Strength and mineralogical behaviour of lime-GGBS-treated acid sulphate soils

Acid sulphate soils (ASSs) exist along the coastline of Australia where the majority of population lives. As Australia’s population is growing rapidly, the coastal cities are experiencing significant expansion which is fuelling construction of new infrastructure (roads, buildings, bridges, buried pipelines etc.). Construction activities on ASSs are challenging due to their low shear strength and high pyrite (FeS2) content, which affects the strength of concrete and treated ASS used as founding media. This study investigates the viability of a soil mixing technique, an environmentally friendly and economical method, as a sustainable solution to foundation problems associated with the ASSs. This study deals with the investigation of the strength and mineralogical behaviour of lime-GGBS treated ASSs. Two types of ASSs are investigated, namely a synthetic ASS prepared by mixing commercially available kaolin and bentonite with pyrite powders, and a natural ASS sourced from the Melbourne CBD, locally known as Coode Island silt. The research first investigates the effects of curing environments on the strength and mineralogical developments of the treated synthetic ASS, followed by a detailed study of the strength, compressibility and mineralogical behaviour of the treated synthetic and natural ASSs. Finally, the sequestration potential of an innovative carbon-negative material, biochar, produced from the slow pyrolysis of green waste in treated soils is investigated. Unconfined compressive strength, 1-D consolidation and basic properties tests (pH, water content, dry density) are conducted to investigate the mechanical behaviour of treated ASSs. X-ray diffraction (XRD) analysis with mineral quantification using the Rietveld refinement method is conducted to study the mineralogical developments of treated soils. Scanning electron microscopy (SEM) with secondary and back-scattered electron imaging is carried out to investigate the poorly crystalline minerals which are unidentifiable by XRD analysis. Energy dispersive spectrum (EDS) analysis is conducted to determine the chemical composition of minerals traced in SEM. This study finds that curing temperature has a substantial effect on the strength behaviour of lime-GGBS treated synthetic ASS, and samples cured at temperature ranges from 19-22°C developed sustainable strength. Moreover, a GGBS-to-lime ratio of 1.5 is effective in developing strength over the 365 day curing period investigated. The addition of greater amounts of pyrite is found to retard the strength development of treated synthetic ASS at lower dosage levels of GGBS, whereas, for the natural ASS the effect of pyrite is found to be favourable at lower dosage levels of GGBS. On the other hand, when greater quantities of GGBS are used, considering the strength development, the presence of greater amounts of pyrite is found to favour the synthetic ASS. Interestingly, for greater GGBS contents, the presence of greater amounts of pyrite shows degradation of strength at longer curing periods for both synthetic and natural ASS. A study of the compressibility behaviour of lime-GGBS-treated ASS has revealed that the influences of different experimental variables on the yield strength are very similar to their influences on the unconfined compressive strength (UCS) values. An increase of GGBS content and curing period increases the yield strength of treated soils, accompanied by a relatively high rate of destructuration. This study also finds a very insignificant effect of pyrite content on the compressibility characteristics of treated soils. The analysis of mineralogical data of treated ASS shows that there is a good correlation between the changes of mechanical properties and their mineralogical developments in terms of the formation of new minerals and their quantities. In general, the increase of strength is associated with the increase of cementitious minerals. This study establishes a link between the formation of deleterious sulphate minerals (i.e., thaumasite) and the degradation of the UCS of soils using high resolution XRD traces and backscattered SEM imaging techniques. Finally, the study focuses on the reduction of the consumption of lime for the stabilisation of ASSs. This study shows that the sequestration of biochar in stabilised ASS not only substantially decreases the amount of lime required for the stabilisation of soils but also permanently buries carbon in soils, which makes stabilisation of soils using chemical additives a greener method.