Ettringite concrete

Ettringite has been widely studied in cement and concrete due to the deleterious expansion, which has been attributed to its formation after the concrete has hardened (Lea 1971 Mehta 1973 Dunstan 1980 Cohen 1983 Monteiro Mehta 1985 Tikalsky 1989 Day 1992 Tishmack et al. 1999). Ettringite also causes expansion in some types of high-S CCPs (see below). 

Day, R. L. 1992. The Effect of Secondary Ettringite Formation on the Durability of Concrete A Literature Analysis. Research and Development Bulletin RD108T, Portland Cement Association, Skokie, IL. 

Mehta, P. K. 1973. Mechanism of expansion associated with ettringite formation. Cement and Concrete Research, 3, 1 -6. 

Solem-Tishmack, J. K., McCarthy, G. J., Docktor, B., Eylands, K. E., Thompson, J. S. Hassett, D. J. 1990. High-calcium coal conversion by-products Engineering properties, ettringite formation, and potential application in solidification and stabilization of selenium and boron. Cement and Concrete Research, 25, 658-670. 

Tishmack, J. K., Olek, J. Diamond, S. 1999. Characterization of high-calcium fly ashes and their potential influence on ettringite formation in cementitious systems. Cement, Concrete, and Aggregates, 21, 82-92. 

Baur, I. Johnson, C. A. 2003/x The solubility of selenate ettringite and selenate monosulfate. Cement and Concrete Research, 33, 1741 -1748. 

Pollmann, H., Kuzel, H.-J. Wenda, R. 1993. Solid solution of ettringites. Part II Incorporation of B(OH) and CrO2- in 3Ca0Al203-3CaS04-32H20. Cement and Concrete Research, 23, 422-430. 

Concrete made with calcium aluminate cement at a properly low w/c ratio is highly resistant to sulphate solutions, sea water or dilute acid solutions with pH > 4, including natural waters in which CO2 is the only significant solute. Resistance may extend to pH 3 if the salt formed is of sufficiently low solubility. Midgley (M96) showed that, for fully converted material exposed to a sulphate ground water for 18 years, penetration with formation of a substituted ettringite was limited to a depth of 5 mm. These properties are consistent with Lea s (L6) view that the resistance is due to the formation of a protective coating of alumina gel, coupled with the absence of CH. No fundamental studies, e.g. on microstructural effects, appear to have been reported. 

Fig. 12.7 SEM (backscattered electron image) of a concrete showing massixe deposits of ettringite (one marked E ), which through sulphate attack has grown on aggregate surfaces, with associated cracking. Courtesy WHD Microanalysis Consultants Ltd, Ipswich, UK (H64).

The A1(0H)4 and OH", and some of the Ca, needed to form ettringite eould be supplied by iinreaeted aluminate or ferrite phase, but the main souree of Al(OH)4 hardened concrete is likely to be AFm phase, represented here as monosulphate ... 

Despite its relatively low solubility, CaS04 present in solution can also attack concrete. In this case, the solution can provide all the ions that are needed to produce ettringite from monosulphate, and little or no dissolution of CH nor decalcification of C -S- H need occur. The saturated solution also slowly attacks C3S pastes (T60) the explanation of this is not clear. The use of abnormally large quantities of finely divided gypsum interground or blended with the cement also produces expansion (029). but little appears to be known about the microstructures that result. 

First Step. When polymer latexes are mixed with fresh cement mortar or concrete, the polymer partides are uniformly dispersed in the cement paste phase. In this polymer-cement paste, flie cement gel is gradually formed by the cement hydration and the water phase is saturated with calcium hydroxide formed during the hydration, whereas the polymer partides dqrosit partially on the surfaces of the cement-gel-unhydrated-cement partide mixtures. It is likely that the calcium hydroxide in the water phase reacts witit a silica surface of the aggre tes to form a calcium silicate layer.I It is confirmed that tire formation of the calcium hydroxide and ettringite in the contad zone between tire cement hydrates and aggregates is attributed to the bond between them.I lPl... 

From the results the tendency of ettringite formation can be observed in those samples that do not have latex. In fact, this phase showed a significant reduction in peak intensity, possibly indicating that the latex inhibits its formation due to reduction of sulfate/aluminate ratio (Figures 6 and 7). Ettringite is a phase that does not remain in the concrete at ages over 28 days with time it gradually incorporates hydrated monosulfate [18]. 

Corrosion is not the only deterioration mechanism in reinforced concrete. Alkali-silica reactivity (ASR), sulphate attack, thurmasite attack, delayed ettringite formation, freeze thaw, thermal movement, settlement and other movement can all lead to concrete damage and their assessment must be included in the surveys. 

Bonin discussed the new results explaining the reasons of paste or concrete stiffening [37]. Apart of false set, occurring as the result of alkalis carbonation, which in turn react with calcium hydroxide with CaCOj precipitation, Bonin considers the case of cement paste stiffening, in which under moisture influence the rate of ettringite formation had changed [37]. 

The thermal treatment of concrete can cause the detrimental effect of delayed ettringite formation, resulting in the deterioration of concrete elements. The problem of delayed ettringite formation belongs to the internal corrosion of concrete and will be discussed in the chapter relating to this corrosion. 

A layer composed of CH and C-S-H gel is formed on the aggregate surface immediately after mixing of concrete components with water [17]. However, Zimbel-man [29] foimd the presence of small ettringite crystals. After the duplex film, the layer of portlandite crystals oriented perpendicularly with c axis to the aggregate surface is present [30,31]. This is an interlayer between the duplex film and cement matrix, and is formed dining 1-2 days after mixing of concrete components with... 

Quite another conditions occur in the case of concrete with porous aggregate, particularly a dry one, which will significantly absorb water. However, the construction of interfacial transition zone has not been studied in such a concrete as yet [10]. One can even suppose, based upon the model studies, that the growth of calcium hydroxide content around the aggregate grains will not be formed. There are some premises indicating the emichment in ettringite [10]. Incase of diy, porous... 

The delayed ettringite formation is rated among the internal corrosion of concrete, which is induced by a heat treatment of concrete, and thus occurring mainly during the precast concrete elements production. However, one carmot exclude the temperature rise up to 70 °C in the interior of massive concrete stracture, as a consequence of heat evolution process in hardened cement paste, and in this condition ettringite can be unstable. 

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