Recent studies on water retention behaviour of biochar amended soil rarely considers the effect of pyrolysis temperature and also feedstock type into account. It is well known that pyrolysis temperature and feedstock type influences the physical and chemical properties of biochar due to stagewise decomposition of structure and chemical bonds. Further, soil density, which is in a loose state (in agricultural applications) and dense (in geo-environmental engineering applications) can also influence water retention behaviour of biochar amended soils. The major objective of this study is to investigate the water retention properties of soil amended with three different biochars in both loose and dense state. The biochars, i.e. water hyacinth biochar (WHB), chicken manure biochar (CMB) and wood biochar (WB) were produced in-house at different pyrolysis temperature. After then, biochars at 5% and 10% (w/w%) were amended to the soil. Water retention behaviour (soil suction and gravimetric water content) was studied under drying and wetting cycle simulated by varying relative humidity (RH, 50–90%). Results show that 10% WHB produced at 300 °C were found to possess highest water retention. CMB is found to possess higher water retention than WB for 10% amendment ratio. In general, the addition of three biochars (at both 300 °C and 600 °C) at 10% (w/w) significantly improved the water retention at all suction ranges in both loose and dense compaction state as compared to that of the bare soil. The adsorption (wetting) and desorption (drying) capacity of biochar amended soils is constant at corresponding RH.

The application of industrial waste in construction reduces the dependency on natural resources. The materials, including red mud (RM) and fly ash (FA), proved to be favorable materials. However, the materials potential together as a geopolymer composite for road applications has rarely been explored. This study will examine the possibility of the replacement of natural materials in subgrade applications. To achieve this, the geopolymer compositions will be prepared by replacing RM with FA at replacement rates of 10%, 20%, and 30% by dry weight basis. The alkaline activator solution of 8 M will be prepared using sodium hydroxide (NaOH) and sodium silicate to develop geopolymer composites. The strength properties will be studied using the California Bearing Ratio (CBR) and unconfined compression strength (UCS) and validated with microstructural analysis using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR). The results reveal that geopolymer composites could achieve a maximum CBR value of 12% and UCS of 2,700 kPa. The microstructural analysis revealed that the formation of dense calcium aluminate hydrate (C-A-H) and calcium silicate hydrate (C-S-H) are the reason for strength improvement. The leaching studies show that the toxic elements were within the permissible limits. Overall, the test results confirmed that the geopolymer composites meet the required strength and could be used as a subgrade material in road construction.

Previous studies revealed that the geotechnical properties of red mud waste (RMW) closely resemble that of conventional soils. Due to the predominant presence of exchangeable sodium, the RMW tends to exhibit high-dispersion behaviour. In this study, mitigation of the dispersion characteristics of RMW using eco-friendly biopolymers was investigated. Two types of biopolymers, namely guar gum (GG) and xanthan gum (XG), in 0·5, 1, 2, 3 and 4% (by weight), were selected for the study. The untreated and polymer-amended RMW samples were subjected to cylindrical dispersion crumb tests for examining the dispersion phenomenon, and the hardened samples were further analysed for microstructure, mineralogical and elemental compositions. Turbidity and pH of water in the crumb tests as well as consistency limits of untreated and polymer-amended RMW were also tested. The microstructural analysis revealed an increase in particle aggregations with an increase in the biopolymer content, while elemental compositions showed gel formation by GG cross-linking with calcium. As such, GG proved to be more effective over XG in mitigating the dispersion characteristics of the RMW. Overall, this study shows that GG biopolymer has potential to be used for mitigating dispersion and controlling dusting in field applications.