**2. Morphology of synthetic C-S-H**

Synthetic C-S-H exhibits varying morphologies depending on types of synthesis techniques and experimental factors encompassing the initial Ca/Si ratio and the types of precursors [13, 14]. Common reaction pathways encompass silica-lime reactions, where lime or hydrated lime is reacted with pozzolans such as silica fume and double decomposition technique, where both calcium nitrate and sodium silicate become decomposed into their constituent ions, the ionic building blocks for C-S-H [14–16].

the "sheaf of wheat" morphology for the reaction between the alkali-silicate gel and supersaturated solution of calcium hydroxide (**Figure 2**). This implies that the unique "sheaf of wheat" morphology can be materialized by mimicking the natural alkali-silica reaction described above.

Hydrothermal synthesis is a common technique used to grow crystalline calcium silicate

tion. Hara et al. synthesized lath-shaped crystals of jennite, with the width of around 1 μm elongated along b-axis based on hydrothermal reactions of fumed silica and lime at 80°C [13]. During the hydrothermal treatment, addition of metal ions such as sodium and aluminum ions, which are commonly present in supplementary cementitious materials including slag and fly ash, also influences the final type and morphology of crystalline calcium silicate hydrate [21]. Nocuń-Wczelik et al. performed the hydrothermal synthesis using the mixture of metal hydroxide, various powder forms of silica and hydrated lime at the temperature range of 160–240°C. It was shown that sodium and silica content exceeding 20 and 50 wt%, respectively, favor the formation of pectrolyte, the sodium-bearing crystalline product, with broom-like morphology. The addition of aluminum ions to the initial mixture comprising calcium hydroxide, silica and sodium hydroxide facilitated the transformation of amorphous C-S-H to crystalline tobermorite, accompanying the morphological change from the interlocked fibers to plate-like morphology. Furthermore, needle-like xonotlite crystals were formed when the initial Ca/Si ratio was set close to 1 during the hydrothermal synthesis.

Tobermorite, the most commonly referred material for the crystalline analogue of amorphous C-S-H, typically has a basal spacing of 1.1 and 1.4 nm. It can be readily synthesized via the

observed in hydrothermally cured concretes (tobermorite synthesis under hydrothermal conditions) [22]. Bell et al. performed the hydrothermal treatment of the mixture containing lime and high-purity quartz at the Ca/Si ratio of 0.83 and the pH of 12.6 at 150°C [22]. The reaction led to the formation of two distinct morphologies for tobermorite, platelets and fibers, with the former possibly induced by the heterogenous nucleation and the latter stemming from the

Galvánková et al. studied the effect of different experimental conditions on the formation of tobermorite [23]. Hydrothermal synthesis was performed using the mixture of silica source and grounded limestone, which had been preheated, at the temperature range between 170 and 190°C.Acicular crystals of tobermorite were observed when silica sand was used as the precursor and the reaction temperature beyond 180°C favored the conversion of tobermorite to xonolite.

Hartmann et al. also investigated the effect of the additive Ca-formate on the morphology of crystalline CSH during the hydrothermal reactions [24]. The authors hydrothermally treated the mixture of quartz, lime and calcium formate at 200°C for 40.5 hours and investigated the effect of varying amounts of calcium formate on final morphology of the resultant CSH. The calcium-bearing additive, even with the lowest amount added, induced the morphological


O16(OH)2

·4H2

O, jennite Ca9

Morphogenesis of Cement Hydrate: From Natural C-S-H to Synthetic C-S-H

, the mineral analogues of amorphous C-S-H from cement hydra-

Si6

O system. 1.1 nm tobermorite is also often

O18(OH)6

http://dx.doi.org/10.5772/intechopen.77723

·8H2

O and

83

Si6

**3. Formation of crystalline CSH**

hydrate phase such as tobermorite Ca5

O17(OH)2

hydrothermal treatment of the ternary CaO-SiO2

homogenous nucleation.

Si6

xonotlite Ca6

Silica-lime reaction can be performed via either mechanochemical synthesis, where the mixture of silica and lime are reacted in solid state under the assistance of mechanical milling, or solution-based synthesis, where the precursors are reacted in the form of solutions [17–19]. Rodriguez et al. performed the mechanochemical synthesis using lime and fumed silica at different starting Ca/Si ratios, and also carried out the solution-based synthesis using preprepared slurries containing lime and fused silica separately [19]. The authors found that both mechanochemical and solution-based synthesis lead to the formation of foil-like C-S-H regardless of the initial Ca/Si ratio. In contrast, when the authors performed the controlled hydration of pure C3 S at a constant lime concentration, the initial Ca/Si ratio exerted a greater effect on the final morphology. As the Ca/Si ratio was increased from the value below 1.58 to the value above 1.58, the morphology transformed from a foil-like morphology to fiber-like status.

Kurtis et al. synthesized C-S-H via the alkali-silica reaction, where silica sources in the form of the alkali-silicate gel acquired from dam, silica fume and silica gel were reacted with saturated solution of calcium hydroxide. In case of the silica gel, another reaction was performed where it was also exposed to a separate solution of sodium hydroxide and calcium chloride [20]. The authors studied each reaction using high-resolution transmission soft X-ray microscopy and observed

**Figure 2.** X-ray image showing the "sheaf-of-wheat" morphology for C-S-H acquired from the alkali-silica reaction. Scale bar is 1 μm [36].

the "sheaf of wheat" morphology for the reaction between the alkali-silicate gel and supersaturated solution of calcium hydroxide (**Figure 2**). This implies that the unique "sheaf of wheat" morphology can be materialized by mimicking the natural alkali-silica reaction described above.
