Effect of Coal Bottom Ash on Strength and Durability Properties of Concrete

Abstract

In India, about 35 million tons of coal bottom ash is produced annually. It is dumped on land adjoining sites of thermal power plants and is becoming environmental hazard to surrounding community. The productive use of coal bottom ash is the best way to alleviate the problems associated with its disposal. The present research work has been carried out to evaluate the feasibility of utilization of coal bottom ash as fine aggregate in concrete. Concrete ‘A’ made with sands having fineness modulus of 1.97 and concrete ‘B’ made with sands having fineness modulus of 2.58 were designed to achieve 28-day compressive strength of 38 and 34 N/mm2, respectively. In both concretes, river sand was replaced with coal bottom ash at 20, 30, 40, 50, 75 and 100% levels. Properties such as workability, bleeding and air content of fresh concrete were evaluated. Tests for compressive strength, splitting tensile strength, modulus of elasticity, water absorption and water absorption, pulse velocity, chloride permeability, abrasion resistance, sulphate resistance, acid resistance and drying shrinkage were performed up to the age of 365 days. Test results show that workability and water loss through bleeding decreased whereas entrained air content increased on inclusion of coal bottom ash as replacement of sand in both concretes tested. For concrete ‘A’ and ‘B’, unit density decreased almost linearly with increase in coal bottom ash content. At early curing age of 7 days, bottom ash concrete mixtures achieved lower compressive strength. 28-day compressive strength of bottom ash concrete mixtures varied between 35.0 and 37.5 N/mm2 for concrete ‘A’ and between 26.7 and 32.2 N/mm2 for concrete ‘B’ as compared to 38.20 and 34.0 N/mm2, respectively, of control concrete mixtures. With age, compressive strength of bottom ash concrete mixtures increased at a faster rate than that of control concrete mixtures. For both concretes ‘A’ and ‘B’, after 180 days, compressive strength of bottom ash concrete mixtures was approximately equal to that of control concrete mixtures. 28-day splitting tensile strength of bottom ash concrete mixtures varied between 2.72 and 2.96 N/mm2 for concrete ‘A’ and between 2.16 and 2.52 N/mm2 for concrete ‘B’. However, 28-day splitting tensile strength of control concrete was 2.67 N/mm2 for concrete ‘A’ and 2.57 N/mm2 for concrete ‘B’. For both concretes ‘A’ and ‘B’, after 90 days, splitting tensile strength of bottom ash concrete mixtures was either equal or more than that of respective control concrete mixture. Modulus of elasticity of bottom ash concrete mixtures was in the range of 27.6 - 23.1 GPa for concrete ‘A’ and 25.4 - 21.3 GPa for concrete ‘B’ against 29.2 and 25.4 GPa, respectively, of control concrete mixtures. Inclusion of coal bottom ash in concrete resulted in better dimensional stability. The effect of coal bottom ash on pulse velocity values of concrete was not significant. Sorptivity of bottom ash concrete mixtures increased with increase in coal bottom ash content. However, the chloride permeability of bottom ash mixtures was comparable to that of control concrete mixtures. It is evident from the test results that after 28-day of curing age, compressive strength, resistance to chloride ion penetration and resistance to abrasion increased significantly due to pozzolanic action of coal bottom ash. Abrasion resistance of bottom ash concrete mixtures was lower than that of control concrete mixtures. Bottom ash concretes of both grades of concrete experienced higher expansion strains than control concretes under external sulfate attack but the total strains were much less than 0.1% at which compressive strength starts decreasing. As such, no loss in 28-day compressive strength was observed. The performance of bottom ash concrete mixtures under external sulphuric acid attack was identical to that of control concrete mixtures. Micro-structural analysis (scanning electron micrograph) reveals that pozzolanic activity of coal bottom ash in bottom ash concrete mixtures started after 28-day of curing age. XRD spectrums indicate that the phase composition of powder concrete paste was not changed qualitatively, however, the change in phase proportions was observed on use of coal bottom ash in concrete.