured through an amplitude sweep at f = 1 Hz. The vertical dashed line delineates the elastic and the plastic regimes. The horizontal plain line indicates the value of the yield stress defined as the value of the shear stress.

Figure 8

Dependence of the yield stress of ACC:V (1:2) paste with the volume fraction. Comparison of the experimental data with the Yodel model [22] (blue curve) and the fractal model of Shi et al. [24] (red curve).