Experimental and Sustainability Assessment of Geopolymer-Stabilized Highway Subgrades Using Industrial By-Products

Author

Adedeji Adekunle Disu, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2023

Degree Name

Doctor of Philosophy

Department

Engineering Science

First Advisor

Kolay, Prabir

Abstract

Geotechnical engineers are often faced with problematic subgrades due to highly compressible fine-grained soils which have been stabilized with conventional lime and ordinary Portland cement (OPC) resulting in improved strength and volume-change properties. However, the huge amount of Carbon dioxide (CO2) emissions from lime and OPC production have posed serious environmental concerns, thereby necessitating the discovery of eco-friendly stabilizers known as Geopolymers. Geopolymers are alkali-activated binders that utilize industrial by-products rich in Silica (Si) and Aluminum (Al) to enhance the engineering properties of weak cohesive soils commonly found in subgrade layers of highway pavements. Despite the availability of extensive literature on various alkali precursors used for geopolymers, there are few references on combinations of Fly ash (FA), Lime sludge (LS) and Ground Granulated Blast furnace Slag (GGBS) based geopolymers. More importantly, with the growing need to incorporate sustainability assessment in geotechnical construction procedures, Life Cycle Assessment (LCA) of geopolymer-stabilized soils is very scarce, if not unavailable. With the enormous annual disposal cost of $90 million required for Lime sludge (LS) generated from water treatment plants in United States, the use of LS as a co-stabilizer with FA will enhance sustainability. Hence, the focus of this research is segmented into two phases: experimental feasibility of FA-LS and FA-GGBS based geopolymers in improving the geotechnical properties of commercially available Edgar Plastic Kaolin (EPK) clay, while the second phase is to assess the sustainability benefits of FA-LS as a replacement for traditional lime and FA-GGBS using the Sustainability Index (ISus) approach, thereby reducing the carbon footprint on the environment. Geopolymer-stabilized EPK samples were characterized with Unconfined Compressive Strength (UCS) testing, volumetric strain, Scanning Electron Microscope (SEM), X-ray diffraction (XRD), and Fourier Transform Infrared (FTIR) spectroscopy. The geopolymer-stabilized EPK samples were prepared using 10 M NaOH (sodium hydroxide) with Na2SiO3 (sodium silicate) as alkali activators with the ratio of Na2SiO3: NaOH = 1.5, while the stabilized soil samples were cured under Room Temperature RT (23°C) and Elevated Temperature ET (70°C) for a period of 7 and 28 days, respectively. Further UCS tests were performed at an intermediate temperature (35C) to observe the effect of temperature on the UCS of geopolymer stabilized clay. Experimental results showed that with increasing FA-LS and FA-GGBS contents; UCS, Young’s Modulus (E) increased, while failure strain (εf) decreased. Maximum UCS was achieved at 15% FA+15% LS and 10% FA+10% GGBS under ET (70°C) curing as compared to RT (23°C) for FA-LS and FA-GGBS geopolymer-stabilized samples, respectively. Statistical analysis of the experimental results conducted showed a strong positive correlation between the UCS and E, and a negative correlation with failure strain (Ɛf). To reduce experimental time in the laboratory significantly, a Multi-Variable Regression (MVR) model was used to develop mathematical predictions for the UCS, given % FA, % LS, Dry density, Bulk density, Curing temperature (Tc), and Curing age (T). The MVR model provided an efficient computational method (R2 = 0.818) of predicting the UCS of geopolymer-stabilized soils, while reducing the uncertainties and common errors in a laboratory to the barest minimum. Micro-structural analysis from Scanning Electron Microscope (SEM) images confirmed the formation of a homogeneous and compact microstructures through geopolymer gels in the stabilized FA-LS and FA-GGBS samples under ET (70⁰C) as compared to RT (23⁰C). X-Ray Diffraction (XRD) tests were conducted on the geopolymer-stabilized EPK samples to detect the mineralogical bonds formed, including new minerals or compounds. Results from the XRD patterns of the FA-LS and FA-GGBS geopolymer-stabilized samples showed that there was formation of new minerals due to direct chemical reaction between the geopolymer precursor and soil minerals. Therefore, the enhancement of mechanical properties was mostly due to a combination of the new minerals formed and the binding effects of geopolymer gels. Fourier-Transform Infrared (FTIR) spectroscopy was conducted to characterize the chemical bonds in the FA-LS and FA-GGBS stabilized samples. From the FTIR spectra of FA-LS geopolymer samples, the prominent bands observed (wavenumber between 950-1080 cm−1), shows a very strong band of asymmetric stretching (va) of the Si-O-T (T stands for Si or Al) dissolved by the SiO4 tetrahedron and N-A-S-H gel, corresponding to the formation of C-(A)-S-H gels in the hydrated products. Also, carbonation CO32−, (880–1478 cm−1), hydroxylation and water molecules (-OH, H-O-H, 3642–3678 cm−1) bands were observed. Generally, it can be the Si-O-Al and Si-O bonds were responsible for the strength improvement. For the FA-GGBS geopolymer samples, the wavenumber between 950-1119 cm−1 represents a very strong band of asymmetric stretching of the Si-O-T (T stands for Si or Al) dissolved by the SiO4 tetrahedron and N-A-S-H gel, corresponding to the formation of C-(A)-S-H gels in the hydrated products. The -OH/HOH symmetric was detected between 3400 to 3650 cm−1, a very strong O-C-O stretching vibration in CO32− between 1421 to 1460 cm−1. Generally, it can be inferred that the Si-O-Al and Si-O bonds were responsible for the strength improvement in both FA-LS and FA-GGBS samples. Sustainability assessment of both geopolymer and lime treatment methods was evaluated using the Sustainability Index framework that incorporates resource consumption, environmental, and socio-economic concerns. Life cycle assessment (LCA) was conducted for traditional lime treatment, FA-LS and FA-GGBS geopolymer-stabilized subgrades to determine the most sustainable treatment method. Results from the sustainability index showed that FA-LS (ISus =12.88) and FA-GGBS (ISus = 29.72) geopolymer treatment of EPK subgrade soils are more sustainable alternatives compared to the traditional lime (ISus = 48.07) treatment method.