Nanostructure of Calcium Silicate Hydrates in Cements

Calcium silicate hydrate (CSH) is the major volume phase in the matrix of Portland cement concrete. Total x-ray scattering measurements with synchrotron x rays on synthetic CSH(I) shows nanocrystalline ordering with a particle diameter of 3.5(5) nm, similar to a size-broadened 1.1 nm tobermorite crystal structure. The CSH component in hydrated tricalcium silicate is found to be similar to CSH(I). Only a slight bend and additional disorder within the CaO sheets is required to explain its nanocrystalline structure.

Figure 1

(a) X-ray structure factors $S_X(Q)$ of synthetic ${\mathrm{Ca}}_{0.13}{\mathrm{Si}}_{0.13}{\mathrm{O}}_{0.51}{\mathrm{H}}_{0.23}$ (CSH, middle) and hydrated ${\mathrm{Ca}}_3{\mathrm{SiO}}_5$ (hyd-C3S, bottom). A similar composition nonhydrated Ca-Si-O glass from 11 (top). The (light) grey lines are the raw data, and (dark) black lines are the back transforms after removal of $r<0.1\mathrm{nm}$ features [10]. The blue line labeled CSH (lit.) from Ref 12, represents CSH with $\mathrm{Ca}/\mathrm{Si}=0.8$. The arrow indicates the 1.1 nm $d$-spacing position of the basal peak in both the CSH patterns. (b) The measured diffraction patterns of hydrated ${\mathrm{C}}_3\mathrm{S}$ paste (top), and the isolated CSH component (2nd top) after subtraction of the crystalline components (lower patterns). The (light) grey $\mathrm{Ca}(\mathrm{OH}{)}_2$ pattern is taken from 19, whereas the blue dotted $\mathrm{Ca}(\mathrm{OH}{)}_2$ pattern has been refined to eliminate Bragg peaks in the remaining CSH (in C3S) pattern [10]. Also plotted is the synthetic CSH(I) pattern (scaled by 0.65) for comparison.

Figure 2

The measured $G_X(r)$ (black line, top) and the equivalently weighted $G_X(r)$ from the Hamid [16] and Merlino et al. [17] crystal structures (blue and grey dotted top lines, respectively). Also shown is the x-ray weighted partial structure factors for the Hamid structure. From top to bottom $\mathrm{\text{Grey}}=\mathrm{CaSi}$, $\mathrm{\text{Black}}=\mathrm{CaCa}$, $\mathrm{\text{Black dotted}}=\mathrm{SiSi}$, $\mathrm{\text{Black}}=\mathrm{SiO}$, $\mathrm{\text{blue}}=\mathrm{CaO}$, $\mathrm{\text{grey dotted}}=\mathrm{OO}$.

Figure 3

The x-ray pair distribution functions of synthetic CSH(I) (top black line, obtained using a Lorch modification function, light grey is the raw transform) and hyd-C3S (lower black and grey lines). The lower dashed blue line shows the intensity damping present due to the finite $Q$ resolution, whereas the upper dashed line approximates the damping present in the CSH(I) sample due to grain size. The middle curves are the distribution functions of 1.1 nm tobermorite before and after refinement to fit the measured data.