AbstractTraditionally, Ordinary Portland Cement (OPC) has been adopted as a binding agent over the past six decades to improve the engineering qualities of soft soils. However, the significant negative environmental impacts associated with their usage besides their potential to cause sulphate heaving of stabilised soils is a global concern. Therefore, this research was concerned with expansive clay stabilisation by partial substitution of OPC and incorporation of minimal quantities of a nanotechnology-produced additive called RoadCEM.
Industrial mix specification guidelines recommend usage of very minimal quantities (1-2% by weight of cement) of RoadCEM to improve the hydration property of OPC in soil stabilisation. However, in order to further reduce the larger OPC proportion that can be potentially used, this research utilised GGBS as a complementary cementitious and environmentally-friendly material while maintaining the industry recommendation of 1% of RoadCEM in the binder mixture. Hence, an investigation into the firmly established sustainability credentials of RC vis-à-vis its potential impact on mostly the volume change and hydromechanical behaviour of stabilised soils formed the major thrust of this study.
Preliminary studies carried out on five expansive soils (having differing Na-montmorillonite contents), enabled two model soils of extreme plastic properties to be adopted for the application of the stabilising agents with the incorporated RoadCEM. OPC proportion in the binder mixtures was reduced by up to 50% and evaluation of the effect of RoadCEM on the geotechnical properties of the stabilised soils was performed. Index property and engineering tests (swelling, consolidation, strength and suction) were employed to study the geotechnical characteristics of both the natural and stabilised soils. Microscopic examination adopting techniques such as SEM, EDS and XRD were also carried out to observe the mechanisms of change in the stabilised samples.
Results of preliminary studies involving an application of the particle size analyser (Mastersizer) and a grain size statistics programme (GRADISTAT) confirmed the presence of clay-sized samples as the major cause of expansive soil swelling within a 95% confidence interval. However, the variability in the engineering properties of the clays when stabilised by OPC was minimal and did not correspond to the percentage proportion of the clay-sized Na-montmorillonite particles present in the clays. Addition of RC to the soil-binder mixture reduced soil swelling to zero with 50% OPC replacement. Furthermore, 50% of OPC substituted by the by-product possessing RC led to more decreased settlement as compared to the stabilised soil without RC inclusion. RC additive caused a reduction in compression index, Cc of approximately 80% in 7 days but with an even greater reduction of about 90% occurring at 28 days of curing. Results of higher water soil moisture retention capacity where exhibited by RC-modified soils and are promising for contaminant encapsulation especially in dredging activities. The most short-term strength development observed with the use of RC was attributed to 40% of the OPC reduced in the soil-binder mixture over 28 days curing duration. Results also indicated the efficacy of RC in reversing sulphate heaving as compared to the stabilised mix containing only the cementitious products. The evolution of extended crystallisation in the hydration mechanism of the stabilised products with the incorporated RC as revealed by SEM indicated a “wrapping” (matrix of interlocking filaments) structure – a phenomenon which is responsible for the improved mechanical properties of the stabilised soils.
|Date of Award||Oct 2020|
|Supervisor||Samson Ngambi (Supervisor), Eshmaiel Ganjian (Supervisor) & Eoin Coakley (Supervisor)|
- ground granulated blast furnace slag
- soil stabilisation
- sulphate-bearing soil
- unconfined compressive strength
- compression index
- soil water characteristic curve