Optimization of fly ash and cement for stabilizing clay soil

Fly ash and cement are commonly used as soil stabilizers, offering environment friendly solutions for soil improvement. This study aims to determine the ideal combination of fly ash and cement to improve the geotechnical behavior of clay soil collected from a future building construction project in Sylhet, Bangladesh. Strength development in soil was assessed across a wide range of fly ash (0–25%) and cement (0–18%) during 3-, 7-, 14-, and 28-day curing periods. Tests included in this research are: Atterberg limits test, standard Proctor tests, unconfined compressive strength (UCS) and scanning electron microscopy tests. Incorporating fly ash, cement, or both decreased maximum dry density (MDD) and increased optimum moisture content (OMC) of the soil, while soil classification remained consistent. The investigation revealed that fly ash initially had a higher reduction rate in MDD of the mixtures up to around 7% binder content, before cement took precedence. Optimal soil-binder strength was achieved at 5% fly ash and 8% cement after curing for 28 days individually. However, the optimal combination of the binders was identified as 6% (3% fly ash and 3% cement), yielding a 99.51% increase in UCS compared to untreated soil after 28 days. Scanning electron microscopy tests show that the compactness of clay soil increases due to the addition of fly ash and cement. This binder combination (3% fly ash and 3% cement) could be suitable for stabilizing clay soil beneath the shallow foundation of a residential building in the study area.