With the aim of reducing the environmental impact associated with cement production, during the last decade, different percentages of clinkers have been replaced in cement by supplementary cementitious materials (SCMs). When new SCMs are incorporated in concrete, it is necessary to evaluate, not only the mechanical properties (as strength and stiffness) and the durability but also the deformations that can generate cracking and decrease the service life of the structures. This paper is focused on the study of volumetric changes at the early ages of pastes made with blended cements with the addition of illitic calcined clays from the Buenos Aires province, Argentina. The objective of this work is to present preliminary studies on the effect of illitic calcined clays on the autogenous and chemical shrinkage of pastes. The studies were made on pastes (water/cementitious material ratio equal to 0.275) using a Portland cement type II/A-L, with the incorporation of different percentages (10%, 20% and 30%) of illitic calcined clays. A device for direct deformation measurement was used to register linear dimensional changes; the general guidelines of ASTM C 1608 were applied for the determination of chemical and autogenous shrinkage. The volumetric changes measured with direct device are the sum of the chemical and autogenous shrinkage accompanied with the expansion due to the heat released during hydration. It was found that pastes incorporating calcined clays had early deformations similar to or lower than reference paste without clay.
Durability in aggressive environments is an important factor to extend the service life of concrete and the use ternary blended cements (limestone filler + calcined clays) can contribute to this purpose. In sulfate environments, the effects of supplementary cementing materials depend on the concentration, Portland cement and the progress of hydration reactions. Low level of limestone filler replacement influences the stabilization of AFt due to formation of monocarboaluminate, but high replacement increases the effective w/c and the capillary porosity promoting sulfate penetration. The use of active pozzolans suppresses the sulfate attack by minimizing both ettringite and gypsum formation. It is generally assumed that curing prior to sulfate exposure should be extended to allow the development of the pozzolanic reaction and subsequent reduction of portlandite content, pore size structure refinement and permeability reduction. However, in most field applications, concretes exposed to sulfate attack are cast in situ and thus, these are exposed to sulfate since early ages. This paper evaluates the sulfate resistance of an illitic-calcined clay and limestone filler when the cement is exposed immediately to aggressive environments. In this paper, the external sulfate resistance of blended cements containing 30% replacement of limestone filler and/or calcined clay (C30F, C30CC and C15F15CC) are analysed. Two different calcined clays from Buenos Aires, Argentina were selected. Mortar prisms and cement paste cubes were fabricated and exposed to a sodium sulfate solution after 2 days. Comparison of sulfate resistance was based on the expansion, mass variation, visual appearance and compressive strength. Furthermore, the evolution of microstructure of blended cements exposed to sodium sulfate solution was characterized by XRD tests on the external surface and the core of cementblended pastes.
Studies of illitic calcined clays are less developed than that corresponding to kaolinitic clays, but illite is one of the more abundant clayed minerals of the earth's crust, as occurs in the Center of the Buenos Aires Province (Argentina) where the largest cement factories are located. Illite clays develop pozzolanic properties when they are thermally treated at 950 °C, causing dehydroxilation and collapse of structure to form a metastable or amorphous aluminosilicate. Illitic calcined clays don't present a significant water demand and the compressive strength of blended cements attains to the corresponding to portland cement at 90 days. It is characterized as slow pozzolana. Illite incorporates certain proportion of reactive alumina and high proportion of alkalis, modifying the pore structure. From durability point of view, the incorporation of illite can affect the sulfate resistance of portland cements or the alkali-silica reaction (ASR). The aim of this paper is to study the behavior of two different illite calcined clay blended cements against chemical attack, like sulfate attack and harmful alkali silica reaction, using the test based on the ASTM C 1012 and ASTM C 441, respectively. For sulfate performance, illite calcined clays was blended with a low C3A in 20% and 40% of weight replacement and a very high C3A cement (white), using a 30% of weight replacement; while a low (Na2Oeq<0.5) and high alkali (Na2Oeq=1.03) cements were used in the ASR-test. After six months, the low and very high C3A cements of both illitic clays shows low expansion in sulfate media (<0.05%) for blended cement without water demand. The ASR-expansion results show that illitic calcined clays reduce considerably the expansion of high alkali cements and it is not harmful to low alkali cement, but long test time results will be conclusive.
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