építôanyag építôanyag  Journal of Silicate Based and Composite Materials Expansivity mitigation of black clay soil using agro-waste based inorganic polymer cement for flexible pavement subgrade anigilaje bunyaMin SALAHUDEEN  Department of Civil Engineering, University of Jos, Jos, Nigeria  bunyamins@unijos.edu.ng Érkezett: 2022. 07. 04.  Received: 04. 07. 2022.  https://doi.org/10.14382/epitoanyag-jsbcm.2023.04 Abstract The use of cement for deficient black clay soil (BCS) improvement is a long time practice. But given the huge negative contribution of cement to carbon emission and climate change and its high demand by the construction industry that led to its scarcity and expensiveness, there is a need for an alternative binder to cement. The possible use of two biomass based inorganic polymer cements (IPC) was investigated herein. Rice husk ash (RHA) and sawdust ash (SDA) from biomass were used for this study. All experimental tests were carried out in accordance with British Standards. Soil samples were mixed with ash-IPC in steps of 0, 5, 10, 15 and 20% of the dry weight of the natural black clay soil. Results indicated significant improvement in all the geotechnical properties tested. The percentage passing the #200 sieve was reduced from 76.25% to 24.34% and 35.51% by RHA-IPC and SDA-IPC respectively at 20% treatment. Peak UCS values of 1123.56 and 954.28 kN/m 2 were respectively recorded for RHA-IPC and SDA-IPC treatments at 20% IPC content and at 28 days curing period. That represents an 813% increment in UCS value at 20% RHA-IPC content and 675% for SDA-IPC. The CBR value increased by 1500 and 1233% respectively for RHA-IPC and SDA-IPC treatments. The Scanning electron microscopy and energy dispersive X-Ray spectrometer EDS results indicated obvious improvement in the particle sizes in the microstructure of the treated BCS. The achieved improvements in all parameters tested are indications that the expansivity of the weak BCS has been reduced to acceptable levels and the effectiveness of using RHA-IPC and SDA-IPC for subgrade soils improvement. Keywords: expansive clay, inorganic polymer cement, sawdust ash, rice husk ash, geopolymer cements, flexible pavements Kulcsszavak: expanzív agyag, szervetlen polimer cement, fűrészpor hamu, rizshéj hamu, geopolimer cementek, rugalmas burkolatok Dr. Anigilaje Bunyamin SALAHUDEEN is a Senior lecturer and researcher at University of Jos, Jos-Nigeria. He obtained his Doctor of Philosophy degree in Civil/Geotechnical Engineering from Ahmadu Bello University. He has co-authored over 70 publications and was certified as Civil Engineer by the Council for the Regulation of Engineering in Nigeria (COREN) in 2013. His research interest include problem soil stabilization using agro-industrial wastes, biomass and geopolymer cements; numerical modelling of geotechnical problems, geotechnical application of artificial intelligence and general computational geomechanics. He is presently researching on solid waste valorisation; alternative and sustainable construction materials and climate change challenges. 1. Introduction Tese days, suitable lands for engineering construction works in urban and metropolitan areas are scarce and expensive. Te need for land for the purpose of housing and urban road networking has drastically increased due to the increase in population and migration to urban areas. Tis makes inevitable the utilization and construction on weak subgrades that are geologically characterized by poor geotechnical properties with low strength characteristics and high compressibility behavior. Improvement of these weak subgrade soils has proved to make more economic sense than replacing them with foreign competent materials [1]. Tis case becomes an issue of major concern when it comes to road networking around urban areas due to the high volume of materials required and haulage constraints, discomfort to the commuting vehicles and expensiveness. In the past, cement was the most commonly used soil improvement-agent. Cement has been used successfully to improve the strength and consistency properties of weak subgrade soils in both cold and hot climates [2-3]. However, due to the huge negative contribution of cement to climate change and emission of greenhouse gasses and its high demand in the construction industry that led to its scarcity and expensiveness, there is an emergency need for alternative binders to cement. A promising, efcient and eco friendly alternative to cement could possibly be agro-industrial waste ash inorganic polymer cements such as rice husk ash-inorganic polymer cement (RHA-IPC) and sawdust ash-inorganic polymer cement (SDA-IPC). Black clay soils (BCS) are inorganic clays of high compressibility. BCS is very hard when dry but completely loses its strength when soaked in water. Tey are characterized by high shrinkage and swelling properties. According to Salahudeen and Akiije [4], the poor geotechnical properties of BCS have made construction on or with them an extreme challenge. Upon drying, cracks of varying widths and depths develop in black clays. Due to wetting and drying processes, heaving and shrinkage take place in the soil deposit. Tese vertical movements lead to failure of structures and fexible pavements built with or on them. Tese failures appear in the form of settlement, heavy depression, routing, cracking and unevenness. BCS are highly expansive with free swell index of over 50% and extremely weak when wet having a CBR value of less than 3% grossly leading to road network distortion as a result of seasonal variation in moisture condition [5-7]. In this study the improvement of BCS was undertaken using two agricultural waste materials: rice husk ash and sawdust ash with alkaline activators. Rice husk is a biomass 26 | építôanyag építôanyag  JSBCM JSBCM  2023/1  Vol. 75, No. 1