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  • In countries such as China where


    In countries such as China where there is a high commercial production of rice, the use of rice husk ash (RHA) as an alternative cementing material can become a great asset for environmental rehabilitation and address the difficult disposal issues associated with this agricultural waste [23]. RHA possesses very good pozzolanic properties which depends on the silica content, silica crystallisation phase and surface area of the ash particles when it is burnt under controlled conditions [24]. The presence of RHA in cement-based materials contributes to the modification of the chemical as well as the physical properties of the materials. RHA as much as 20% has been used to replace cement in the treatment of sediment contaminated with Cu, Cr and Pb [25,26]. The use of RHA for the s/s of waste form have been found to be promising. However, the knowledge this site and the chemical aspects of the s/s technique require further research to increase our understanding when RHA is used [27]. To the authors’ knowledge, most of the studies on the use of RHA for s/s have focused on the mechanical performance and the short-term leaching characteristics of stabilised waste form [28]. There are limited studies aimed at increasing our understanding of the processes driving the long-term leaching behavior of a low permeability cement-RHA stabilised waste form containing high concentration of heavy metals. The ASTM C618 recommends that for a natural pozzolan such as RHA to possess a greater activity the combination of the amount of SiO2, Al2O3 and Fe2O3 should have a minimum value of 70% while the SO3 and loss on ignition (LOI) should possess a maximum value of 4 and 10 respectively. Based on these criteria most of the RHA used for the partial replacement of cement in the literature were pre-treated to meet the ASTM criteria for a natural pozzolanic material. There are scanty reports on studies using RHA as obtained from the field even if they do not meet these standardized criteria.
    Materials and method
    Results and discussion
    Conclusions The s/s of Pb and Cu contaminated slurry using cement and rice husk ash (RHA) was investigated. This study investigated the potential of replacing 20%–40% of the cement used for the s/s of Cu and Pb with RHA. It was observed that an increase in the binder content from 100 kg/m3 – 300 kg/m3 was effective for the immobilisation of Pb in the stabilised slurry through the reduction of the apparent diffusion coefficient (i.e. reduced from to 2.13 × 10−14 m2/s to 2.54 × 10-19). However, a minimum binder content of 200 kg/m3 was also sufficient to attain a diffusion coefficient of Cu as low as 2.1 × 10-18  m2/s at RHA content of 40%. The reduction of the required cement for s/s by 40% and then replaced with a cheaper agricultural waste product RHA will save several developing countries many dollars in rehabilitation of contaminated sites. The LX determined for all the samples was very high (above 9) giving an indication that treatment was very effective. The main leaching mechanism controlling the immobilisation from the waste form was diffusion and long-term kinetically controlled dissolution. The long-term kinetically controlled dissolution processes are predominately precipitation and sorption. Aside these two processes, physical encapsulation was also very important physical mechanism which helped in the immobilisation of Cu and Pb. In addition, the microscopic study using the MIP and the XRD test showed that the incorporation of the RHA initiated changes in the stabilised waste form. However, other microscopic study such as the use of scanning electron microscope (SEM) could also be employed for further studies. The pH controlled leaching as simulated by the batch leaching experiment also showed that the leaching of both Cu and Pb were pH sensitive. The amount of binder content was however important for the immobilisation of Cu and Pb under changing pH conditions. It should be noted that the incorporation of RHA could lead to order of magnitude increase in the leachability of Cu under near neutral pH.