Calcium reaction with aluminate hydrates

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 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. 

Many cements used today are composites of Portland cement and industrial waste materials that can enter into the hydration reactions and contribute to the strength of the hardened product. These substances include pulverized fuel ash (PFA) from burning of pulverized coal in thermal power stations, crushed blast-furnace slag (Section 17.7), and natural or artificial pozzolanas—that is, volcanic ash and similar finely particulate siliceous or aluminosilicate materials that can react with the Ca(OH)2 in Portland cement to form hydrated calcium silicates and aluminates. As noted earlier, the solubility of Ca(OH)2 is such that the pH of pore water in Portland cements will be about 12.7, at which the Si-O-Si or Si-O-Al links in the solid pozzolanas will be attacked slowly by OH- to form discrete silicate and aluminate ions and thence hydrated calcium silicate or aluminate gels. 

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],... 

Reaction with silica and alumina. Hydrated lime reacts with pozzolans (materials containing reactive silica and alumina) in the presence of water to produce hydrated calcium silicates and aluminates. The reactions may take months to proceed to completion at ambient temperatures, as in mortars (section 26.6) and lime treated soil (section 26.3), but proceed within hours at elevated temperatures and water vapour pressures (e.g., in steam at 180 °C and a pressure of 10 bar — see sections 26.10,26.11 and 26.12). This pozzolanic reaction is the basis of the strength generated by hydraulic quicklimes (section 26.9). 

Stabilisation is a much slower process, which occurs progressively over several months, and involves the reaction of lime with the siliceous and aluminous components of the soil. The lime raises the pH to above 12, which results in the formation of calcium silicates and aluminates. These are believed to form initially as a gel, which coats the soil particles, and which subsequently crystallises as calcium silicate/aluminate hydrates. Those hydrates are cementitious products, similar in composition to those found in cement paste. The rate of crystallisation is temperature dependant and may take many months to reach completion. The resulting gain in strength (measured by the California Bearing Ratio Test [26.11]) is progressive, as illustrated in Fig. 26.2. 

Gypsum is important as it reacts with aluminate to give etringite - calcium aluminium sulphate hydrate, Ca6Al2S30ig 32H2O - on the surface of the aluminate grains. This slows the reaction of aluminate with water and allows the wet cement paste to be worked for longer. 

The sensitivity of concrete towards the action of seawater is above all due to the presence of calcium hydroxide and hydrated calcium aluminates, which are both susceptible to reactions with sulfate ions from the seawater. Expansive effects decrease, therefore, as the percentage of CjA in the cement or the content of Ca(OH)2 in the concrete diminishes. The traditional approach to avoid these reac-... 

In the C-A-H system, protected against the CO2 influence, there is a large number of so-called hexagonal hydrates, ciystalhzed in the form of hexagonal plates. These are the metastable phases, because cubic CjAHg is the only stable calcium aluminate hydrate [83, 84]. This phase is, however, formed in the reaction of calcium aluminates with water only at temperature higher than 45 °C [85]. At lower temperatures... 

As it has been mentioned, the hydration of CjA has a decisive impact on the rheological properties of fresh paste. The high rate of reaction with water leads to the saturation of solution with aluminate and calcium ions and as a consequence to the crystallization of C AHj. This corresponds to the quick stiffening of paste, determined as flash set. All the substances modifying the rate of CjA reaction with water by adsorption on the surface of this phase or by the change of the ions concentration in the liquid phase will have a great impact on the rheological properties of paste. 

However, Kondo [3 ] is of the opinion that the hydrolysis of the glass in water occurs and the calcium ions are released initially to the liquid phase. Simultaneously on the surface of slag grains an acid, colloidal shell of silica-alumina gel is formed. This shell has low permeability and hence the further slag reaction with water is hindered. In the presence of Ca(OH)2, added as alkaline activator, the siUcon and aluminum from the shell are released to the solution (Fig. 8.2). The solubiUty of aluminum compounds becomes considerably increased in the solution of pH higher than 12.5 because in this condition the Al(OH) ions are formed [4]. The concentration of aluminum in the liquid phase is increasing because the calcium aluminates crystallize a httle later, primarily the C-S-H (1) is formed. Simultaneously the solubility of hydrates formed in this condition is reduced. 

The calcium aluminate cements are not resistant to the attack of alkalis, because the aluminum hydroxide is soluble in the water solutions of sodium and potassium hydroxides, with which the calcium aluminates are also reacting with. The alkali metals carbonates react with calcium aluminate hydrates and these reactions cause destmction of concrete ... 

Finally, it is known that the increase of ferrite phases content in clinker is advantageous. They are formed at low temperatures and produce the melt, which promotes the calcium oxide reaction with silica. The ferrite phases react quickly with water, giving with lime the hydrated componnds, analogous to the aluminate hydrates. 

In the system Portland cement-fly ash-water the initial hydration of the clinker phases, especially those of tricalcium silicate and tricalcium aluminate, progresses much faster than that of the ash. Just as in the hydration of pure Portland cement, a C-S-H phase, ettringite, and calcium hydroxide are formed as the first hydration products. Any alkali sulfate salts, commonly present in the clinker, also dissolve and convert to alkaU hydroxides and calcium sulfate in a reaction with calcium hydroxide. The alkalinity of the liquid phase increases, and may exceed pH=13. 

Ettringite cements contain an aluminate donor such as monocalcium almninate, tricalcium aluminate, tetracalcium trialmninate sulfate, or tetracalcium aluminate ferrite, together with calcium sulfate dihydrate and in some instances also calcium hydroxide. In the hydration of such mixes ettringite is formed as the main or sole reaction product. If allowed to react umestricted, the hydrating paste exhibits a significant expansion however, hardened pastes with strengths comparable to those of other cements may be produced if the hydration is allowed to take place under mechanically restricted conditions. This may occur in completely closed steel molds, by which measure undesired expansion of the paste m be effectively prevented (Odler and Yan, 1994). 

When the cement comes into contact with water, chemical reactions start between the clinker minerals (CaO = C, Si02=S, AI2O3 = A) and the water (abbreviated in the terminology as H ), forming colloidal, insoluble reaction products such as calcium silicate hydrates (CSH), calcium aluminate hydrates (CAH) ... 

The aggregates may be contaminated with salts, such as Na2S04 and react with calcium aluminate hydrate to produce ettringite and also CaS04 2H20. These reactions products damage concrete by... 

In CAC, the CA reacts with water to form a series of calcium aluminate hydrates. These include CAHjq, C2AHg, C3AH6, and AH3 (an amorphous phase). The metastable hydrates, CAHjq and C2AHg, convert to C3AH6. The following scheme summarizes the eonversion reactions. 

The essentials of modern cement manufacture have been followed for the last 200 years (Lea 1970). A calcium source (usually limestone) and an aluminosilicate source (usually shale or clay) are mixed together and fired at high temperatures (1300 to 1500°C) to produce calcium silicates, aluminates and alumino-ferrites. The calcium silicates hydrate on contact with water to produce CSH gels which are the chief binding agents in the cement. Cements containing CSH gel (including pozzolanic cements) are termed hydraulic because they set and harden on reaction with water and once set, continue to harden if placed underwater (Taylor 1990). 

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