PEER-REVIEWED ARTICLE bioresources.com Keskin et al. (2023). “Blast furnace slag for soils,” BioResources 18(1), 2045-2063. 2045 An Environmentally Friendly Approach to Soil Improvement with by-Product of the Manufacture of Iron İnan Keskin, a İbrahim Şentürk, a Halil İbrahim Yumrutaş, a, * Ermedin Totiç, b and Ali Ateş c Blast furnace slag has been used for many years in various applications related to civil engineering. Many studies have created a wide variety of cost-effective and environmentally friendly solutions for this industrial by- product. This study aims to contribute to the performance evaluations of the usability of the blast furnace slag for soil improvement and the effects of the additive ratio and curing time. Bentonite samples were prepared with the addition of blast furnace slag at 5%, 10%, 15%, and 20% ratios by weight at optimum water content (wopt). Results were evaluated using the liquid limit, plastic limit, unconfined compressive strength, and swelling tests performed after 1, 7, 14, and 28 days of curing time. Results revealed that the liquid limit value decreased, and the unconfined compressive strength increased with increasing curing time and blast furnace slag ratio in the mixture. Additionally, swelling pressure generally decreased with increasing slag contribution and curing time. The lowest values of the unconfined compressive strength were observed on the 7 th day of curing time, and the minimum value was obtained at 10% mixing ratio. The highest unconfined compressive strength values were observed on the 28 th days of curing time. The optimum mixing ratio was 5%. DOI: 10.15376/biores.18.1.2045-2063 Keywords: Waste material; Environmentally friendly; Slag; Bentonite; Soil improvement Contact information: a: Department of Civil Engineering, Karabuk University, Box 78050, Karabuk, Turkey; b: Department of Civil Engineering, Bartın University, Box 74110, Bartın, Turkey; c: Department of Civil Engineering, Abant İzzet Baysal University, Box 14030, Bolu, Turkey; * Corresponding author: iyumrutas@karabuk.edu.tr INTRODUCTION Waste material management and disposal are global environmental endeavors. Due to the rapid growth of the population, urbanization, economic growth, and increasing consumption patterns, the problem has attracted the attention of scientists, governments, institutes, and companies. Industrially developed countries have made significant efforts to overcome this matter. However, it is obvious that the problem is growing steadily, and more action needs to be implemented. The USA created 292.4 million tons of municipal solid waste in 2018. The corresponding number for 27 European countries is approximately 69 million tons. The amount of waste materials seems to be increasing incessantly. The generation of solid waste materials is predicted to increase by about 70% and to reach 3.4 billion metric tons all over the world according to various projections (EPA 2018; Eurostat 2018; Statista 2020; Van et al. 2021). Blast furnace slag is a by-product of the manufacture of iron. This industrial by- product also has great potential to be used as a stabilizing agent in civil engineering. The world generated about 100 million tons of blast furnace slag (Mehta 1999). The global