Effects of ${\mathrm{Ca}}^{2+} \to {\mathrm{Mg}}^{2+}$ substitution on the properties of cementitious tobermorite

Using density-functional-theory calculations we assess the effects of the isovalent ${\mathrm{Ca}}^{2+}\to {\mathrm{Mg}}^{2+}$ substitutions on the mineral structure of tobermorite, a major analog of the main hydrated cement phase, calcium-silicate-hydrate (CSH). From the structural point of view, due to its smaller ionic radius, Mg substitution leads to an overall decrease of the lattice parameters for the 9 Å (dry) and 11 Å (hydrated) tobermorite structures used in the study. Furthermore, Mg doping at intralayer sites leads to a considerable distortion of the unit cell due to a changing oxygen coordination number. With the increasing amount of Mg doping, chemical bond analysis shows an overall increase in the bond strength with the crystal cohesion enhancement. In addition, Mg sites in the dry and hydrated tobermorite are characterized by an enhanced reactivity, which may be useful in capturing $\mathrm{C}{\mathrm{O}}_2$. With regard to the elastic properties, Mg doping leads to an overall stiffening of the elastic moduli. However, depending on the particular site, Mg doping may give rise to an increased elastic anisotropy. Overall, the present results indicate that Mg-based tobermorite structures may be useful prospects in the search for alternative components toward the development of high-performance, environmentally friendly cementitious materials.

Figure 1

Side view of layered structures of tobermorites. (a) Dry 9 Å tobermorite. (b) Hydrated 11 Å tobermorite. Schematic comparison of the wollastonite-like chain between the (c) single (dry tobermorite) and (d) double silica chains (hydrated tobermorite).

Figure 2

Visualization of the simulation cells, with panels (a) and (b) showing the atomic site types for the dry and hydrated tobermorite structures, respectively.

Figure 3

Schematic visualization of ${\mathrm{Ca}}^{2+}\to {\mathrm{Mg}}^{2+}$ substitutions for dry tobermorite structure. For clarity, only ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ ions are shown. T9A-0Mg represents the Original dry tobermorite structure. T9A-Mg-inter the substitution of the interlayer ${\mathrm{Ca}}^{2+}$ ions only. T9A-Mg-inter the substitution of the intralayer ${\mathrm{Ca}}^{2+}$ ions only. And T9A-Mg-all the substitution of all ${\mathrm{Ca}}^{2+}$ ions.

Figure 4

Schematic visualization of ${\mathrm{Ca}}^{2+}\to {\mathrm{Mg}}^{2+}$ substitutions for hydrated tobermorite structure. For clarity, only ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Mg}}^{2+}$ ions are shown. T11A-0Mg represents the original hydrated tobermorite structure, T11A-Mg-inter represents the substitution of the interlayer ${\mathrm{Ca}}^{2+}$ ions only, T11A-Mg-inter represents the substitution of the intralayer ${\mathrm{Ca}}^{2+}$ ions only, and T11A-Mg-all represents the substitution of all ${\mathrm{Ca}}^{2+}$ ions.

Figure 5

Visualization of (a) the undoped dry tobermorite (T9A-0Mg) structure and its Mg-doped variants, (b) T9A-Mg-inter, in which all interlayer Ca ions have been replaced by Mg, (c) T9A-Mg-intra in which all intralayer Ca ions have been replaced by Mg, and (d) T9A-Mg-all in which all Ca have been substituted by Mg.

Figure 6

Visualization of (a) the undoped hydrated tobermorite (T11A-0Mg) structure and its Mg-doped variants, (b) T11A-Mg-inter, in which all interlayer Ca ions have been replaced by Mg, (c) T11A-Mg-intra in which all intralayer Ca ions have been replaced by Mg, and (d) T11A-Mg-all in which all Ca have been substituted by Mg.

Figure 7

Bond length distribution for each bond pair in (a) the dry tobermorite and (b) the hydrated tobermorite. Results are shown for T9A-0Mg/T11A-0Mg (blue), T9A-Mg-inter/T11A-Mg-inter (green), T9A-Mg-intra/T11A-Mg-intra (red), and T9A-Mg-all/T11A-Mg-all (orange).

Figure 8

(Left) Bond strength energies for dry and (right) hydrated tobermorite. Results for undoped T9A-0Mg and T11A-0Mg structures are shown in blue, interlayer substitutions in T9A-Mg-inter/T11A-Mg-inter in green, intralayer substitutions in T9A-Mg-intra/T11A-Mg-intra in red and the fully substituted T9A-Mg-all/T11A-Mg-all in orange.

Figure 9

BOD and TBO contributions for (a) dry and (b) hydrated tobermorite structures. Scales on the right represent the TBO values percentages for the Si-${\mathrm{O}}_{\mathrm{p}/\mathrm{b}/\mathrm{h}}$ (blue), $\mathrm{Ca}/{\mathrm{Mg}}_{\mathrm{inter}/\mathrm{intra}}$-O (green), and the O-H-O, O–H, and O-H bonds (orange). Scale on the left represents the BOD values (pink).

Figure 10

Relative changes in elastic stiffness constants for doped structures with respect to undoped crystals for dry (a) and hydrated (c) tobermorites. Variation of Voigt-Reuss-Hill averages for the bulk moduli $B$, shear moduli $G$, and Young's moduli ($E$) due to Mg doping in the dry (b) and hydrated (d) tobermorite structures.

Figure 11

Visualization of the anisotropies of Young's moduli $E$, shear moduli $G$, and the Poisson's ratio $\nu$ projected in the $ab$ plane for dry tobermorite and its Mg-doped variants. For $G$ and $\nu$ blue and green contours represent the maximum and minimal values of these elastic parameters, respectively (see text).

Figure 12

Visualization of the anisotropies of Young's moduli $E$, shear moduli $G$ and the Poisson's ratio $\nu$ projected in the $ab$ plane for hydrated tobermorite and its Mg-doped variants. For $G$ and $\nu$ blue and green contours represent the maximum and minimal values of these elastic parameters, respectively (see text).