**4. Synthesis of cubic C-S-H and morphology-induced improvement in mechanical properties**

The aforesaid control over the morphology of calcium silicate hydrate is somewhat limited to crystalline CSH grown in hydrothermal conditions or amorphous C-S-H with the restricted scope of final shapes encompassing fibrils, plates and foil. Therefore, the extensive control over a wide range of morphologies for gel-like C-S-H had not been accomplished and the range of experimental techniques somewhat lacked diversity. Considering that the surfactantassisted, template-based synthesis had been widely applied to generate compositionally similar calcium-silicate glass particles with well-defined shapes, the similar techniques could be applied to synthesize C-S-H with a wider range of morphologies than described above [25, 26].

Moghaddam et al. accomplished for the first time the well-defined rhombohedral and cubic morphology for C-S-H, concomitantly proving beneficial properties arising from the specific morphology in the context of construction industry [27]. Herein, the authors employed the surfactant assisted, seed-mediated technique to materialize various well-defined morphologies for C-S-H, where the naturally formed calcium carbonate particles were exploited as seed particles for C-S-H nucleation and growth.

When the silicate source, sodium silicate and calcium source, calcium nitrate were dissolved in water solution under sonication, atmospheric carbon dioxide was also dissolved in the reaction mixture, releasing carbonate ions. The authors hypothesized based on the free energy of formation that in the presence of two types of anions, the silicate and the carbonate ions, calcium ions combine selectively with carbonate ions, thereby forming calcium carbonate seeds [28]. The cationic surfactant, cetyltrimethylammonium bromide (CTAB) stabilized the nanosized seeds, promoting their combination and growth in [104] directions to form microsized seeds with cubic/rhombohedral shapes. Owing to the formation of the seeds described above, the amount of available CO3 2− ions naturally decreased, thereby prompting the subsequent reaction between calcium and silicate ions to form C-S-H. The formation of C-S-H started with the heterogenous nucleation on the microsized CaCO3 seeds as the seed-mediated nucleation is more energetically favorable than homogenous nucleation. The subsequent growth led to the formation of C-S-H with well-defined cubic and rhombohedral morphologies.

Experimental factors such as the initial Ca/Si ratio, temperature of the reaction medium and the types of surfactants all exerted the significant influence on final morphology of the asformed C-S-H.

Selecting the appropriate type of surfactant was the critical factor for achieving the final well-defined morphology. Cationic surfactants including cetylammonium bromide (CTAB) could undergo electrostatic interactions with silicate ions and stabilize them, directing the reaction pathway toward the formation of cubic particles. Similarly, tetra(decyl)ammonium bromide (TDAB) ultimately induced a greater variety of morphologies ranging from cubic to rods. In contrast, anionic surfactants such as sodium dodecylsulfate, owing the inherent negative charges, repel the silicate ions and thus could play the stabilizing role similar to CTAB. It is likely that they exerted the undesired electrostatic attraction with calcium ions, disrupting the formation of cubic calcite seeds. Consequently, this led to the formation of highly aggregated, irregularly shaped C-S-H. Also, it was found that using nitrate ions as counterions was the most favorable for the formation of cubic and rhombohedral morphologies, while chloride or hydroxide ions resulted in irregular particles and sheets, respectively.

change of tobermorite crystals from typical acicular shape to bent needle-like morphology. This morphological change is likely to have arisen from the adsorption of formate ions on growing (001) faces during the synthesis, thereby impeding the normal growth process of tobermorite. This switches the major growth direction from [001] axis to [010] axis, leading to

**4. Synthesis of cubic C-S-H and morphology-induced improvement** 

The aforesaid control over the morphology of calcium silicate hydrate is somewhat limited to crystalline CSH grown in hydrothermal conditions or amorphous C-S-H with the restricted scope of final shapes encompassing fibrils, plates and foil. Therefore, the extensive control over a wide range of morphologies for gel-like C-S-H had not been accomplished and the range of experimental techniques somewhat lacked diversity. Considering that the surfactantassisted, template-based synthesis had been widely applied to generate compositionally similar calcium-silicate glass particles with well-defined shapes, the similar techniques could be applied to synthesize C-S-H with a wider range of morphologies than described above [25, 26]. Moghaddam et al. accomplished for the first time the well-defined rhombohedral and cubic morphology for C-S-H, concomitantly proving beneficial properties arising from the specific morphology in the context of construction industry [27]. Herein, the authors employed the surfactant assisted, seed-mediated technique to materialize various well-defined morphologies for C-S-H, where the naturally formed calcium carbonate particles were exploited as seed

When the silicate source, sodium silicate and calcium source, calcium nitrate were dissolved in water solution under sonication, atmospheric carbon dioxide was also dissolved in the reaction mixture, releasing carbonate ions. The authors hypothesized based on the free energy of formation that in the presence of two types of anions, the silicate and the carbonate ions, calcium ions combine selectively with carbonate ions, thereby forming calcium carbonate seeds [28]. The cationic surfactant, cetyltrimethylammonium bromide (CTAB) stabilized the nanosized seeds, promoting their combination and growth in [104] directions to form microsized seeds with cubic/rhombohedral shapes. Owing to the formation of the seeds described above,

reaction between calcium and silicate ions to form C-S-H. The formation of C-S-H started with

is more energetically favorable than homogenous nucleation. The subsequent growth led to

Experimental factors such as the initial Ca/Si ratio, temperature of the reaction medium and the types of surfactants all exerted the significant influence on final morphology of the as-

Selecting the appropriate type of surfactant was the critical factor for achieving the final well-defined morphology. Cationic surfactants including cetylammonium bromide (CTAB)

the formation of C-S-H with well-defined cubic and rhombohedral morphologies.

2− ions naturally decreased, thereby prompting the subsequent

seeds as the seed-mediated nucleation

the observed morphological change.

84 Cement Based Materials

**in mechanical properties**

particles for C-S-H nucleation and growth.

the heterogenous nucleation on the microsized CaCO3

the amount of available CO3

formed C-S-H.

Aside from the types of surfactants, the final morphology was also influenced by various reaction conditions such as precursor concentration and temperature. An increase in the precursor concentration prompted the formation of larger particles due to the greater availability of ionic building blocks to participate in growth of C-S-H. This also resulted in a greater proportion of twins, triplet and multiplet particles with poorly defined morphologies. On the other hand, when the precursor concentration was low, the as-formed C-S-H seed particles assembled to form dendritic structures instead of serving as the cubic seeds for nucleation and semiepitaxial growth of C-S-H. This hypothesis was verified by the observable cubic subunits, which constitute tails and edges of the dendritic structures under SEM.

The types of counterions within the calcium source also affected the final morphology of C-S-H. The use of calcium nitrate yielded the best results in terms of the cubic/rhombohedral morphology. However, the use of calcium chloride and calcium oxide resulted in C-S-H with poorly defined irregular shapes and crumpled sheets, respectively.

Overall, this sonication-assisted, in situ seed-mediated pathway mapped out the complex, morphology-oriented synthesis of semicrystalline C-S-H using four reaction parameters, encompassing the initial Ca/Si ratio, types of counterions within a calcium source, types of surfactants and the mixing method (**Figure 3**).

The authors then verified the morphology-induced enhancement in mechanical properties based on the combination of nanoindentation technique and compressive testing. Although mechanical properties of synthetic C-S-H have been linked to its final Ca/Si molar ratio or silicate polymerization before, the authors herein proved for the first time the shape-dependent mechanics from the scale of a single particle to assembled states [29, 30]. Compared to the previous reports, where the mechanics of C-S-H had been often evaluated using compacted samples, the authors devised a de novo matrix-based strategy to probe the mechanics of individual C-S-H particles first. The results showed that the individual cubic particles exhibit approximately 650 and 300% increase in hardness and stiffness, respectively, compared to the control C-S-H with irregular morphology. Nanoindentation was also performed on a pressure-induced sample using a (10 × 10) grid, thereby exhibiting ~83 and ~30% increase in the average values of hardness and elastic modulus for cubic samples compared to the control samples, which consisted of irregular C-S-H compacted under external pressure. Furthermore, the compressive toughness and ductility of the cubic samples were ~300 and 77% higher compared to the control samples.

**Figure 3.** Morphology map as a function of surfactant concentration and the initial Ca/Si molar ratio [27].
