Carbon capture potential in modified soil

Abstract

Clay soils present a range of challenges in geotechnical engineering. In addition to addressing the problematic nature of clay soils concerning ground stability, geotechnical engineering has a role in the context of climate change. As far as possible, geotechnical design should mitigate the effects of increases in carbon dioxide (CO2) and other greenhouse gases (such as methane, nitrous oxide, fluorinated gases), that are released to the atmosphere, thus causing the Earth to become warmer. Lime modification of clay soils has attracted a significant amount of interest, due to its potential to improve soils for construction purposes. This tackles the issue of waste reduction, reduces the need for imported fill, and thereby reduces the CO2 emissions associated with the traffic movements. However, the production of lime itself produces additional CO2 emissions. This thesis addresses the use of lime (Ca(OH)2) in ground stabilisation, assessing the associated formation of calcium carbonate (CaCO3) and the extent to which this can mitigate the CO2 emissions associated with the production of the Ca(OH)2 without overly severe impacts on engineering properties. Experimental treatment of kaolin with lime shows that average carbonate content values from 4.70-10.08% dry mass of CaCO3 for 4-8% Ca(OH)2 contents in samples at 10% air voids were achieved, with a maximum recovery of 93% of CO2 lost during lime manufacture. Based on 7 days cured specimens with a combination of 6% Ca(OH)2 and 10% air voids content, a compressive strength development of 280 kPa was achieved for carbonated treated kaolin, compared to 170 kPa for non-carbonated equivalents which is a substantial increase in strength of approximately 60%. This strength is equivalent to California bearing ratio (CBR) value of 29 %, greater than the minimum CBR required for a stabilised capping layer (15%), suggesting that carbonated treated kaolin is suitable for use as a stabilised capping layer. The increases in strength and stiffness for saturated carbonated lime treated specimens are much reduced compared to what might be predicted from the literature for some non-saturated noncarbonated equivalents. However, the increases are sufficient for application to capping layers. The freeze-thaw (FT) resistance for carbonated treated kaolin was found to be approximately 24%, and is suggested sufficient for a capping layer, when viewed in the context of the less ii stringent requirements for FT durability for capping material. Treated kaolin compacted to air voids content from 3% to 15% achieves permeability values of 1.8 × 10-9 m/s to 7.4 × 10-9 m/s. Another research focus in this thesis is to use imaging techniques to detect and quantify the amount of CaCO3 formed and the voids content of the carbonated kaolin sample. X-ray computed tomography (XRCT) analysis, using ImageJ software, showed the presence and distribution within the clay sample of CaCO3 and air voids. Furthermore, using this technique it was possible to quantify the air voids content and the amount of CaCO3 formed, with good agreement with chemical methods (calcimeter, TGA). At 8% Ca(OH)2, 25% air voids, the highest amount of carbonate content of 9.82 ± 0.06% was detected by the XRCT. The presence of CaCO3 formation in carbonated soils may be determined using the scanning electron microscope (SEM). Based on SEM results, calcium carbonate grains of about 2-3 µm in size were found on the surface of the kaolin. The results of this study have shown that this method of combined modification and carbonation treatment of clays has the potential to offset up to 93% of the CO2 released from lime production for stabilisation (representing 0.03% global CO2 emissions), alongside improving the compressive strength of the clays. This could be effectively used for a combined carbon capture function and engineering function such as the capping layer in road pavement. A design specification for carbon capture and ground improvement is developed, assessing the benefits in terms of carbon sequestration. If combined modification and carbonation is to be adopted in practice, then an addendum needs to be included in the specification (such as the Highways Agency, 2007; MWCH 1, 2009: Series 600) for limestabilisation for engineering purposes. Compaction requirements to achieve 10% air voids would give combined strength and carbon capture benefits. The combined modification and carbonation application to lime treated clay has shown the potential to mitigate climate change alongside ground stability improvement of soft clay.