Cements hydration

The expansive component C A SI in Type K expansive cements hydrates in the presence of excess sulfate and lime to form ettringite is... 

The ionic bond is the most obvious sort of electrostatic attraction between positive and negative charges. It is typified by cohesion in sodium chloride. Other alkali halides (such as lithium fluoride), oxides (magnesia, alumina) and components of cement (hydrated carbonates and oxides) are wholly or partly held together by ionic bonds. 

When the water is added to the final dry cement material, the hydration of the cement begins immediately. The water is combined chemically with the cement material to eventually form a new immobile solid. As the cement hydrates, it will bond to the surrounding surfaces. This cement bonding is complex and depends on the type of surface to be cemented. Cement bonds to rock by a process of crystal growth. Cement bonds to the outside of a casing by filling in the pit spaces in the casing body [163]. 

For example, when cementing a casing string run to shallow depth or when setting a directional drilling kick-off plug, it is necessary to accelerate the cement hydration so that the waiting period will be minimized. The most commonly... 

When mature concrete is contaminated by chloride, e.g. by contact with deicing salts, the cement chemistry is more complex, and less chloride is taken up by the cement hydrate minerals and a larger proportion is free in the pore solutions and can therefore pose a greater hazard. When embedded steel corrodes, the production of a more voluminous corrosion product pushes the concrete from the steel with resultant cracking and spalling of the concrete. 

To allow sufficient water to be available for cement hydration... 

A. N. Scian, Lopez. J. M. Porto, and E. Pereira. Mechanochemical activation of high alumina cements— hydration behavior Pt 1. Cement Concrete Res, 21(l) 51-60, January 1991. 

For solid wastes to be suitable as a full or partial replacement for components in other applications, it should be free of objectionable material such as wood, garbage, and metal that can be introduced at the foundry. It should be free of foreign material and thick coatings of burnt carbon, binders, and mold additives that could inhibit product manufacture, such as cement hydration. It may be necessary to crush the solid waste to reduce the size of oversized core butts or unclasped molds. Magnetic separation is a good solution to producing a suitable coarse or fine aggregate product. 

Gypsum, 4 582-601 5 467, 785t 23 576 forms and composition, 4 583t hardness in various scales, 7 3t in Portland cement, 5 467 in Portland cement hydration, 5 477t thermal reduction of, 23 577 thermodynamics and kinetics of formation- decomposition, 4 586-588 Gypsum board, 4 600-601 Gypsum processes, obtaining sulfur from, 23 576-577... 

The rheological characteristics of cement pastes are related to the nature of the attractive and repulsive forces which exist between cement and cement hydration product particles and can be categorized as follows ... 

The various types of water-reducing admixtures possess different but characteristic adsorption isotherms which qualitatively reflect their effect on cement hydration kinetics, as shown in Fig. 1.17. 

In the absence of knowledge of the surface area of cement hydrates available for adsorption at the time of addition, it is difficult to estimate how many layers of water-reducing admixture molecules are adsorbed, but attempts have been made [40] indicating that over 100 layers may be formed with calcium lignosulfonate and salicylic acid at normal levels of addition. However, these calculations were based on specific surface areas of 0.3-1.0 m g-l, whereas other studies [27, 38, 39] have indicated... 

The addition of a water-reducing admixture to a cement suspension can be shown to disperse the agglomerates of cement particles into smaller particles [33,38, 47] and can be seen clearly in photomicrographs as shown in Fig. 1.21. Maximum dispersion occurs at a level of 0.3-0.5% by weight of calcium lignosulfonate [33, 34] which would indicate the presence at the surface of about 0.2-0.4% calcium lignosulfonate. The separation of particles results in an increase in the surface area of the system by 30-40% [33, 38], which may explain the more rapid rate of cement hydration after the initial retardation period. 

The higher the molecular weight of SNF, the greater the retardation of cement hydration. 

Fig. 2.14 Conduction calorimetric curves for Portland cement hydrated in the presence of SMF.

There is some retardation of cement hydration but at 28 days the products of C3S hydration are essentially the same as in an unsuperplasticized cement system. The C3A/gypsum reaction products may be changed morphologically to a cubic rather than a hexagonal form. 

There is little published data on the effect of air-entraining agents on the chemistry and morphology of cement hydration. However, the limited studies [15] indicate that the normal hydration pattern under isothermal conditions for ordinary Portland cement shown in Fig. 3.14 is modified as follows ... 

The chemical materials used to produce dampproofers are able to form a thin hydrophobic layer within the pores and voids and on the surfaces of the concrete in one of three ways (1) reaction with cement hydration products ... 

Materials which react with cement hydration products... 

Reaction with cement hydration Stearic acid C17H35COOH ... 

Fig. 4.4 The influence of sodium oleate on cement hydration (Edwards).

There are no recorded data to indicate that materials of this type would alter the stiffness of the concrete into which they are incorporated. However, the fact that these materials are associated with the matrix/air interface, and not the cement hydrates themselves, would suggest that the physical properties of the bonding constituents of the hardened cement would remain unchanged. 

Conduction calorimetric curves of Portland cement hydrated isothermally containing various quantities of triethanolamine are shown in Fig. 5.3 [8]. On initial contact with water each sample evolves heat (not shown in figure) that can be attributed to heat of wetting, hydration of free lime and reaction of C3 A with gypsum to form... 

The drying shrinkage of concrete containing calcium chloride is increased in comparison to plain concrete, even though the amount of moisture lost is less [22]. This is illustrated in Fig. 5.37 and it is thought that the reduced moisture loss will be due to the more advanced state of hydration in the specimens containing calcium chloride. The increased shrinkage must, therefore, be a characteristic of the type of cement hydration products formed. Under saturated conditions, such as total water immersion, the amount of expansion of the concrete is reduced when calcium chloride is present. 

The mechanism of the inhibitive action of LiOH proposed by Stark et al. [7] is attributed to the formation of lithium silicate that dissolves at the surface of the aggregate without causing swelling [7], In the presence of KOH and NaOH the gel product incorporates Li ions and the amount of Li in this gel increases with its concentration. The threshold level of Na Li is 1 0.67 to 1 1 molar ratio at which expansion due to alkali-silica reaction is reduced to safe levels. Some workers [22] have found that when LiOH is added to mortar much more lithium is taken up by the cement hydration products than Na or K. This would indicate that small amounts of lithium are not very effective. It can therefore be concluded that a critical amount of lithium is needed to overcome the combined concentrations of KOH and NaOH to eliminate the expansive effect and that the product formed with Li is non-expansive. 

Cement hydration and epoxy polymerization occur simultaneously to form a structure that is similar to the latex-modified cementitious system. Epoxy systems develop high strength, adhesion and have low permeability, good water resistance and chemical resistance. A major advantage of this system is that it can be cured under moist or wet conditions. According to a recent study, the epoxy-modified mortars can be made without the hardeners with superior properties to those obtained with conventional epoxy mortars [89, 90]. 

The properties of a latex depend on the nature of polymers in the latex, particularly the monomer ratio in copolymers and the type and amount of plasticizers. The monomer ratio affects the strengths of the latex modified mortars to the same extent as the polymer-cement ratio [87, 92]. Mechanical and chemical stability, bubbling and coalescence on drying all depend on the type and amount of surfactants and antifoamers and the size of dispersed polymer particles. It is important that the use of selected antifoamers and surfactants as stabilizers or emulsifiers produces no adverse effect on cement hydration. 

Surfactants enable the polymer particles to disperse effectively without coagulation in the mortar and concrete. Thus, mechanical and chemical stabilities of latexes are improved with an increase in the content of the surfactants selected as stabilizers. An excess of surfactant, however, may have an adverse effect on the strength because of the reduced latex film strength, the delayed cement hydration and excess air entrainment. Consequently, the latexes used as cement modifiers should have an optimum surfactant content (from 5 to 30% of the weight of total solids) to provide adequate strength. Suitable antifoamers are usually added to the latexes to prevent excess air entrainment increased dosages causes a drastic reduction in the air content and a concurrent increase in compressive strength [87, 92-94]. 

Have ability to form continuous films in mortar or concrete, due to a lower minimum fihn-forming temperature than the application temperature, which adhere well to the aggregates and cement hydrates and possess good alkali and water resistance. 

Modification of mortar and concrete in the presence of a latex occurs by concurrent cement hydration and formation of a polymer film (coalescence of polymer particles and the polymerization of monomers). Cement... 

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