Civil Engineering and Architecture 13(1): 331-348, 2025 http://www.hrpub.org DOI: 10.13189/cea.2025.130120 Mechanical Properties and Durability of Concrete Containing Indonesian Coal Fly Ash Fadhilah Muslim 1,* , Rachmat Hermawan 2,3 1 Department of Civil Engineering, Faculty of Engineering, Universitas Indonesia, Indonesi 2 Department of Metallurgical and Material Engineering, Faculty of Engineering, Universitas Indonesia, Indonesia 3 PT. PLN (Persero) Head Office, South Jakarta, Indonesia Received July 21, 2024; Revised October 10, 2024; Accepted November 13, 2024 Cite This Paper in the Following Citation Styles (a): [1] Fadhilah Muslim, Rachmat Hermawan , "Mechanical Properties and Durability of Concrete Containing Indonesian Coal Fly Ash," Civil Engineering and Architecture, Vol. 13, No. 1, pp. 331 - 348, 2025. DOI: 10.13189/cea.2025.130120. (b): Fadhilah Muslim, Rachmat Hermawan (2025). Mechanical Properties and Durability of Concrete Containing Indonesian Coal Fly Ash. Civil Engineering and Architecture, 13(1), 331 - 348. DOI: 10.13189/cea.2025.130121. Copyright©2025 by authors, all rights reserved. Authors agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Abstract The use of cement, as the main construction material, is a major contributor to escalating global CO 2 emissions as cement production increases. To address this challenge and align with Sustainable Development Goals (SDGs), fly ash from boiler combustion has a potential to replace a partial percentage of cement as a waste material. In line with Indonesian Carbon neutral’s strategy, biomass cofiring of Coal Fire Power Plant (CFPP) also contributes to net-balanced CO 2 emissions. This research explores the potential benefits of incorporating fly ash derived from coal and biomass co-firing at power plants on the properties of concrete. This paper presents a comprehensive investigation of the impact of co-firing on the mechanical properties and durability of concrete. Three concrete mixes prepared using Portland Cement CEM I and cement replacement by 10%, 20% and 30% fly ash (FA) were subjected to rigorous testing, including assessments of density, porosity, sorptivity, and electrochemical analysis. The results revealed that replacing 10% of cement with fly ash yielded the most optimal outcome, with the concrete exhibiting remarkable corrosion resistance. Specifically, it exhibited a density of 2.226 kg/m 3 , porosity of 26%, sorptivity of 0.3 mm/s 0.5 , and a corrosion rate of 0.26 mm/year. Additionally, the Electrochemical Impedance Spectroscopy (EIS) test demonstrated enhanced corrosion resistance of the reinforcing steel across all variations of concrete specimens. This was further confirmed by the carbonation test, which indicated the absence of carbonation (indicated by pink areas). In conclusion, the utilization of fly ash derived from coal and biomass co-firing in coal-fired power plants presents a promising alternative to traditional cement in concrete. This alternative material offers superior properties compared to concrete composed solely of Ordinary Portland Cement (OPC). Keywords Fly Ash, Cofiring, Concrete, Mechanical Properties, Corrosion Resistance, Durability 1. Introduction In terms of supporting the Sustainable Development Goals (SDGs) to accelerate the target of the new and renewable energy mix of 23% by 2025 [1], PT PLN (Persero), an electricity national company in Indonesia has conducted a strategic program CFPP. The fuel used is based on a mixture of biomass and coal which has the criteria of being easy, clean, sustainable, and based on people's economy. For this reason, Suralaya CFPP uses sawdust as a biomass base which is used with a mixture of 5% biomass and 95% coal. In the process of burning coal, cofiring CFPP will produce fly ash which is classified as Fly Ash and Bottom Ash (FABA). FABA from the combustion process at CFPP is non-B3 waste according to government regulation number 22 of 2021 about the implementation of environmental protection and management [2]. Fly ash also could be utilized for construction raw materials such