Pemanfaatan Limbah Bottom Ash sebagai Substitusi Agregat Halus untuk Mengurangi Pasir Alam pada Beton

Abstract

The demand for natural fine aggregate continues to increase along with infrastructure development, while excessive sand extraction may cause adverse environmental impacts. Meanwhile, coal combustion in power plants generates coal bottom ash (CBA), which has the potential to be utilized as an alternative material in concrete. This study aimed to investigate the effect of CBA as a partial substitute for fine aggregate on the properties of fresh and hardened concrete. The novelty of this study lies in an integrated evaluation of CBA as the sole fine aggregate substitute, without pozzolanic additives, in normal concrete of 25 MPa design strength, relating the reductions in slump and unit weight and the increase in water absorption to the permissible substitution level for structural and non-structural applications. A laboratory experimental method was employed using CBA substitution levels of 0%, 10%, 20%, 30%, and 40% by weight of fine aggregate in concrete with a design compressive strength of 25 MPa. The investigated parameters included slump, fresh concrete unit weight, compressive strength at 7, 14, and 28 days, and water absorption at 28 days. The results showed that increasing the CBA content reduced the slump from 85 mm to 48 mm and the unit weight from 2,380 kg/m³ to 2,225 kg/m³ at the 40% substitution level. The 28-day compressive strength increased from 26.2 MPa for the control concrete to 27.4 MPa and 28.1 MPa for the 10% and 20% substitution levels, respectively, and subsequently decreased to 24.3 MPa and 20.5 MPa at 30% and 40% substitution, respectively. Water absorption increased from 3.8% for the control concrete to 6.5% at the 40% substitution level. The 20% CBA substitution level was identified as the optimum level solely on the basis of the highest 28-day compressive strength (28.1 MPa), although it was not statistically different from the 10% substitution (27.4 MPa) among the tested parameters, with water absorption increasing to 4.7%; it does not represent an evaluation of all sustainability aspects. The results indicate that CBA has potential for use as a partial fine aggregate substitute, although its appropriate replacement level should be determined according to the material characteristics and concrete mix design. The maximum unit weight reduction of 6.5% does not make the concrete a lightweight concrete. The contribution of this study is empirical data on the permissible CBA substitution level in normal concrete that balances strength, unit weight, and water absorption, as a basis for further durability and environmental assessment.