The utilization of waste materials in soil improvement is the potential for the reduction of the negative effect on the environment and the construction cost as well. One of the waste materials is rice husk ash (RHA), which is an agricultural-by product and can be utilized for soil improvement. This paper aims to present a comprehensive review of the numerous investigations on rice husk ash and its utilization in soil improvement. Based on the literature review, there are some findings as follows: (i) burning rice husk ash in incinerator under a controlled temperature of 500÷8000C and time of 1÷4 hours will produce RHA with high pozzolanic activity; (ii) the RHA can be considered as a stabilizer for different types of soil, and it can be used in combination with different chemical binders and waste materials for soil improvement; (iii) from the view of engineering purposes, the RHA from about 3 to 20%, the lime from 2 to 9%, the cement from 2 to 15% were mostly suggested to improve the soil for pavement, road (base and sub-base layers), and building houses in rural areas. However, the research on the utilization of RHA in soft ground improvement by deep mixing method using lime and cement is still limited, and it is therefore recommended for further research. In addition, the experimental field research on the utilization of RHA for soil improvement in engineering practice needs to be conducted.

The paper presents the compressive strength of mortar samples when replacing natural sand by bottom ash of An Khanh and Cao Ngan thermal power plants (TPPs) in Thai Nguyen province. Experimental research is the main method applied in this research. Research results show that replacing natural sand by bottom ash in the two TPPs, the compressive strength of samples reached the required mortar’s strength. The compressive strength of the mortar sample replaced all-natural sand, particularly for masonry mortar and plastering mortar, is 10.9 MPa and 16.4 MPa respectively for An Khánh TTP, and 12.7 MPa and 12.6 MPa for Cao Ngan TTP, while the compressive strength of the motar used all natural sand, reached 13.2 MPa and 8.6 MPa.

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