Document Type: Research Paper


1 Faculty of Engineering, Hamedan Branch, Islamic Azad University, Hamedan, I. R. of Iran

2 Faculty of Engineering, Science and Research Branch, Islamic Azad University, Tehran, I. R. of Iran


Expansive soils may cause disaster if not adequately taken care of. Lime continues to be commonly used for modification of these types of soils although it may have limited success in some applications. Thus, the present study was performed to address the viability of using silica fume (SF) as industrial waste to modify the behavior of expansive soils. This achieves the double objectives of overcoming the restrictions associated with lime treatment, and also of providing reliable data for using SF in the field of geotechnics to reduce its environmental cost. The additives including lime, SF, and lime-silica fume (LSF) mixture were separately added to the expandable smectite clay at wide ranges from 2% to 30% by mass, respectively. A set of laboratory tests including Atterberg limits, swelling, unconfined compression strength, permeability, electrical conductivity, and pH measurement were carried out at various curing periods to evaluate different influences of the additive types on the soil performance. The SSA and sedimentation analyses were conducted to assess the soil microstructure changes. The micro level structures of natural and modified clayey soil samples and their chemical composition were also studied using scanning electron microscope (SEM) equipped with Energy Dispersive X-ray (EDX) microanalysis. The results show that the geo-mechanical properties of highly expansive soil can be modified by the large content of lime. Besides, with the addition of lime and inadequate curing, where mainly flocculation occurs, the permeability of soil is negatively affected and slight increase in strength is observed. On the other hand, the addition of SF alone, even up to 30%, has less effect on the swelling power and produces a negligible change in the soil strength, regardless of the curing periods. It is found that the defects arising from the lime treatment can be greatly enhanced by the use of lime-silica fume mixture. Based on the results of macro and microstructure tests, the LSF blend improves the engineering parameters of smectite with a lower amount of lime and shorter curing time as compared with lime treated samples. This can occur due to extending the synthesis of the new pozzolanic compounds. It also significantly decreases the soil permeability through physicochemical interactions and induces a sharper decline in free lime, which results in the decrease of post instability problems in chemically modified soil. The study concludes that the combination of silica fume and lime can be successfully utilized as an additive to increase the efficiency of soil stabilization from economic, technical and environmental point of views.