A large amount of coal fly ash produced in thermal power plants is disposed of in landfills which causes many environmental problems. The utilization of fly ash can be encouraged in geotechnical engineering projects. In this paper, the effects of class C and class F fly ash on the mechanical and microstructural behavior and stabilization of clay soil were evaluated through a program of laboratory experiments. The experiments included compaction, unconfined compressive strength, consolidated-undrained triaxial, one-dimensional consolidation tests, and scanning electron microscopy analysis on samples of fly ash-stabilized clay soil after 1, 7, and 28 days of curing. The tests were conducted on mixtures of clay with class C or class F fly ash, ranging from 0% to 30% of the soil. Experimental results showed that the strength parameters and permeability of the stabilized soil improved while the compression and swelling indices decreased by the addition of fly ash and by the increase of curing days. The results obtained from the mechanical tests agreed with the results from the SEM analysis. Based on the results, the soil could be successfully stabilized by using class C fly ash. The improvements in strength, swelling, and permeability parameters of the stabilized soil were higher with the class C fly ash compared with class F fly ash.

Fly ash is a waste material obtained from burning of coal in thermal power plants. Coal consumption is still very high and is expected to remain above 38% globally. Therefore, large volumes of fly ash are produced every year that need to be managed as waste. Improper disposal of fly ash can lead to surface water and ground water pollution and adversely affect human health and environment. The use of fly ash as an agent to stabilize soil has recently become popular in geotechnical engineering due to its many benefits such as being eco-friendly and cost-effective, and improving the geotechnical characteristics of the soil. This paper presents a review of the geotechnical properties of fly ash-stabilized fine-grained soils. Several features of fly ash, including classification, physical, geotechnical, chemical, and mineralogical properties, health concerns, disposal, availability, and cost are analyzed. The effects of fly ash in improving a wide range of mechanical properties of soils including unconfined compressive strength, shear strength, CBR value, consolidation and/or swelling characteristics, and permeability are reviewed in detail. It is shown that fly ash can be a substitute material for use in soil stabilization, leading to substantial economic and environmental benefits.

This study investigated the improvement in the behaviour of a clay soil due to the addition of alkali-activated fly ash as a stabilising agent, and the effects of different activation factors such as alkali dosages and silica moduli. The alkali activator solution used was a mixture of sodium silicate and sodium hydroxide. Class F fly ash was used as the precursor material for the geopolymerisation process. Soil samples stabilised with non-activated class F fly ash were prepared and tested to compare the results with samples stabilised with alkali-activated fly ash. Compaction tests, unconfined compressive strength tests, X-ray diffraction analysis, and scanning electron microscopy analysis were carried out on samples cured 1, 7, and 28 days at room conditions. The results showed that the compressive strength of stabilised soil significantly increased when the fly ash was activated. The optimal activation parameters to stabilise the soil were found to be alkali dosages in the range of 12% to 16% and a silica modulus of 1.25. The highest compressive strength recorded was at 1293 kPa with an alkali dosage of 16% and a silica modulus of 1.25, while for the non-stabilised soil, it was at 204 kPa at 28 days of curing. Mineralogical analysis showed a decrease in the peak intensities of kaolinite and illite, while microstructural analysis indicated an alteration in soil texture with the addition of the alkali-activated fly ash.

Geopolymers are typically cured either at ambient temperature (~25 °C) or subjected to short-term heat curing before being stored under ambient conditions until testing. However, in hot-arid regions, the daily ambient temperature may exceed 45 °C during summer months. Therefore, such conditions should also be considered as high ambient curing, and their influence on low-calcium geopolymer performance needs to be investigated. In this study, pumice- and fly ash-based geopolymer mortars were produced to evaluate the effects of different curing regimes. In the pumice-based mixtures, 10 wt% of pumice was replaced with metakaolin to enrich the alumina content. Three curing conditions were applied: ambient curing, high ambient curing, and heat curing. Setting times of geopolymers were determined based on each curing regime. Physical properties, including density, water absorption, and sorption coefficient, were assessed. Compressive strength development was evaluated over 90 days. In addition, durability performance was assessed through water resistance, freeze–thaw durability, and resistance against sulphuric and hydrochloric acid. Fourier transform infrared spectroscopy and X-ray diffraction confirmed that geopolymerisation reactions continued significantly up to 90 days under high ambient curing, while mercury intrusion porosimetry showed a reduction in porosity. These findings explain the continuous increase in compressive strength. Pumice-based geopolymers cured under this condition exhibited significantly better long-term strength than those cured under other regimes. High ambient-cured fly ash-based geopolymers, a 3-day strength of 40.3 MPa was achieved, eliminating the need for heat curing. Thus, high ambient curing enables the in situ use of these geopolymers and offers a cost-effective and eco-friendly alternative.

Fine-grained soils may have undesired characteristics such as high swelling potential and low strength, thus requiring improvements. One of the stabilization methods involves the use of fly ash. Fly ash is a waste material obtained from burning coal in thermal power plants. The use of fly ash is encouraged as an alternative material for soil stabilization, due to its features such as pozzolanic properties and economic availability. This paper describes the results of an experimental study on stabilization of a clayey soil with fly ash. Unconfined compressive strength (UCS), triaxial and consolidation tests were carried out on samples of kaolinite mixed with class C fly ash at different percentages and cured for 1, 7, and 28 days, in order to study the effects of class C fly ash on the mechanical behaviour of the stabilized soil. The results showed that the inclusion of fly ash significantly improves the strength characteristics of the soil. Curing time was also found to have a significant effect on improving the properties of the soil.