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Mortars or concretes that have been cured at high temperature. This phenomenon is usually referred to Delayed Ettringite Formation (DEF) because ettringite deposits have been observed in expansive structures. However no correlation has been established between the amount ettringite detected in expansive heated mortars and the degree of expansion. Ettringite is prevalent in old structures regardless of the occurrence of deterioration. This study is aimed at investigating the expansion mechanism and in particular the role of ettringite relating to expansion. The results indicate that there is no direct cause between the ettringite detected by XRD and expansion. Instead, evidence is presented for the possible implication of the C-S-H of expansive and non-expansive heated mortars that is the presence of ettringite formed in the outer C-S-H from calcium monosulfoaluminate, present within the outer C-S-H, and released sulfate from the C-S-H. Such ettringite can cause expansion because its formation takes place in site in the outer C-S-H. The paste expands and detaches from the non-expansive components such as aggregates forming gaps at the paste/aggregates interfaces.

A number of unusual microscopic features have been documented in studies of the alteration of permeable concretes undergoing sulfate attack in semi-arid climates. The high S/C of the concretes examined here has permitted complete penetration of sulfate-bearing ground water through the concrete, and deposition of crystallized salts on evaporative surfaces, including the upper surfaces of slabs. The microstrucrual alterations observed in response to this penetration of sulfate-bearing ground water include the deposition of secondary ettringite and the accompanying local expansion and cracking and the deposition of gypsum, but are much more extensive and complicat4d than that. Calcium hydroxide is often entirely removed throughout the concrete, leaving open areas, particularly in the interfacial zones around aggregates. Various new compounds are deposited in these spaces, and elsewhere within the paste. In addition to ettringite and gypsum, the substances deposited often include monosulfate; brucite and magnesium silicate hydrate may be formed where the ground water is rich in magnesium; and thaumasite may be developed where carbonation accompanies sulfate attack. Partial decalcification of the C-S-H gel is common, and magnesium silicate hydrate may locally replace it. Even the large residual unhydrated cement grains, usually stable indefinitely in most concretes, are profoundly altered. The C2S and C3S in such grains may be completely dissolved and the spaces thus provided within the outlines of the cement grains may also be filled by secondary deposits. It is evident that sulfate attack produces profound internal changes in the paste microstructure, leading to the softening, exfoliation, and other external symptoms of sulfate attack reported in the literature.

Paste and mortar specimens were manufactured by using ordinary portland cement (OPC), C3A-free portland cement, slag cement and pozzolan cement. A carbonaceous or siliceous filler (10% by cement weight) was blended with each of the above portland cements. Limestone or quartz sands were used for mortar mixtures. Four different water-cement ratios (w/c) were adopted: .55, .50, .45, and .40. After a 28-day wet curing, paste and mortar specimens were immersed in MgSO4 aqueous solutions with a SO4 concentration of 350, 750, and 3000 mg/l, corresponding to chemically aggressive exposures 5a, 5b, and 5c respectively, according to the European Norms (ENV 206). The deterioration of cement paste specimens was studied by X-ray diffraction analysis to detect ettringite and /or thaumasite formation in relationship with the visual observation of sulfate attack. The deterioration of mortar specimens was studied by measuring elastic modulus and compressive strength at different periods of aggressive exposure (from 1 month to 5 years). After 5 years of exposure to the sulfate attack, paste and mortar specimens with slag and pozzolan cements were undamaged independent of the sulfate concentration, sand type, and w/c. On the other hand, paste and mortar specimens with blended limestone-portland cements showed surface damage when exposed to the 3000 mg/l of SO4 aqueous solution. However, they did not show loss of either compressive strength or elastic modulus in the 5 years of sulfate exposure. The surface damage was mitigated when OPC was replaced by the C3A-free portland cement and completely eliminated when this cement was blended with a siliceous filler rather than with a limestone case. Thaumasite and ettringite are responsible for the surface attack. The amount of thaumasite was a little higher in the presence of blended limestone portland cement.

The behavior of ordinary concrete and high strength concrete under chemical attack was studied over a period of more than 5 years. The concrete specimens were immersed in a variety of chemical solutions including 5%, 2.5%, .5% and .1% ammonium sulfate, and 10%, 5%, 1% and .1% ammonium nitrate. The strength and weight changes, and the penetration depths of the attacking ammonium nitrate ions into the specimens were measured. The influence of the initial concrete strength and the concentration level of the aggressive solution on the behavior of the immersed concretes were evaluated.

A model for predicting the initiation period of reinforcement corrosion for marine concrete structures was presented at the CANMET/ACI Int. Confr. On Durability, Sydney, Australia 1997. The theory has been refined to be valid also for short time curing and exposure. The objective of the program aimed at giving input data to the model for estimating the initiation period of reinforcement corrosion. Furthermore, the programmed should document the chloride ingress into various practical concretes made with lightweight aggregates, depending on a number of variables in curing and exposure conditions as well as concrete composition and materials. The most important variable were 1) curing time before exposure, 2) curing temperature, 3) exposure temperature, 4) exposure time, 5) type of exposure, 6) salt concentration in exposure medium and 7) type of binder. The most important conclusion was that the results fitted very well to the earlier presented hypothesis for the prediction of the initiation period independent of type of aggregate. Additionally the following main conclusions shall be mentioned: Surface chloride content, C is the environmental load, and it increases with the exposure time during the first years, and reduces with increased curing time and the introduction of slag, and independent of curing and exposure temperature. The achieved diffusion coefficient, D is independent of curing and exposure temperature, and decreases with an increase of the exposure time and the introduction of slag. The parameter a expresses the time dependency of D with exposure time. a is independent of curing time, curing and exposure temperature, and it increases somewhat with increased salt concentration in the exposure water and introduction of slag.