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This study evaluates effect of metakaolinite on the chemical resistance of standard cement mortars produced from portland cements of different C3A content, ranging from 3.3% to 11.4%. The metakaolinite was produced by burning of kaolin clay at different time and temperatures, specially selected to ensure the highest pozzolanic activity, as measured by ASTM method. The chemical resistance was evaluated through the measurements of strength, shrinkage and expansion tests on the samples stored in water and chemical solutions. The porosity and microstructure was also investigated. As it appeared, the metakaolinite admixture did not change substantially the standard properties of mortars, though their chemical resistance was markedly improved.

SP192 In 2000, CANMET, in association with ACI, the Japan Concrete Institute, and several other organizations in Spain and Canada, sponsored a fifth international conference held on June 4-9, 2000, in Barcelona, Spain. More than 120 papers from 35 countries were received and peer reviewed in accordance with the policies of the American Concrete Institute; 73 were accepted for publication. The accepted papers deal with all aspects of concrete durability. In addition, several sessions dealing with sulfate attack, superplasticizers and supplementary cementing materials, and near surface testing for the durability of concrete were organized. In addition to the papers that have been published in the refereed proceedings, more than 30 papers were presented at the conference.

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.