Rice husk ash is an attractive pozzolan. Due to its low cost and high activity it has a promising perspective in sustainable construction. In combination with lime, its effect in soil improvement can be equal to cement treatment but its production process consumes much less energy. The main component of the rice husk ash is silica, which is the element that governs the reactivity of the ash. A delicate burning process is required to eliminate the organic components in the rice husk but keep the silica to be amorphous so that a highly reactive rice husk ash can be obtained. A too high temperature would transform amorphous silica to crystalline silica, which would reduce the reactivity. The suggested burning process in literature is 2 hours at 500oC. However, due to the exothermic property of the burning rice husk it is difficult to control the exact burning temperature, hence there is still a possibility that the carbon and the crystallized silica are present and hinder the activity of the rice husk ash. Based on the silica state and the carbon content, the rice husk ash is classified in three types: C-RHA which is collected from a quick and open-air burning and contains a large amount of carbon; Cr-RHA which is collected form slow burning at above 600oC and contains a large amount of crystallized silica; and A-RHA which is collected from the suggested burning process which is 500oC in 2 hours and is considered to be the most active. The activity of these three types of rice husk ash and their effect in soil improvement were tested. As expected, the higher reactivity of the A-RHA compared with the Cr-RHA confirmed the capability of these burning conditions. Surprisingly, the C-RHA appeared to be the most reactive and its effect to the soil was also the most positive despite of the large carbon content and the detected crystalline silica. The high reactivity of the C-RHA derives that there is a hierarchy of the solubility depending on the burning duration so that although all the three types of ash were mainly amorphous, the C-RHA is the most soluble. From the experiments, the role of the carbon was seen in only the term of quantity as it reduce the proportion of the silica, but might it have any support to the reactivity of the material in those experiments then it needs more investigation. The results of the treated soil showed that the rice husk ash need the lime to be activated, but then it helped to enlarge the possitive effect of the lime. The immediate effect of the additives to the plasiticity of the soil were seen to be the results of the lime only, but the long-term effect of the strength and the compression of the soil were seen to be the results of the combination between rice husk ash and lime. Especially in the case of C-RHA, it can reduce half the amount of lime in the case of 6% lime mixing to give similar undrained shear strength which was about 410 kPa. Because the carbon is not a serious harmful factor in the soil, and the soil can also take the advantage of the firmness of the quartz, so between the two cases, the rice husk ash with large carbon content and also considerable amount of quartz but higher reactivity, and the rice husk ash which is almost purely amorphous silica but less reactive, the former is the preferred for the soil improvement purpose.