STUDY OF STRENGTH, DURABILITY AND MICROSTRUCTURAL PROPERTIES OF FLY ASH AND GGBS BASED GEOPOLYMER STABILIZED HIGH PLASTIC CLAY

Author

Sudarshan Shrestha, Southern Illinois University CarbondaleFollow

Date of Award

8-1-2025

Degree Name

Master of Science

Department

Civil Engineering

First Advisor

Kolay, Prabir

Abstract

Soil Stabilization is a well-established technique in civil engineering which aims to improve the physical and mechanical properties of soil such as its strength, durability and load-bearing capacity, which are critical for the stability of infrastructure like roads, foundations, embankments, and pavements. Traditionally, cement and lime were extensively used materials for stabilization, which are often associated with significant environmental impacts and economic drawbacks. Geopolymers have emerged as an eco-friendly stabilizer that can be a feasible substitute for cement and lime in stabilization of weak soil. Lot of research has been done in soil stabilization using fly ash and Ground Granulated Blast Furnace Slag (GGBS) based geopolymer, where fly ash and GGBS, rich in silica and alumina are activated using alkali activator solution usually Sodium Hydroxide (NaOH) and Sodium Silicate (Na2SiO3) solution. The preparation of alkali activator solution is usually done a day prior due to the exothermic nature of the NaOH. Several researchers have reported that geopolymerization reactions are ideal at elevated temperatures. The current research focuses on the efficiency of dry mixing approach in stabilization of locally available clayey soil using fly ash and GGBS based geopolymer. In dry mixing approach alkali activator powder is mixed with the geopolymer precursor uniformly and water is added to it for the preparation of Geopolymer samples. The primary focus of this study is to make use of the heat generated due to the exothermic nature of NaOH for enhancement of strength and durability properties of clayey soil. The current study focuses on use of two geopolymer precursor: GGBS a glassy, granular material which is a by-product of the iron and steel industry produced by rapidly cooling molten blast furnace slag using water or air and fly ash, a fine powdery material which is a by-product of burning pulverized coal in thermal power plants. Both fly ash and GGBS is used as 5%, 10%, 15% and 20% of dry mass of soil to understand the collective effect of both precursor in the improvement of strength and durability properties of clayey soil. Referring to conclusion and recommendations of various past literatures molarity of sodium hydroxide (NaOH) was set to 10M. The ratio of NaOH to Na2SiO3 was kept 1:1.5 and the liquid to solid ratio was kept 0.55 for better workability. The study includes a comprehensive series of tests like particle size distribution test, specific gravity test, Atterberg limit test, and miniature Proctor test. Carbondale soil was classified as a clayey soil with high plasticity with Liquid limit (LL) of 60 and Plasticity Index (PI) of 34. The Maximum Dry Density (MDD) and Optimum Moisture Content (OMC) of Carbondale soil was found to be 27% and 1.44 g/cm3 respectively. Specific gravity test was conducted to determine the specific gravity of Carbondale soil, fly ash and GGBS as 2.65, 2.665 and 3.181, respectively. Unconfined Compression Strength (UCS) test was conducted on untreated and geopolymer stabilized soil samples after curing the samples under high moisture curing conditions by wrapping up in a plastic film, kept inside a Ziploc bag and stored in a moisture tank. Samples were cured for 7 days and 28 days at an ambient temperature. The stress strain response for geopolymer stabilized samples exhibited brittle behavior as compared to ductile response of untreated Carbondale soil with higher peak stress and better initial stiffness. With the increase in the content of geopolymer precursor, the stress strain response becomes more brittle and stiffer. The behavior of geopolymer-stabilized samples becomes more brittle and stiffer as the curing period increases. The UCS value for Carbondale soil was found to be 227.40 kN/m2 and 333.76 kN/m2. For 7 days curing period, UCS value increased to 388.48 kN/m2, 549.49 kN/m2, 756.23 kN/m2, 834.78 kN/m2 for 5%, 10%, 15% and 20% FA-GGBS based geopolymer stabilized soil. This is an increase of 70.83%, 141.64%, 232.55% and 267.09% for corresponding FA-GGBS based geopolymer stabilized soil. For 28 days curing period, UCS value increased to 423.83 kN/m2, 578.07 kN/m2, 798.20 kN/m2 and 967.89 kN/m2 which exhibit an increase of 26.99%, 73.20%, 139.15% and 190.00% for corresponding FA-GGBS based geopolymer stabilized soil. The findings indicate that the stabilizer content and curing time have a major impact on the strength of geopolymer stabilized soil. Two types of Durability test, Wetting - Drying test and Freezing - Thawing test were conducted on untreated and geopolymer stabilized soil samples cured for 7 days under high moisture curing conditions. Samples stabilized with 10%, 15% and 20% FA-GGBS based geopolymer exhibited superior resistance against wetting drying action by surviving all 12 cycles with minimal cracks, bulking and physical deterioration. However, no combination survived all the cycles of the freezing thawing test. Samples were discontinued due to extreme deterioration at the top portion, loss of cohesion between the particles, development of excessive cracks, significant physical disintegration and bulking of the samples. Comparatively, 10% and 15% FA-GGBS based geopolymer stabilized soil performed better against freezing thawing action. Microstructural analysis was performed through Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) test to study the micro-structure fabric of untreated soil, GGBS, fly ash and geopolymer stabilized soil. SEM images confirm the formation of dense and compact microstructures, resulting from the development of Calcium Aluminosilicate Hydrate (C-A-S-H) and Sodium Aluminosilicate Hydrate (N-A-S-H) gels. With the increase in the content of FA-GGBS, microstructural image gets more densified and compact with fewer voids and fewer untreated particles. A greater Si/Al ratio than the Ca/Si ratio and a larger concentration of Na are revealed by energy dispersive X-ray spectroscopy (EDX), which promotes the creation of the N-A-S-H geopolymer network over the C-A-S-H bond formation.