Cement-water reactions

Calcium phosphate-based systems have wide applications in biomedical areas. Brown has outlined the similarities between the hydration of calcium silicates and calcium phosphates. The hydration products in both systems have high surface areas, variable composition, and poor crystallinity. Pozzolanic reactions and Hadley-like grains form in both systems. The primary cement-water reactions for C3S and tetracalcium phosphate are as follows ... 

Calcium aluminate cements (CAC) have a wide range of alumina content (38 to 90%). The chemistry and principle cement-water reactions for high alumina cement—a widely used non-portland eement—are described in detail in Ch. 9. The primary binding phase is calcium monoaluminate (CaAl204 or CA). Refractory cements contain higher alumina contents (70 to 90%). 

In AB cements water does not merely act as solvent for the setting reaction. It also acts as an important component of the set cement. For example. 

The cement-forming reaction will be similar to glass polyalkenoate cement. The cement matrix will consist of metal polyacrylates, but boric acid will be produced instead of silica gel. Since boric acid has a water solubility oil-1 % compared with the near insolubility of silica gel, it would... 

Crisp et al. (1978) were able to follow the course of the cement-forming reaction using infrared spectroscopy and to confirm previous observations. They found that the technique could be used to distinguish between crystalline and amorphous phases of the cement. Hopeite shows a number of bands between 1105 and 1000 cm this multiplicity has been explained by postulating a distortion of the tetrahedral orthophosphate anion. (Two-thirds of the zinc ions are tetrahedrally coordinated to four phosphate ions, and the remainder are octahedrally coordinated to two phosphate and four water ligands.)... 

Vashkevitch Sychev (1982) have identified the main reaction product of the cement-forming reaction between copper(II) oxide and phosphoric acid as Cu3(P04)2. SHjO. The addition of polymers - poly(vinyl acetate) and latex - was found to inhibit the reaction and to reduce the compressive strength of these cements. However, impact strength and water resistance were improved. 

The CHgO-Zn coordinate bond in the zinc eugenolate chelate is very weak (Gerner et ah, 1966) and the chelate has poor stability thus, the cement-forming reaction [9.1] can be reversed. This occurs when the cement is placed in water, when the matrix is easily hydrolysed to eugenol and zinc hydroxide (Figure 9.2a) (Wilson, 1978 Wilson Batchelor,... 

Water quality standards, 21 581-583 Water reactions, supercritical, 24 16-17 Water-reducers, for cement, 5 485 Water-reduction strategies, in papermaking, 16 126-127 Water removal... 

Modem concretes often incorporate a mixture of chemical and mineral admixtures, each of which may interact with the various constituents of cements and influence cement hydration reactions. The admixture-cement interactions may in fact be viewed as the reaction between two complex chemical systems - the multicomponent, multiphasic inorganic materials in the cement and the organic compounds of multicomponent admixture systems. For example, lignosulfonate water-reducers are intrinsically complex mixtures of chemical compounds derived from the chemical degradation of lignin, while synthetic admixtures such as superplasticizers contain species with a broad distribution of molecular weights, reaction products, or other chemicals added for a specific purpose [125]. The performance of an admixture in concrete is highly dependent on many... 

Abstract. Polyvinylalcohol (PVA) is a polymer soluble in hot water, it has the property of film formation and it can improve the concrete performance. The effects of PVA modified with nano clay on the cement hydration reaction have been investigated by means of semiadiabatic calorimeter, FTIR spectroscopy and SEM. FTIR spectroscopy was employed to monitor chemical transformation of cement. The morphology of the different samples was compared by means of SEM micrographs. With the semiadiabatic calorimeter the hydration kinetic was measured to compare the heat rate of the admixtures materials. Fixing the water-cement ratio, w/c, in 0,45, the ratio of polymer to cement (p/c) was 2 wt% and the ratio of clay to polymer was 4 wt% (0.8wt.% related to cement). The polymer and modified polymer admixtures produced a retardation effect on the kinetic of cement hydration, but the clay acts as nucleating agent. The increase of the temperature with time was measured and a new model with four parameters was employed and the kinetic parameters were determined for each sample. 

Contraction depends on the phase composition of cement. CjA has the highest contraction and C2S— the lowest one. Contraction can be calculated from the molecirlar mass and densities of the substrates and of hydration products of cement corrstituents reactions with water. At simplified assumption that the tobermorite C3S2H3, with derrsity 2.44 g/cm is the product of C3S and C2S reaction with water we have ... 

At early age of cement paste reaction with water the plastic shrinkage occurs, which consists in evaporation of water from the surface layers of concrete [108]. The volume of fresh concrete is thus reduced. This process is not related to the properties of cement itself however it can also result in the formation of cracks. This situation is the consequence of substantial humidity gradient in concrete. The plastic shrinkage, equal 2.5 mm/m, observed in the 7 cm thick surface layer of concrete, was two times higher than the shrinkage in the core of element [108]. 

When Portland cement is added to water, reactions take place to form calcium silicate hydrates and release calcium hydroxide. These hydrates act as the glue in mortars and concretes to hold the matrix together. 

Both natural and artificial pozzolans can be used to make cements by reaction with lime in the presence of water. 

Concrete consists of a mixture of cement, water and coarse and fine aggregates. During the reaction between the cement and the added water, the concrete hardens and gains strength. The chemical reactions between cement and water - the so-called hydration - are marked by considerable heat development. If concrete hardens under adiabatic conditions, i.e. without exchanging heat with its surroundings, the concrete temperature will typically increase by 50-70 °C due to the heat developed. 

Cement hydration Reaction of the anhydrous cement phases with water to form cement hydrates. The reaction is a dissolution-precipitation process involving both the participation and the mediation of an aqueous phase. 

The first equation is an example of hydrolysis and is commonly referred to as chemical precipitation. The separation is effective because of the differences in solubiUty products of the copper(II) and iron(III) hydroxides. The second equation is known as reductive precipitation and is an example of an electrochemical reaction. The use of more electropositive metals to effect reductive precipitation is known as cementation. Precipitation is used to separate impurities from a metal in solution such as iron from copper (eq. 1), or it can be used to remove the primary metal, copper, from solution (eq. 2). Precipitation is commonly practiced for the separation of small quantities of metals from large volumes of water, such as from industrial waste processes. 

Barium carbonate prevents formation of scum and efflorescence in brick, tile, masonry cement, terra cotta, and sewer pipe by insolubilizing the soluble sulfates contained in many of the otherwise unsuitable clays. At the same time, it aids other deflocculants by precipitating calcium and magnesium as the carbonates. This reaction is relatively slow and normally requites several days to mature even when very fine powder is used. Consequentiy, often a barium carbonate emulsion in water is prepared with carbonic acid to further increase the solubiUty and speed the reaction. 

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