STRENGTH AND DURABILITY ASPECTS OF SILICA FUME BLENDED CONCRETE IN ACIDIC ENVIRONMENT

Abstract

Acidic environment represents one of the most severe conditions to which structures may be subjected and it is found due to sub soil, industrial waste disposal, housing waste disposal, ground water, rainy water. Acidic environment is harmful to concrete as they can lead to increase in concrete volume and consequent cracking. Concrete structures are also used for storing liquids, some of which are harmful for concrete. In industrial plants, concrete floors come in contact with liquids which damage the floor. In damp conditions SO2 and CO2 and other acidic fumes present in the atmosphere affect concrete by dissolving and removing part of the set cement. In fact, no Portland cement is acidic resistant. Concrete is also attacked by water containing free CO2. Sewerage water also very slowly causes deterioration of concrete. Sulphates are generally found in ground water and sub-soil. Sea water also contains large quantity of sulphates. Sulphates can be naturally occurring or could be as a consequence of industrial waste disposal. Calcium, sodium, magnesium and ammonium sulphates (increasing order of hazard) are harmful to concrete as they can lead to increase in the concrete volume and consequent cracking. Calcium sulphate reacts with calcium aluminate present in cement hydrates forming an expansive ettringite. Sodium sulphate reacts with calcium hydroxide and forms expansive gympsum in presence of aluminates and may turn lead to formation of ettringite. Magnesium sulphate reacts with cement compounds decomposing the cement itself and subsequently producing gypsum and ettringite. For minimizing the danger of sulphate attack, low C3A content are recommended. Sulphate resisting cement with very low C3A is most suitable. However, if chlorides are also present in the ground water and sub soil in addition to sulphate then it is not recommended for the use in view of the vulnerability of low C3A cement to chloride ion diffusion. Blended cements are x most preferred when both sulphates and chlorides are present in an environment at the same time. Blended cements have low C3A content and also enable production of pastes containing small amount of calcium hydroxide. The pozzolana cements have also shown high sulphate resistance which is probably due to the composition and the structure of the pores in hydrated pastes. The resistance of concrete to sulphate attack can be tested by storing the specimens in a solution of sodium or magnesium sulphate or in a mixture of these two. Using of silicafume in concrete it improves very high strength and durability characteristic of hardened concrete. In the present study the influence of acidic environment with time on the mechanical behaviour of concrete has been investigated. The cement concrete produced with silicafume replaced with 5% and 15% of cement quantity of grade M-35 and M-45 concrete. Super plasticizer is used in concrete mix to make high workability of slump (100-120 mm). These mixes are used to prepare specimens of cube of size 150 x 150 x 150 mm, cylinders 150 mm diameter and 300 mm height, beams 100 x 100 x 500 mm Half nos. of specimens are cured in potable water and half nos. of specimens are cured in 0.2 N H2SO4 Acidic water for 7 days, 28 days and 90 days. Then the mechanical properties are determined which includes compressive strength, flexural strength, split tensile strength, loss in mass, rebound number and ultrasonic pulse velocity test. It is concluded that compressive split tensile strength and flexural strength of the specimen are reduced after acidic environment exposure.