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Interest in blended cements is growing because of its advantages as increased production economy, reduced CO2 emissions and fuel consumption.
Cement replaced with supplementary cementitious material fly-ash has a positive effect on the strength development, water permeability and rapid chloride penetration resistance of concrete over the curing age of 365 days.
A study of three different grades of concrete M38, M33 and M28, each with seven different fly-ash replacement level, 10%, 20%, 30%, 40%, 50%, 60% and 70% were carried out. Ordinary Portland cement concrete was also used as reference concrete. Coefficient of permeability as well as rapid chloride penetration resistance was determined at an age of 28, 56, 90,180 and 365 days whereas strength at 3, 7, 28, 56, 90, 180 and 365 days.
Test results show that permeability of concrete decreases with the increase of fly-ash level up to an optimum value and then start to increase, whereas strength of concrete increases with the increase of fly-ash up to an optimum level and then starts to decrease.
Among all the concretes studied, the optimum amount of cement replacement is reported to be 30%.
The study reveals that fly-ash concrete is less permeable and achieves higher strength due to pozzolanic activity of fly-ashes that creates more durable calcium silicate hydrate and fills capillaries and bleed water channels occupied by water soluble lime. As a result fly-ash concrete inhibits the penetration of aggressive species including chloride which can effectively reduce the corrosion of the embedded steel reinforcement.
