Viability of Utilizing Paddy Straw Ash as a Supplementary Cementitious Material in M20 Concrete

Abstract

This study evaluates the feasibility of using paddy straw ash (PSA) as a supplementary cementitious material in M20 grade concrete. Paddy straw was converted into ash through controlled combustion at 600 ± 50 °C for 2 h, followed by grinding and sieving through a 90-micron sieve. Eleven concrete mixtures were prepared by replacing cement with PSA from 0% to 20% at 2% intervals. Fresh and hardened properties were assessed using slump, compressive strength at 7, 14 and 28 days, ultrasonic pulse velocity at 28 days, and drying shrinkage measurements. The incorporation of PSA progressively reduced workability, with slump decreasing from 95 mm in the control mix to 45 mm at 20% replacement, indicating increased water demand due to the porous and finer nature of the ash. Compressive strength improved up to an optimum replacement level of 8%, where the 28-day strength reached 36.95 MPa compared with 32.40 MPa for the control mix, corresponding to an increase of 14.04%. At higher replacement levels, strength gradually decreased, although all mixes achieved compressive strength above 20 MPa at 28 days. Ultrasonic pulse velocity followed a similar trend, with the highest value of 4.38 km/s recorded at 8% PSA, indicating improved internal compactness at moderate replacement levels. Drying shrinkage decreased consistently with increasing PSA content, from 585 microstrain in the control mix to 453 microstrain at 20% replacement. The results indicate that PSA can be used effectively as a partial cement replacement in M20 concrete, with 8% replacement providing the most balanced performance in terms of strength, quality and dimensional stability.