Secondary minerals to replace cement in stabilising an alluvium

Abstract

Ordinary Portland cement (CEM-I) is widely used across the construction industry. It is the most commonly used cementitious binder for ground improvement applications such as deep dry soil mixing (DDSM) in the UK, due to its high strength performances. However, CEM-I production is one of the world’s most energy intensive and expensive industrial processes; contributing up to 7% of the world’s total CO2 emissions (McLellan et al., 2011). Hence, there is now significant pressure on the cement and construction industries to greatly reduce their CO2 emissions by developing “greener” alternatives to CEM-I, which are both more environmentally and financially sustainable in the long-term. Alkali activated industrial waste materials, known as geopolymers have been identified as potential alternatives to CEM-I. There are numerous advantages in recycling industrial waste materials such as ground granulated blast furnace slag (GGBS) and pulverised fly ash (PFA), including avoiding the need to transfer such materials to landfill, their abundant supply, negligible or zero production costs and for calcium-bearing wastes such as slags, their recently determined potential for carbon capture and storage (CCS). This thesis presents recent laboratory research which focussed on the potential for utilising alkali activated industrial waste materials as sustainable binders in DDSM to enhance the geotechnical properties of soft soils. The laboratory testing programme deployed geotechnical and mineralogical tests to determine the performance of the binders when incorporated into a soft alluvial soil, typically found in abundance across the UK. Comparisons with the strength and durability of untreated and stabilised soils have been made. The study indicates that from the by-products tested, soils stabilised with sodium hydroxide (NaOH) activated GGBS resulted in the greatest strength and durability improvements; with other materials tested showing smaller improvements. The addition of NaOH has been observed to allow pozzolanic reactions to occur, leading to improved mechanical properties; primarily strength, which increased with time. Abstract Secondary Minerals to Replace Cement in Stabilising an Alluvium ii The effectiveness of DDSM treatment in stabilising sections of high-speed railway lines with ground conditions dominated by soft and highly compressible soils, has previously been well demonstrated (Holm et al., 2002; Hughes and Glendinning, 2004). Traditionally, the monotonic strength properties of stabilised soils have been used to assess their suitability for stabilising railway embankments. However, the dynamic strength properties of such materials require investigation in order to provide better estimates of their field behaviour when subjected to complex loading conditions associated with high-speed railway embankments and high-frequency train traffic. Hence, this thesis combines monotonic and dynamic triaxial testing techniques to assess the suitability of the new GGBS-NaOH binder for stabilising high-speed railway embankments. After 28 days curing, the binder successfully demonstrated itself as an effective countermeasure against significant track displacements after the simulated passage of a typical InterCity 125 high-speed train. This thesis advocates that there is great potential for using GGBS-NaOH as a more environmentally and financially sustainable binder over CEM-I for DDSM projects in the UK, such as the proposed HS2 and HS3 rail links. However, further research and collaboration with the construction industry is still required before the new binder may be used on a commercial scale.