Ettringite-monosulfate reaction

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. 

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]. 

For sodium-oleate-based air-entraining agents, the C3S peak is not affected, but the C3A peak is accelerated and splits into two up to a 10 times normal dosage level. It is believed that the ettringite and monosulfate reactions... 

Unsaturated fatty acids, such as oleates and others used as water-proofers, afFeetthe hydration of cement. The tricalcium silicate peak in the calorimetry is unaffected, but the ettringite and monosulfate reactions are affected substantially. ... 

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. 

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

Mixes of C4A3S and CSH2 (gypsum) yield ettringite alone, if mixed at a molar ratio of at least 1 2, and a combination of ettringite and monosulfate if the amount of gypsum is reduced. Simultaneously aluminum hydroxide is formed as a reaction product ... 

If mixed with water, the existing calcium hydroxide, or that formed in the hydration of added lime, dissolves in the liquid phase and reacts with the glass phase of the ash. An amorphous C-S-(A)-H phase is formed as the product of reaction, which—at appropriate water/solid ratios of the starting mix—causes setting and hardening of the system. Limited amounts of ettringite (AFt) and monosulfate (AFm) may also be formed in the reaction, if sulfate ions are present in the ash. 

If a hardened ordinary Portland cement paste is in contact with a solution that contains ions in combination with Na or K, two principal reactions take place, a conversion of the monosulfate into ettringite and the formation of gypsum ... 

The mechanism of the sulfate attack on Portland-slag cement is broadly similar to that observed in pure Portland cement pastes. In an attack by alkali sulfates the dominant reactions are the partial decalcification of the C-S-H phase and the conversion of the existing monosulfate to ettringite. The amount of ettringite formed is lower than in the case of Portland cement, as monosulfate is the only phase that can serve as a direct source for ettringite formation, whereas neither Al incorporated in the C-S-H phase nor that present in hydrotalcite are available for such a reaction, jnst like Al in ettringite already formed independently of snUate attack (Gollop and Taylor, 1992-1996). 

For industrial Portland cements, tetracalcium aluminum monosulfate hydrate (C ASH ), which is a major component of fully hydrated hardened cement, reacts with gypsum to form secondary ettringite (C ASjHj ). This reaction also causes expansion of 57 vol.% of the solid components ... 

Since ettringite forms a diffusion barrier around the C3A grains, the hydration reaction is initially delayed. Only after about 12 h will this barrier be broken by the onset of transformation of ettringite into monosulfate (Figure 5.7). This reaction is reversible, as the monosulfate can retransform to ettringite in the presence of sulfate ions in the groundwater, leading to the stmctural destruction of concrete, as can be observed on many urban concrete structures. In modern sulfate-resistant... 

In the conduction calorimetric curves of the aluminate-gypsum system,the first stage is characterized by a peak at 30 minutes for the formation of ettringite. In the second stage, the peak occurs at 24-48 hours when the monosulfate phase forms by the reaction between ettringite and aluminate. 

Several complex reactions occur when concrete is exposed to sea water. Compounds such as aragonite, calcium bicarbonate, calcium monosulfate hydrate, ettringite, Ca-Mg silicate hydrates, magnesium silicate, thaumasite, etc., have been identified by the application of thermal analysis in conjunction with other techniques. Thus, the factors leading to the deterioration of concrete can be established. 

Problem. Make a rough determination of the temperature tr (°C) at which it can be expected that the ettringite formed by the reaction (a) becomes unstable and is transformed into monosulfate by the reaction (b) ... 

Figure 5.21. Hexagonal crystals of monosulfate may be formed by reaction of ettringite with C3A the investigation suggests that this transformation will be spontaneous at high temperatures during the early hardening of the concrete.

Reaction with gypsum to give monosulfate. b Reaction with gypsum to give ettringite. 

Figure 4.13 Hydration stack over time of a zeolite-blended portland cement as measured by synchrotron radiation indicated are ettringite (AFt), monosulfate (AFm), clinoptilolite (zeolite Cli), portlandite (CH) and alite (C3S). (Reprinted from Cement and Concrete Research, 40, Snellings, R., G. Mertens, O. Cizer and J. Elsen (2010), Early age hydration and pozzolanic reaction in natural zeolite blended cements Reaction kinetics and products by in situ synchrotron x-ray powder diffraction, 1704-1713, Copyright 2010, with permission from Elsevier.)...

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