Grain hydrating cement

C-S-H gel of the similar morphology to the outer product, occurring as pseudo-morphoses of cement grains hydrated entirely,... 

The mechanism by which the spines grow is fascinating (Fig. 20.4). The initial envelope of hydrate on the cement grains, which gave setting, also acts as a semi-... 

The admixture may react with cement constituents to precipitate insoluble products, and these may form slightly permeable films or coatings on the cement grains, acting as protective barriers with respect to further hydration. 

Figure 1. Schematic of hydrating cement grains, illustrating growth of C-S-H gel around them ana construction of interconnected, water-filled interstices. Matrix is enlarged relative to scale.

Fig. 7.2 Backscattered electron image of a mature Portland cement paste, aged 2 months. Successively darker areas are of unreacted cement grains (bright), sometimes with visible rims of hydration products, CafOH), other ( undesignated ) regions of hydration products, and pores (black). Scrivener and Pratt (S28).

CH can be observed as areas darker than the unreacted clinker phases but brighter than the other hydration products. As in calcium silicate pastes, these appear to have grown in regions initially occupied by water. Although the areas appear discrete on two-dimensional sections, they are not necessarily so in the three-dimensional material. They can engulf small cement grains. 

The first study using a wet cell, made at high w/c ratios, showed tubular growths radiating from the cement grains, which were considered to have formed by a silicate garden mechanism (D14). Later work showed that they were rich in calcium, aluminium and sulphur, and that they did not form if CjS was substituted for cement (BlOl). They have not been observed in the more recent studies made at normal w/c ratios, and do not appear to be a significant feature of normal cement hydration. 

Adsorption of the admixture on the hydrating cement grains could decrease flocculation in at least three ways (D44). The first is an increase in the magnitude of the ( -potential if all the particles carry a surface charge of the same sign and sufficient magnitude, they will repel each other. The second is an increase in solid- liquid affinity if the particles are more strongly attracted to the liquid than to each other, they will tend to disperse. The third is steric hindrance the oriented adsorption of a non-ionic polymer can weaken the attraction between solid particles. 

H53.H62). The properties were attributed to a combination of effects. The particles of microsilica, being much finer than those of the cement, partially fill the spaces between the cement grains, and this, together with the superplasticizer, allows the latter to pack more uniformly. They also provide nucleation sites for hydration products, undergo pozzolanic reaction and probably improve the paste aggregate bond. 

When acrylic polymers are added into a cement mortar mix, their spheres coalesce to form a continuous polymer matrix which coats the hydrating cement grains and aggregate. This polymer matrix acts as a barrier which helps to improve the hydration of the cement and also provides a polymeric network which increases the toughness and durability of the finished product. 

In all cases, the operations for preparing PCC are similar to those used for common concrete and mortar. Curing, however, is different. After only 1 day of moist cure, the surface of PCC work can be uncovered. The film already formed on the surface retains the necessary amount of moisture for the full hydration of the cement. In principle, after only a few days of air cure at around the normal ambient temperature (20°C), the PCC can be put in service. The polymer latex used for making a PCC must be able to form a film under ambient conditions, coat cement grains and aggregate particles, and form a strong bond between the cement particles and aggregates. 

Fig. 3.12 The scheme of cement hydration (according to [20]) (a) cement grains in water, (b) gel layer formed on cement grains, (c) break-up of gel layer under the osmotic pressure and the growth of secondary C-S-H gel, (d) filling of pores in the paste after longer period of time, caused by gel formation...

The reaction of clinker phases with water, together with the crystalUzation of hydrates, brings about the substantial changes of rheological properties of the paste. The formation of hydrated phases and, according to Locher [28], the morphology of hydrates crystallized in the liquid phase or on the surface of cement grains, leads to the reduction of distance between them and the viscosity of paste is enhanced. At... 

The analysis of the rheological properties of cement pastes is complicated additionally by the interaction of solid and liquid phase. In Table 5.1 the approxiniate size of hydrating cement grains and the distances attributed to the particular types of interactions are shown according to Massazza [21],... 

The application of classic double layer model to the cement pastes is questionable because the surface in this case is not in thermodynamic equilibrium. The surface of cement grains reacts continuously with water and, as a result, the releasing of different ions into the liquid phase occurs and the surface charge varies all the time. Therefore opposite to the classic double layer its irmer part changes continuously. For this reason appeared the concept to replace the classic potential by the dynamic potential, which is changing continuously dining the hydration process [26]. However, the potential of hydrating cement is often measured and an example of these measurements results is shown in Fig. 5.18 [27]. 

Illston [134] found a good relation between the maximum continuous pore radius, measured with mercury porosimeter and permeability (Fig. 5.61). The maximum continuous pores radius is defined as a pore size at which the maximum on the pore distribution curve occurs (see Fig. 5.27). This maximum radius of continuous pore system decreases with the time of hydration, as a space between cement grains is filled with hydration products. The term continuous pores was firstly used by Winslow and Diamond [135] they attributed it to the maximum on the pores size distribution curve, as it could be derived from the flow of mercuiy through the main, continuous pore chaimels. However below the peak mercuiy is intmding only to the local pore channels [135]. Mehta and Manmohan [136] are considering... 

Outside this duplex film there is a porous zone where the following components are observed primary—coarse and secondary— fine crystalline CatOH), C-S-H particles and hydrated cement grains the latter ones are present often in the forms... 

Fig. 6.15 Effect of silica fume addition on the construction of paste-aggregate transition zone eg—cement grains,. sf silica fume, (a, b) c, d hydration progress. (According to [23])...

The soluble alkali content in cement is of great importance for the aggregate expansion reaction. Sodium and potassium occur in cement in the form of sulphates and form the solid solutions in CjA and C2S (see Sect. 2.5.5.3). Sodium and potassium sulphates are very quickly dissolved. Alkalis from C3A solid solution are easily soluble too, while those from CjS are practically inactive, because of the slow CjS hydration. The release of alkalis is also affected by distribution of particular phases in the polymineral cement grain one can imagine C3A encapsulated in brownmil-lerite coating its reaction with water will be retarded. 

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