Available online at www.CivileJournal.org Civil Engineering Journal (E-ISSN: 2476-3055; ISSN: 2676-6957) Vol. 10, No. 11, November, 2024 3603 Effect of Coal Combustion Waste on Cement-Treated Clay Soewignjo A. Nugroho 1* , S. P. Retno Wardani 2 , A. S. Muntohar 3 , Syawal Satibi 1 1 Department of Civil Engineering, Faculty of Engineering, Universitas Riau, Pekanbaru, Indonesia. 2 Department of Civil Engineering, Faculty of Engineering, Diponegoro University, Semarang, Indonesia. 3 Department of Civil Engineering, Engineering Faculty, Universitas Muhammadiyah Yogyakarta, Yogyakarta, Indonesia. Received 16 January 2024; Revised 05 October 2024; Accepted 12 October 2024; Published 01 November 2024 Abstract High plasticity clay is soil with poor material characteristics; one of them is the large shrinkage condition due to its high plasticity. Therefore, it is necessary to improve the soil using cement and coal-combustion waste (CCW). The purpose of this research is to determine the effect of mixture addition on the CBR value of the specimen on stabilization. Variations of 6%, 9%, 12%, and 15% of CCW and cement content vary by 3% and 5%. Based on the test results, there is an increment in the CBR bearing capacity, which was initially 0.80% to 18.75% to 42.90% by adding cement and CCW between 6% and 15%, respectively, after 7 days of curing. This increment is the largest of all variations in the CBR test. The percentage of increment in CBR value is quite large, i.e., a multiple of 200. This proves that the mixture is able to work effectively in increasing the bearing capacity of the soil. Based on microscopic testing of CBR samples, it is obtained that the higher the levels of Bottom Ash Fly Ash (BAFA) and cement, the rougher and paler the samples became as the structure changed. And vice versa, the lower CCW content and cement content made the sample structure become smoother and brighter in color, closer to the original color of natural clay. Keywords: Bottom Ash; Fly Ash; Cement; High Plasticity Clay; Stabilization. 1. Introduction Coal Combustion Waste (CCW) is a type of additional material (additive) obtained from coal residue or waste material from burning coals. The main difference between bottom ash and fly ash (BAFA) is the amount of compound elements such as calcium, silica, aluminum, and iron. According to data released by the Ministry of Energy and Mineral Resources in 2019, national coal production was 548 million tons, and the amount of coal production in 2018 resulted in BAFA waste reaching 5% to 6% of total production, or 27 million tons to 37.4 million tons. Several companies in Riau Province, both national and private companies, use coal as their main fuel, including PT IKPP, PT RAPP, and PLTU Tenayan. Nainggolan & Muhardi [1] and Indriyati et al. [2] studied the chemical composition of fly ash (FA) and bottom ash (BA) from PT IKPP, with the findings as displayed in Table 1. The CBR value has a direct proportional relationship to the degree of density. When the CBR value is greater, it tends to have higher soil density. The similarity of their density degree demonstrates the relationship between CBR in the field and CBR in the lab. In order to obtain them, use Equations 1 and 2, and Table 2 shows the qualitative value of the density degree. CBR lab = P (0.1′/0.2′) P standard × 100% (1) * Corresponding author: nugroho.sa@eng.unri.ac.id http://dx.doi.org/10.28991/CEJ-2024-010-11-010 © 2024 by the authors. Licensee C.E.J, Tehran, Iran. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).