Optimal process of grinding activation of rice husk ash


Student thesis: Doctoral Thesis

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  • Weiting XU


Awarding Institution
Award date2 Oct 2013


The utilization of rice husk ash (RHA) as a pozzolanic material in cement mortar and concrete provides several advantages, such as improved strength and durability properties, reduced materials costs due to cement savings, and environmental benefits related to the disposal of waste materials. Reactivity of RHA is attributed to its high content of amorphous silica, and to its very large surface area governed by the porous structure of the particles. Generally, reactivity is favored also by increasing fineness of the pozzolanic material. The retained amorphous silica and high fineness can be achieved by controlled burning and grinding, respectively. Therefore, systematically studies on controlled burning and grinding activation process are carried out to provide a highly active RHA that enhances the strength gain of cement mortar and concrete. For the burning activation of RHA, the rice husk calcined at 500, 600, 700 and 800 °C, respectively for 2 hours, was tested for compressive strength and X-ray diffraction (XRD) analysis. Results show that the optimal burning condition for obtaining highly active RHA is 600 °C for 2 hours, and such highly active ash can be used as a partial cement replacement material. The origin of high activity of RHA was also investigated at nano-level in this study. The scanning electron microscopy (SEM) and transmission electron microscope (TEM) analysis were carried out on the RHA sample. SEM and TEM images show that RHA has three-layered structure i.e. the inner, outer and interfacial with honeycombed and interstitial pores. The interstitial pores with size less than 50 nm are actually the main reason for very large specific surface area and very high pozzolanic activity of RHA. For the grinding activation of RHA, the batch dry grinding process was achieved in a laboratory ball mill. The RHA was ground to different particle size through different grinding durations. Optimum duration of the grinding process was explored along with particle size analysis, chemical composition, mineralogy, and morphological aspects of the ground RHAs. Results reveal that the pozzolanic activity of RHA is increased with increase in fineness of the ground RHA at a certain grinding durations. The optimal grinding duration of RHA is 30 minutes, which corresponds to mean particle size of 5.45 μm and specific surface area of 19.558 m2/g. In order to see whether the processed RHA can be used at high percentages in cementitious products, the performance of its incorporated mortar and concrete is examined. The strength contribution of RHA in cement mortar/concrete, and the durability contribution of RHA in concrete were investigated. The results show that the mortar/concrete incorporating up to 10% cement replacement by the processed RHA (burning at 600 °C for 2 hours and grinding for 30 minutes) is found to perform better than normal cement mortars as early as seven days. Furthermore, the concrete incorporating 10% cement replacement by such RHA also showed similar strength and durability enhancement compared with the concrete made of silica fume. The experimental results in this study demonstrate that controlled burning and grinding is a suitable mean to increase utilization of RHA as cement replacement. The processing technique is technically been demonstrated to yield quality RHA for use in concrete, thus reducing the amount of RHA that needs to be disposed of in landfills.

    Research areas

  • Size reduction of materials, Cement, Rice hull ash, Additives