Cement gel, 3.28

It is interesting that this cement has been known for over 100 years and yet certain features of its chemistry remain obscure. The exact nature of the matrix is still a matter for conjecture. It is known that the principal phase is amorphous, as a result of the presence of aluminium in the liquid. It is also known that after a lapse of time, crystallites sometimes form on the surface of the cement. A cement gel may be likened to a glass and this process of crystallization could be likened to the devitrification of a glass. Therefore, it is reasonable to suppose that the gel matrix is a zinc aluminophosphate and that entry of aluminium into the zinc phosphate matrix causes disorder and prevents crystallization. It is not so easy to accept the alternative explanation that there are two amorphous phases, one of aluminium phosphate and the other of zinc phosphate. This is because it is difficult to see how aluminium could act in this case to prevent zinc phosphate from crystallizing. 

Concrete is a composite material composed of cement paste with interspersed coarse and fine aggregates. Cement paste is a porous material with pore sizes ranging from nanometers to micrometers in size. The large pores are known as capillary pores and the smaller pores are gel pores (i.e., pores within the hydrated cement gel). These pores contain water and within the water are a wide variety of dissolved ions. The most common pore solution ions are OH", K+ and Na+ with minor amounts of S042" and Ca2+. The microstructure of the cement paste is a controlling factor for durable concrete under set environmental exposure conditions. 

Mixtures of cement gel and unhydrated cement pa rtides enveloped with a close-packed layer of polymer partides... 

These results show that the Al and Fe present in the cement gel do I not replace Si, except perhaps to a minor extent. This conclusion is reinforced by most of the results of the silicate anion studies described in Section 7.2.6. If there was any significant replacement of silicon at a crystal chemical level, the average anion size and Si04 connectivity would both be. reduced. Further evidence of a similar nature is provided by the observation that the average anion size in C-S-H preparations made in suspension is not significantly affected by the presence of AF (S73). The silicate anion evidence also makes it appear unlikely that there is significant replacement of silicon by sulphur it is, in any case, improbable that such replacement would occur in a dimeric or chain anion, as a Si-O-S link would be readily hydrolysed. 

Sulphate present in the gel could be accommodated as interlayer anions associated with AFm layers. C-S-H can, however, take up CaS04 in the absence of AH or Fe, and thus of AFm layers. Odler (017) showed that CaS04 thus bound was very readily removed, and suggested that the Ca and S04 ions were adsorbed. The and Na ions that are found in the cement gel by X-ray microanalysis are partly the result of deposition from the pore solution when the latter evaporates during specimen preparation or in the high vacuum of the instrument, and these, too, with equivalent amounts of OH , may be adsorbed after deducting the fractions deposited from the pore solution, the K/Ca and Na/Ca ratios in the gel are typically 0.01 and <0.01, respectively (Section 7.5.2). 

For a Portland cement with 0.2% total NUjO and 0.6% total K O, at w/c = 0.5 and an age of 1 year, calculation by the above method indicated that 59% of the Na released and 48% of the released were taken up by the products. The ratios of alkali cations to Ca in the products are sufficiently low (typically < 0.01 for Na and 0.01 for K ) as to suggest that the ions, with an equivalent quantity of OH, are adsorbed. As Glasser and Marr (G63) concluded earlier, Na appears to be more strongly held than K. As was noted in Section 7.3.2, the ratios of alkali cations to found in the cement gel by X-ray microanalysis are higher than those in the solid products, because they include the materials dissolved in the pore solution, which are deposited when the latter evaporates. 

In this model, hep is assumed in the general case to comprise three components from the volumetric standpoint, viz. (a) unreacted cement, (b) hydration product and (c) capillary pores. Individual solid phases are not considered, whether in the cement or in its hydration products, which were collectively called cement gel . This term may be found confusing, because it includes the CH, which forms relatively large crystals and cannot reasonably be considered part of a gel we shall substitute the term hydration product . The water present in the paste was categorized as evaporable or non-cvaporable, the latter being defined in the later work (P34) as that retained on D-drying. Evaporable water, when present, was considered to reside partly in the capillary pores, and partly in so-called gel pores within the hydration product. This latter part was called gel water. 

The space occupied by the concrete should, as much as possible, be filled with solid aggregate and cement gel free from honeycombing. Compactness may be the primary criteria for those types of concrete which intercept nuclear radiation. 

Note B/C = Bentonite Cement Gel = Silicate Basalt Gel Percentages are by volume of soil treated... 

This is not to say that more highly ionized polymers, such as conventional polyelectrolytes, are not of technological importance and interest. In fact, just the opposite is true. Polyelectrolytes have historically been utilized as ion-exchange resins, but a number of novel applications such as cements, gels, and encapsulation membranes are under development. Several applications of these materials, such as polyelectrolyte complexes and ionic biopolymers, are also included in this review. 

It is believed that a cx)-matrix phase which consists of cement gel and polymer films is generally formed as a binder according to a three-step simplified mcxlel shown in Fig. Grosskurth proposed a similar... 

First Step. When polymer latexes are mixed with fresh cement mortar or concrete, the polymer partides are uniformly dispersed in the cement paste phase. In this polymer-cement paste, flie cement gel is gradually formed by the cement hydration and the water phase is saturated with calcium hydroxide formed during the hydration, whereas the polymer partides dqrosit partially on the surfaces of the cement-gel-unhydrated-cement partide mixtures. It is likely that the calcium hydroxide in the water phase reacts witit a silica surface of the aggre tes to form a calcium silicate layer.I It is confirmed that tire formation of the calcium hydroxide and ettringite in the contad zone between tire cement hydrates and aggregates is attributed to the bond between them.I lPl... 

Latex-modified mortar and concrete are made by using a composite binder of inorganic cements and organic polymer latexes, and have a network structure which consists of cement gels and microfilms of polymers. Consequently, the properties of the latex-modified mortar and concrete are markedly improved over conventional cement mortar and concrete. The properties of the fresh and hardened mortar and concrete are affected by a multiplicity of factors such as polymer type, polymer-cement ratio, water-cement ratio, air content, and curing conditions. 

The detailed studies of the stracture of cement paste and its influence on the physical properties we owe to Powers [31, 36]. According to his assumption, the cement gel , it means the C-S-H phase composed of colloidal particles, is a basic binding component of the paste. This colloidal status of C-S-H is proved by an... 

Based on this model. Powers related the strength of paste with the cement gel content. The C-S-H share was determined by Powers as a ratio ... 

Lea [39] is accepting the hypothesis that carbonation shrinkage can be explained by decrease of the non-evaporable water content in cement gel. However, Powers (according to Lea [3 9]) attributed this shrinkage to the dissolution of Ca(OH)2 in the zone of shrinkage stress caused by drying, and precipitation of calcium carbonate in the zones of lower stress. 

Neither a model of creep, nor the mechanism of this process is known as well. Obviously, the creep relates to the properties of cement gel and the presence of adsorbed water. The creep growth with moisture content in a paste, while the dried samples do not show this effect [127]. However, this latter statement is not quite sure perhaps only a substantial reduction of creep occurs [128]. The hypotheses assume the squeezing of water from the paste under load. 

The improvements in properties noted may be linked to several factors the reduction made possible in the water/cement ratio, a reduction in gross porosity, the entrainment of air as a fine dispersion, and specific interactions or bonds with the cement gel. Using electron microscopy, Wagner (1965, 1966, 1967) and Aignesberger et al (1969) have shown that melamine and vinyl resins form networks that interpenetrate the network formed by the cement gel. Also, using similar techniques, Pierzchala (1969) concluded that poly(vinyl acetate) effectively becomes a constituent of the cement gel, and makes it chemically resistant, even to hydrofluoric acid. 

This equation shows that the strength of a gel can be high enough for stractural applications if the particle size is small enough. For example, a cement gel with particle size of lOnm and a work of adhesion of 0.1 would have a... 

---