Cement grains

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-... 

Fig. 20.3. The setting and hardening of Portland cement. At the start (a) cement grains ore mixed with water, H. After 15 minutes (b) the setting reaction gives a weak bond. Real strength comes with the hardening reaction ( ), which takes some days.

Obviously, too, the rate will depend on the total surface area of cement grains available for reaction, and thus on the fineness of the powder. So hardening is accelerated by raising the temperature, and by grinding the powder more finely. 

The ability to consistently produce HFSCC has been made possible by the development of a new type of VEA. Most viscosity-enhancing admixtures (VEA) are readily adsorbed on cement grains and often form bridges between the cement particles. Consequently, they strongly affect... 

The pivotal role that superplasticizers play in the formulation of self-leveling mortars is due to the dramatic effects they produce on flow behavior. Such effects are believed to be derived by the adsorption of the admixture on the surfaces of cement grains, thereby providing surfaces of a similar or zero charge which are mutually repulsive. They thus fully disperse cement particles, freeing more water for lubrication and reducing interparticle attraction. Both yield stress and plastic viscosity are decreased and the decrease is greater for yield stress it may be completely eliminated if sufficient admixture is added so that Newtonian behavior is observed (Fig. 7.25) [75, 76]. 

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.

The depth of reaction of the cement grains after a given time has been... 

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).

The brightest areas are of unreacted clinker phases. Individual cement grains are usually polymineralic and, as in a clinker (Section 4.3.1), the different phases within them can be distinguished by their differing grey levels or by X-ray microanalysis. 

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. 

Stubby rods of AFt phase are also seen (D25 D27). They are typically some 250 nm long and 100 nm thick. Studies using wet cells show them to occur both on the surfaces of the grains, and at some distance away (S41,S68) (Fig. 7.6b). They are probably more abundant near to the surfaces of the aluminate phase, and appear to nucleate in the solution and on the outer surface of a layer of gel. On drying, this layer shrinks, and the AFt crystals fall back onto the surfaces of the cement grains. The early products thus differ in morphology and composition from the exfoliating foils or honeycombs of C-S-H that have been observed in CjS pastes. 

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. 

It is widely agreed that water reduction is effected through improved dispersion of the cement grains in the mixing water flocculation is decreased or prevented, and the water otherwise immobilized within the floes is added... 

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. 

Ernsberger and France (E7) showed that addition of calcium lignosulpho-nate causes cement grains to develop a negative -potential. Daimon and... 

Retardation is probably caused by the formation of protective layers over the cement grains this has been demonstrated for lead (T54) and zinc (A27) salts and carbonates (U24). In the latter case, it was shown that the layer formed at low concentrations, which caused retardation, was more compact than that formed at higher concentrations, when acceleration occurred. 

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. 

Type 3 (vadose) Very fine to medium-tabular cemented grained clayey to silty sand unit... 

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