Ettringite, 1.4

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

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. 

Ettringite (6-calcium aluminate trisulfate, 8-hydrate), 5 479t... 

Ettringite (6-calcium aluminate trisulfate, 32-hydrate), 5 479t Ettringite cement applications, 5 500t hydration, 5 477, 477t E-type inks, 14 326 Eucalyptol, 24 528... 

The sulfate ion concentration normally diminishes rapidly, whereas in the presence of a water-reducing admixture, the high concentration is maintained due to the reasons given in (1) which, in turn, can be related to a delay in the reaction of the gypsum by the C3A phase to form ettringite. Indeed, if the cement... 

C3A + H2O = C4AH 9 (quickly stopped by formation of a protective coating of ettringite). 

The C3A phase is subjected to competitive reaction between water and gypsum to form a small amount of the initial C3A hydrates covered by a protective coating of ettringite, and also a proportion of the C3S hydrates to form a calcium silicate hydrate of low calcium content in the region of 100 nm thick. 

In this final peak, the remaining tricalcium aluminate reacts with both gypsum and water to form ettringite and subsequently the tricalcium aluminate monosulfate. C3A hydrates formed are of varying composition but... 

It can be seen, therefore, that on typical paste shrinkage in the region of 4000 microstrain, that some 30-40% of this could be accounted for by the sulfate component. An important point to note is that the sulfate reaction has a negative influence on shrinkage and, therefore, acts as a restraint to creep and shrinkage as the reaction proceeds. It was noted in an earlier section that the addition of a water-reducing admixture delays ettringite reactions and could be a possible mechanism by which the volume deformations are increased. 

Table 2.2 Amounts of ettringite and monosulfate hydrate formed in the C3A-CaS04. 2H2O system...

Addition of dampproofers based on caprylic, capric or stearic acids, stearates or wax emulsions do not have any effect on the setting characteristics of hydration products of Portland cement. However, the unsaturated fatty acid salts, such as oleates, although not affecting the tricalcium silicate hydration, have a marked effect on the ettringite and monosulfate reaction [12] and this is illustrated in the isothermal calorimetry results in Fig. 4.4. It is possible that a calcium oleoaluminate hydrate complex is formed involving the double bond of the oleic acid. 

The subsequent conversion of the ettringite to monosulfate by reaction with C3A is also accelerated by TEA. 

The expansion mechanism associated with the formation of ettringite in cement is not clear and several hypotheses have been advanced [77]. A... 

Subsequent reaction of the monosulfate with gypsum produces acicular crystals of ettringite. Monosulfate apparently does not contribute to expansion, whereas the formation of ettringite involves expansion. A recently issued patent [75] covers the use of prehydrated high-alumina cement (H-HAC), lime and gypsum mixtures. Particle type, size, thickness of protective coating, and presence of moisture determine the rate and extent of expansion [Fig. 6.11]. 

The following factors affect the reaction which forms ettringite and the resultant expansion. 

Type and amount of aluminates, calcium sulfate and free lime present in the mixture governs the rate at which ettringite forms. The presence of lime is reported to be essential for both the initial and subsequent stages of ettringite formation because it maintains a solution phase saturated with calcium ions [81]. Crystal habit, particle size and range of particle size in CSAs determine its rate of hydration and duration of expansion (Fig. 6.11). 

The silica fume is said to decrease the formation of ettringite by reducing the concentration of calcium (Ca" +) and hydroxyl (OH ) ions involved in the formation of ettringite [146]. The expansion of lime-based admixtures is not affected by water-reducing admixtures to the same extent as observed for CSA-based admixtures. However, set retardation may result due to the increased amounts of calcium hydroxide produced in the presence of the WRA. 

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