**3. Formation of crystalline CSH**

**Figure 2.** X-ray image showing the "sheaf-of-wheat" morphology for C-S-H acquired from the alkali-silica reaction.

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

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

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

Scale bar is 1 μm [36].

of pure C3

82 Cement Based Materials

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

ions, the ionic building blocks for C-S-H [14–16].

Hydrothermal synthesis is a common technique used to grow crystalline calcium silicate hydrate phase such as tobermorite Ca5 Si6 O16(OH)2 ·4H2 O, jennite Ca9 Si6 O18(OH)6 ·8H2 O and xonotlite Ca6 Si6 O17(OH)2 , the mineral analogues of amorphous C-S-H from cement hydration. 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 hydrothermal treatment of the ternary CaO-SiO2 -H2 O system. 1.1 nm tobermorite is also often 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 homogenous nucleation.

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 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 the observed morphological change.

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

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

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

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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 sub-

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

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

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

units, which constitute tails and edges of the dendritic structures under SEM.

poorly defined irregular shapes and crumpled sheets, respectively.

surfactants and the mixing method (**Figure 3**).

77% higher compared to the control samples.

sheets, respectively.
