Experimental Investigation On Expansive Soil Modified By Nano Cement And Simulation Of Montmorillonite

Expansive soil is a type of special clay in geotechnical engineering which has a high tendency of swelling and shrinking.The characteristic of expansion when absorbing water and shrinkage when losing water will affect buildings built on it.This will cause economic losses and security risks,so it is important to modify the expansive soil.The most widely used method is cement solidification upon to now.However,there are some shortcomings in the cement-treated expansive soil.It is found that a better improvement effect can be obtained when adding nano silica into cement-modified soil.The soil we use in this paper consists of bentonite and kaolinite with dry mass radio 1:1.We prepared 4 groups of samples which are expansive soil alone,expansive soil adding 5%cement,expansive soil adding 4% cement and 1% nano silica,expansive soil adding 3%cement and 2% nano silica.Unconfined compression strength before and after soil improvement are compared.Further microscopic investigation is carried out to explain the modification mechanisms.Unconfined compression test results reveal that unconfined compression strength improves significantly when cement and nano silica are added to soil.Meanwhile the improvement effect of mixture of cement and nano silica is better than that of cement only,but the strength does not increase so much as the portion of nano silica increases.This conclusion can be used to decide optimum dosage of cement and nano silica.At the same time,X-ray diffraction tests and scanning electronic microscope tests are carried on four groups of samples after unconfined compression test to investigate mineral composition and microstructure changes before and after treatment.Results show that calcium silicate hydrate emerges when cement is added to soil,but cement and nano silica treated soil does not produce new phase compared with cement-treated soil,just changes the content of quartz and cement hydration product.SEM tests show that cement-treated soil and nano silica cement-treated soil has a more dense structure than expansive soil,but cement and nano silica treated soil has a higher strength than cement-treated soil,this may be due to the unique properties of nano silica leading the connection of CSH,which contributes to a higher strength of soil.In this research,molecular simulation with MS is carried out to simulate Ca-montmorillonite with interlayer of one,two and three layers of water,respectively.Simulation results show that as the water content increases the layer spacing and volume increase at the same time,which indicate the swelling properties of montmorillonite when absorbing water.Meanwhile,the simulation result indicates that different ions have different location and self diffusion coefficient,and it is related with water content in the interlayer.