International Concrete Abstracts Portal

Since the past 25 to 30 years, preferably heat treated precast concrete members (> 70 “C) manufactured with high early strength Portland Cements with higher sulphate contents, under adverse exposure conditions have some-times exhibited structural damage in the form of map cracking and loss of strength. These damages are characterized as Damaging Late Ettringite Formation (DLEF) (1). The cause for the increased occurrence of DLEF starting about 1970 is to our opinion the world wide increase of the permissible sulphate contents of the Portland-Cements (2-5). It is important that 1 wt.-% SO3 can form 5 wt.-% of ettringite or 7.7 wt.-% thaumasite. In “German directions“ a maximum heat treatment (HT) temperature of 80°C is specified for concrete exposed to dry environmental conditions. For concrete‘ exposed to intermediate or permanent wet conditions, a maximum HT temperature of 60°C is specified. Though the standard does not provide clear directions, if is believed that cements containing pozzolanic admixtures can be subjected to more intensive HT (6). DLEF is caused by a formation, destruction and a later renewed forma-tion of ettringite occuring preferably after HT at > 70°C of pastes, mortars and concretes made with high strength Portland cements. - Very early start of HT showed no significant influence on DLEF. Late or repeated HT resulted in more severe or repeated damaging. - Humidities < 95 % at 20°C resulted up to 780 d of starage in no DLEF. - After treatment with interim some FTC and/or cured at low temperatures provoked more early and severe destruction.

Changes in the cement manufacturing process such as the use of higher sulfur fuels have tended to raise clinker sulfate levels and SO3/alkali ratios. As a consequence, interground gypsum additions to cement have dropped because more sulfate is available from the clinker. Also, these clinker sulfates tend to be available as double sulfate salts; calcium langbeinite instead of potassium sulfate. What is the impact of clinkers with high SO, level on concrete performance; mainly on its workability and durability ? The aim of this study is to provide some answers to this question. Cements made from either high SO, clinker or low SO, clinker and gypsum or hemihydrate, but with a given chemical composition, have been simulated by pure phase materials and hydrated up to one hour. Calcium langbeinite is rapidly dissolved. Because of its dissolution rate and ability to form “blocking ettringite”, high calcium langbeinite clinkers should provide improved rheological properties. Moreover, cements made with clinkers containing significant quantities of calcium langbeinite should have a similar workability and durability to a cement made with a low sulfate clinker to which larger quantities of gypsum have been added. The dissolution rate of anhydrite potentially existing in very high SO3/alkali clinker has also been simulated. Experiments indicate that it dissolves and reacts quite quickly so that it should not provide any durability problem if present in cement and concrete.

The volume stabilities of mortars have been monitored for periods up to 5 years. In addition to controls maintained at 20°C, prisms were subjected to high temperature curing regimes, typically 12 hours at 70, 80 or 9OO C. Curing temperature was found to be the dominant factor in determining whether significant (>O. 1%) expansion took place during subsequent water storage at 20°C. Most of the mortars expanded after curing at 90°C but very few at 80°C and none at 70°C. Analysis of the results for mortars cured at 90°C indicated that expansion increased with cement fineness and levels of alkali, C3 A, C3 S and MgO and showed a maximum at a cement SO3 content of -4%. The pessimum SO3 content increased with cement alkali content. The incorporation of ground limestone accelerated expansion but did not affect the ultimate value. Cement replacement levels of 30% or 40% with siliceous fly ash and 30% or 50% with ground granulated blastfurnace slag appeared to prevent expansion. A sample of Type IS cement gave only moderate expansion. The mortars that expanded after curing at 80°C had high alkali levels, through the incorporation of KOH or K2 SO4 in the mixing water. At 20°C no long-term expansions were observed, despite the inclusion of cements with total SO3 levels up to 5.6% and clinker SO3 levels up to 2.6%. The Type K cement expanded by -0.05% within the first 7 days and was then stable. Cements based on a high C3 A clinker gave early expansions (-56 days) which increased as the cement SO3 content increased and as the specific surface area decreased, although all expansions were <0. 1%.

Paste, mortar, and concrete cured at temperatures above certain limits may exhibit expansion and cracking during subsequent exposure to varying moist conditions. This phenomenon originally became known as delayed ettringite formation, DEF. DEF results in a typical microstructure which is demonstrated with examples from field samples and laboratory-made samples. The microstructure is compared with examples of internal sulfate attack in laboratory samples. These typical features include gaps around the aggregate where the paste shows an almost perfect replica of the individual aggregate surfaces. Expansion of the paste on a scale which is homogeneous relative to the aggregate would lead to such features. The chemistry in DEF is similar to that of sulfate attack. A mechanism involving hydrates of aluminates and possible unhydrated cement clinker particles is discussed.

The stability domain of stoichiometrically prepared ettringite and monosulfate hydrate in water at 30 and 1OOoC also in the presence of lime, alite, monocarboaluminate hydrate, or each other is reported. The formation conditions of both calcium sulfoaluminate hydrates in pure ordinary portland cement dealing mainly with the hydration behavior of the tricalcium aluminate and the tetracalcium aluminoferrite in the presence of gypsum, also of lime, alite, or each other at 300C is summarised. The type of hydrate formed in sodium -hydroxide, sulfate solution or as a result of alkali doping in the structure of the anhydrous phase is given. The ettringite/monophase formation and duration in the three stages of C3 A and C4 AF hydration (instantaneous reaction, dormant, and acceleration period) are described. The present review focused on the stability of the calcium sulfoaluminate hydrates is a summary of 13 publications.