Durability of concrete and cement composites

Background Thaumasite form of sulfate attack

Although thaumasite had been known as a rare calcium carbonate-silicate- sulfate hydrate of the composition CaCO3.CaSiO3.CaSO4.15H2O since 1878, it was not until the 1960s that it first became more generally known as a deterioration product of cement and concrete. The first major report that clearly documented thaumasite as a concrete deterioration product was published in the Highway Research Record in the United States (Erlin and Stark, 1965). This article mentioned occurrences in two sewer pipes, in a grout and in a pavement core base. For example, in the sewer pipes, the deteriorated areas contained brucite, calcite, gypsum and thaumasite. The presence of thaumasite, frequently in association with ettringite, appeared to represent situations involving attack by sulfate solutions over a number of years. No technical investigation on how the thaumasite had been formed had been undertaken at the time.

The first known occurrence of thaumasite sulfate attack in Europe occurred in the UK in February 1969 in a mortar containing a Portland masonry cement inside some new houses being constructed at Stoke-on-Trent during wintertime. The masonry cement, which contained a limestone filler and an air-entraining admixture, had been applied to the internal walls of these houses as a rendering and been covered with gypsum plaster as a finish. Where the mix had been improperly dispersed and applied under cold damp conditions, blistering arose within six to eight weeks with the cold damp conditions remaining prevalent. The blisters were sometimes up to 3 cm in diameter and contained both ettringite and thaumasite that had clearly arisen as a result of sulfate attack. The thaumasite was in a very poor crystalline form and the limestone filler had entered into some chemical reaction (Bensted, 1977a, 2000). Some of the thaumasite crystals formed here appeared to produce over- growths on some of the ettringite crystals. However, there was no evidence for any extensive degree of solid solution between these two minerals. The lack of expansive reaction at the plaster-cement interface with the properly dispersed mix under ordinary conditions was attributed to low porosity, limiting the ingress of sulfate ions, which requires continuous moisture-filled pores to be available. The expansive effects were not observed when the masonry cement containing limestone filler had been properly rendered and coated with plaster under conditions that had not been cold and damp (Bensted, 1977a).

An extensive search of the technical literature at the time revealed no experimental work explaining how thaumasite can form. As a result, a detailed study was undertaken to synthesise thaumasite. More than 200 small-scale experiments were carried out for up to four years at 1±4ëC in air with excess water present. Thaumasite was found to form generally under these conditions, when there were carbonate (including atmospheric CO2 in some of the experiments), silicate and sulfate ions with sufficient calcium ions and excess water available (Varma and Bensted, 1973; Bensted and Varma, 1973a). Under these cold, damp conditions, samples of Portland cement (including Sulfate Resisting Portland cement) and Portland masonry cement formed some thaumasite. No thaumasite was formed in any experiments undertaken at ambient temperature (ca. 20ëC). During the early 1970s, examples of thaumasite in deteriorated building materials were reported in the technical literature (Leifeld et al., 1970; Fleurence et al., 1972).

What is serious about thaumasite sulfate attack is that the main binding constituent of hardened cement, C-S-H, becomes converted into thaumasite, which is a non-binding powdery material having no inherent compressive strength. The key question that had been raised was: why was thaumasite formation facilitated at low temperatures? The answer to this question became apparent from a wide range of instrumental investigations involving X-ray crystallography (Lafaille and Protas, 1970; Edge and Taylor, 1969, 1971), infrared spectroscopy (Bensted and Varma, 1973a; Moenke, 1964; Bensted, 1977b, 1994) and laser Raman spectroscopy (Varma and Bensted, 1973; Bensted, 1988, 1999), which demonstrated that the thaumasite structure contains silicon surrounded by six hydroxyl groups and not the more usual number of four. Six-co-ordination of silicon by hydroxyl (OH groups) or directly by oxygen is very rare (Bensted and Varma, 1973b) and normally needs high pressure or low temperature to facilitate such a molecular arrangement. An important characteristic of thaumasite is that, once formed, it is stable up to ca. 110ëC, when it decomposes sharply to a disordered structure known as thauma- site glass (Bensted and Varma, 1973a); it is actually more stable to temperature changes associated with gentle heating than is ettringite, which starts to decompose below 100C.

With the benefit of hindsight it is apparent that, in the past, thaumasite was often confused with carbonated ettringite because of discrepancies in data reported in the older technical literature for DTA, optical microscopy and infra- red spectroscopy. X-ray diffraction is normally the best technique to employ for characterising thaumasite, because the main crystal lattice d-spacings for thaumasite and ettringite are sufficiently far apart to enable independent identification of both these minerals to be clearly made (Bensted, 1977b). Since then, instrumental techniques have improved and such identification is now easier. Not surprisingly, thaumasite has now been identified in many countries as a deterioration product (Macphee and Diamond, 2003). De Ceukelaire, at the University of Ghent, Belgium, made the important discovery that the expansive capability of thaumasite formed from ettringite via the woodfordite route (Bensted, 2003a) ± as explained below in Section 4.3.2 ± is much less than the volumetric expansion of ettringite. He found that thaumasite only occupies about 45% of the volume of ettringite from which it has been derived (De Ceukelaire, 1989, 1990).

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