**1. Morphology of C-S-H observed during the hydration of PC or PC-based blend**

Before reviewing the morphology of naturally formed, semicrystalline C-S-H acquired during the hydration of PC, it is necessary to review its nucleation and growth mechanism during the hydration process. Despite the decadelong efforts, the complete picture for the mechanism of C-S-H formation during the hydration of cement is yet to be acquired, and several nucleation, growth and structural models have been proposed [1, 2]. The consensus is that the initial stage is comprised of dissolution of cement phases such as tricalcium silicate (C3 S) and dicalcium silicate (C2 S), releasing calcium, hydroxide, and silicate ions [3, 4]. Jennings et al. studied morphological development of hydrating C3 S, one of the major phases of cement, using the combination of multiple electron microscopic techniques, transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and scanning electron microscopy (SEM) [5]. The authors herein found that the morphology of C-S-H formed during the hydration of C3 S varies between the early, middle and the late stages of the cement hydration. At the early stages of the hydration, fibrous products were observed on the surface of the grains. During the middle stages, where the rapid exothermic reaction takes place, a mixture of different morphologies was observed. The complete layer of amorphous gel product along the boundaries of the C3 S particle was found while needles with the lengths of 0.75–1.0 μm radiating from the grain and tapered fibers with the length of 0.25–0.5 μm were also observed. In the late stages, the authors found that crumpled foils and dense inner products dominate.

formed within the larger grains. Richardson et al. further divided the outer product into two morphologically distinct types exhibiting different Ca/Si ratios. The higher Ca/Si ratio corresponded to the fibrillar, directional morphology, while the lower Ca/Si ratio corresponded

**Figure 1.** TEM image showing fine, dense morphology of the inner C-S-H and less dense, fibrillary morphology of the

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

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

81

S [9].

Taylor et al. also found that the age of the sample also significantly affects the morphology of C-S-H found in PC and PC-slag blends [10]. The authors compared the morphology of C-S-H existing in 20-year-old neat PC and PC-slag blend samples with those found in similar samples, which are only 14 months old [10]. The authors found that the outer product found in the 20-year-old neat PC sample is finer, showing little variation in morphology between the inner and the outer product in contrast to the samples, which are 14 months old. Furthermore, the morphology of the outer product C-S-H was different for the PC-slag samples containing different amounts of slag. As the amount of slag was increased from 0 to 90%, the fibrillar morphology was gradually transformed to foil-like morphology and only the crumpled foil

Alkali-silica reaction, which is the common reaction between reactive silica species and alkalis found in cementitious materials, also produces C-S-H with the unique "sheaf of wheat" morphology [11]. Zampini et al. studied the evolution of the wet cement paste-aggregate interface from 5 minutes to 10 days using environmental scanning electron microscopy (ESEM) and found that the C-S-H with a "sheaf of wheat" morphology was formed from the alkali-silica reaction [12]. The authors concluded that formation of this specific morphology is favored at water-to-cement ratio of 0.5, and also facilitated by the inclusion of silica fume. This in turn implies that the presence of silicate ions in a supersaturated solution of hydrated lime is criti-

to the foil-like morphology.

outer C-S-H formed during the hydration of C3

was observed for the slag-only paste at the end.

cal in inducing the aforesaid "sheaf of wheat" morphology.

The amorphous, gel-like layer found on the grain at the middle stage of C3 S in Jenning's study above support that the initial reaction products form via heterogenous nucleation and grow outward into water-filled pore solutions. Herein, the rates of nucleation and growth of C-S-H are heavily influenced by the degree of supersaturation of the constituent ions encompassing calcium and silicate ions [4]. It was later found via multiple studies that natural C-S-H found during the PC hydration can be divided into two types showing distinct morphologies. C-S-H gel that occupies the boundary region of the anhydrous cement grain is called an "inner product" and that forms in the originally water-filled pore spaces is called an "outer product." Those two types form at different stages of hydration and exhibit distinct morphologies [6, 7]. Richardson et al. found via TEM analysis that the outer product C-S-H has a fibrillar morphology and that the aspect ratio of the corresponding fibrils depends on the amount of available space in pore spaces [8, 9]. Coarse fibrils with the high aspect ratio were present in larger pore spaces and vice versa. On the other hand, the inner product exhibited a dense, homogeneous morphology with the significantly decreased porosity compared to the outer product (**Figure 1**). Furthermore, within each of the inner and outer type of C-S-H, the size of the anhydrous cement grain also affected the final morphological features [8]. The inner product formed within the smaller cement grains, with each grain being less than around 5 μm, exhibited less density and higher porosity compared to that Morphogenesis of Cement Hydrate: From Natural C-S-H to Synthetic C-S-H http://dx.doi.org/10.5772/intechopen.77723 81

**Keywords:** cement hydrate, shape-controlled synthesis, cubic cement, calcium silicate

Before reviewing the morphology of naturally formed, semicrystalline C-S-H acquired during the hydration of PC, it is necessary to review its nucleation and growth mechanism during the hydration process. Despite the decadelong efforts, the complete picture for the mechanism of C-S-H formation during the hydration of cement is yet to be acquired, and several nucleation, growth and structural models have been proposed [1, 2]. The consensus is that the initial stage

combination of multiple electron microscopic techniques, transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and scanning electron microscopy (SEM) [5]. The authors herein found that the morphology of C-S-H formed during the

At the early stages of the hydration, fibrous products were observed on the surface of the grains. During the middle stages, where the rapid exothermic reaction takes place, a mixture of different morphologies was observed. The complete layer of amorphous gel product along

radiating from the grain and tapered fibers with the length of 0.25–0.5 μm were also observed. In the late stages, the authors found that crumpled foils and dense inner products dominate.

study above support that the initial reaction products form via heterogenous nucleation and grow outward into water-filled pore solutions. Herein, the rates of nucleation and growth of C-S-H are heavily influenced by the degree of supersaturation of the constituent ions encompassing calcium and silicate ions [4]. It was later found via multiple studies that natural C-S-H found during the PC hydration can be divided into two types showing distinct morphologies. C-S-H gel that occupies the boundary region of the anhydrous cement grain is called an "inner product" and that forms in the originally water-filled pore spaces is called an "outer product." Those two types form at different stages of hydration and exhibit distinct morphologies [6, 7]. Richardson et al. found via TEM analysis that the outer product C-S-H has a fibrillar morphology and that the aspect ratio of the corresponding fibrils depends on the amount of available space in pore spaces [8, 9]. Coarse fibrils with the high aspect ratio were present in larger pore spaces and vice versa. On the other hand, the inner product exhibited a dense, homogeneous morphology with the significantly decreased porosity compared to the outer product (**Figure 1**). Furthermore, within each of the inner and outer type of C-S-H, the size of the anhydrous cement grain also affected the final morphological features [8]. The inner product formed within the smaller cement grains, with each grain being less than around 5 μm, exhibited less density and higher porosity compared to that

The amorphous, gel-like layer found on the grain at the middle stage of C3

S), releasing calcium, hydroxide, and silicate ions [3, 4]. Jennings et al. studied

S varies between the early, middle and the late stages of the cement hydration.

S particle was found while needles with the lengths of 0.75–1.0 μm

S) and dicalcium

S in Jenning's

S, one of the major phases of cement, using the

**1. Morphology of C-S-H observed during the hydration of PC or** 

is comprised of dissolution of cement phases such as tricalcium silicate (C3

morphological development of hydrating C3

hydrate

80 Cement Based Materials

**PC-based blend**

silicate (C2

hydration of C3

the boundaries of the C3

**Figure 1.** TEM image showing fine, dense morphology of the inner C-S-H and less dense, fibrillary morphology of the outer C-S-H formed during the hydration of C3 S [9].

formed within the larger grains. Richardson et al. further divided the outer product into two morphologically distinct types exhibiting different Ca/Si ratios. The higher Ca/Si ratio corresponded to the fibrillar, directional morphology, while the lower Ca/Si ratio corresponded to the foil-like morphology.

Taylor et al. also found that the age of the sample also significantly affects the morphology of C-S-H found in PC and PC-slag blends [10]. The authors compared the morphology of C-S-H existing in 20-year-old neat PC and PC-slag blend samples with those found in similar samples, which are only 14 months old [10]. The authors found that the outer product found in the 20-year-old neat PC sample is finer, showing little variation in morphology between the inner and the outer product in contrast to the samples, which are 14 months old. Furthermore, the morphology of the outer product C-S-H was different for the PC-slag samples containing different amounts of slag. As the amount of slag was increased from 0 to 90%, the fibrillar morphology was gradually transformed to foil-like morphology and only the crumpled foil was observed for the slag-only paste at the end.

Alkali-silica reaction, which is the common reaction between reactive silica species and alkalis found in cementitious materials, also produces C-S-H with the unique "sheaf of wheat" morphology [11]. Zampini et al. studied the evolution of the wet cement paste-aggregate interface from 5 minutes to 10 days using environmental scanning electron microscopy (ESEM) and found that the C-S-H with a "sheaf of wheat" morphology was formed from the alkali-silica reaction [12]. The authors concluded that formation of this specific morphology is favored at water-to-cement ratio of 0.5, and also facilitated by the inclusion of silica fume. This in turn implies that the presence of silicate ions in a supersaturated solution of hydrated lime is critical in inducing the aforesaid "sheaf of wheat" morphology.
