Calcium aluminate hydrates formation

Spmng and Rechenberg [132] state that in the pastes of cements rich in alkalis C3A dissolves quickly because of the reduced Ca(OH)2 solubility. At low gypsum content the quick set can be observed as a result of the hexagonal calcium aluminate hydrates formation In the presence of gypsum excess the similar effect can be the consequence of ettringite formation. 

The reactions in the regulated-set cements containing Cjj A3CF2 (note mixed notation) as a principal phase resemble those in ordinary Portiand cements. Initial reaction rates are controlled by ettringite formation. Setting occurs with formation of the monosulfate, along with some transitory lower-limed calcium aluminate hydrates that convert to the monosulfate within a few hours. 

There is a significant number of metastable calcium aluminate lydrates. Then-identification is difficult because of a laige number of polymorphs and high susceptibility to the formation of caiboaluminates under the influence of CO2, although they occur as well ciystallized hydrates. In cement paste they can also form the nanometric mixtures with the C-S-H phase. Therefore it will be convenient to begin the discussion of calcium aluminate hydration from the presentation of the CaO-Al203-H20 system (Fig. 3.34). 

The orientation of CH crystals is disturbed when some irregularities occur, such as rough aggregate surface, as well as the formation of calcium aluminate hydrate... 

The volume of calcium aluminate hydrates formed in the hydration of CA is smaller than the sum of the volumes of the original CA and the water participating in the reaction. This chemical shrinkage for the hydration of monocalcium aluminate amounts to 16 vol.% in the formation of CAHj and to 25 vol.% if CjAHg and AHj are formed as products of hydration. 

In Portland cement, although the average C3A content is about 4-11%, it influences significantly the early reactions. The phenomenon offlash set, the formation of various calcium aluminate hydrates, and calcium carbo-and sulfoaluminates involve the reactions of C3A. Higher amounts of C3A in portland cement may pose durability problems. For example, a cement which is exposed to sulfate solutions should not contain more than 5% C3A. 

Particulates in the form of different calcium aluminate hydrates, e g., CAHio, C2AH8, and CjAHg, or pre-hydrated CAC/CH (H-CAC/CH) have varying effects on ettringite formation and expansion of expansive cement pastes. 

Mixtures containing different calcium aluminate hydrates can be prepared by hydrating ealcium aluminate cement (CAC) at different temperatures (Table 2). Either CAHjo, C2AHg, or CjAHg is the major component for each different mixture. The major component of the H-CAC/CH mixture also is C3 AHg. Addition of CHto CAC appeared to promote C3 AHg formation. The use of ealcium aluminate mixtures prepared from CAC hydration rather than pure S5mthetic calcium aluminates is based on practical considerations. 

Observation of the hydration process indicated the following. In the first few minutes of the dissolution of free lime, the hydration of anhydrite and hemihydrate and the formation of calcium aluminate hydrate and monosulfate hydrate occur. Ettringite is formed within one hour, monosulfate hydrate within 2-6 hours and C-S-H gel within 1-16 hours, with the maximum heat ofhydration of C3S at about 10 hours. Measurements of the non-evaporable water indicate that the amount of combined water is 60-80% of the theoretically determined total amount of combined water at complete hydration. (The total amount of combined water was estimated to be 36%.) The amount of ettringite in the paste is estimated to be about 18-25% for the period one hourto seven days. The monosulfate content increases from about 10% at six hours to about 15-25%inone day. The alite in Reg Set cement paste is approximately 65-70% hydrated in one day and 8 0-95% hydrated in seven days. It is suggested that fluoride is tied up as A1(0H)2F. The possibility of fluoride-substituted ettringite and the formation of halo-aluminate hydrates of the form ( A CaXj wHjO is conceivable. 

Other reactions taking place throughout the hardening period are substitution and addition reactions (29). Ferrite and sulfoferrite analogues of calcium monosulfoaluminate and ettringite form soHd solutions in which iron oxide substitutes continuously for the alumina. Reactions with the calcium sihcate hydrate result in the formation of additional substituted C—S—H gel at the expense of the crystalline aluminate, sulfate, and ferrite hydrate phases. 

The banning of calcium chloride during the last decade provided the impetus for the development of alternative materials which accelerated the hydration of cement without the potential for corrosion. A number of inorganic and organic compounds including aluminates, sulfates, formates, thiosulfates, nitrates, silicates, alkali hydroxides, carbonates, nitrites and calcium salts have been evaluated. Commercialization and field experience, however, is limited to only a few of these materials. 

The dry product is ground to a powder and then a little calcium sulfate (CaS04) is added to slow down the setting rate of the cement. When water is added to the mixture, slow complex chemical changes occur, resulting in the formation of a hard interlocking mass of crystals of hydrated calcium aluminate and silicate. 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

Mineral additions may be broadly categorized as pozzolanic materials or latent hydraulic cements. Neither type reacts significantly with water at ordinary temperatures in the absence of other substances. Pozzolanic materials are high in Si02 and often also in AI2O3, and low in CaO they are sufficiently reactive that mixtures of them with water and CaO produce C-S-H at ordinary temperatures and thereby act as hydraulic cements. If they contain AI2O3, calcium aluminate or aluminate silicate hydrates are also formed. Because they are low in CaO, this component must be supplied in stoichiometric quantity. In a composite cement, it is provided by the Portland cement through decreased formation of CH and decreased Ca/Si... 

Calcium aluminate cements harden rapidly as soon as the massive precipitation of hydrates begins. This may be attributed to the fact that, unlike those of Portland cement, the major hydration products are crystalline. Relatively high proportions of water are taken up in the hydration reactions, the theoretical w/c ratios needed for complete hydration of CA being 1.14, 0.63 and 0.46 for the formation of CAHj, CjAHg -F AHj and CjAH -F 2AHj, respectively. For this reason, and also because of the rapid heat... 

Various methods have been used to obtain cements that set and harden rapidly. They include the use of Portland cement with admixtures and of mixtures containing both Portland and calcium aluminate cements, described in Sections 11.5 and lO.I.IO, respectively. Another approach has been the manufacture of clinkers containing either CuAy CaF, or C 4A, S. both of which hydrate rapidly under appropriate conditions with the formation of ettringite. 

Cement binders are based on silicates, aluminates, aluminium silicates, aluminium ferrites of alkaline earth metals (mainly calcium). The hydration of these compounds or their mixtures with the formation of... 

Figure 19. Cement hydration process. Calcium silicate hydrates to form C-S-H, a quasi-amorphous gel of composition close to C3S2H3. The excessive rate of hydration of the aluminate phase is controlled by gypsum through the formation of a calcium trisidfoaluminate hydrate, ettringite. (Reproduced with permission from reference 5. Copyright 1991 Elsevier.)...

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