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  • Of all the residues of plants the ash


    Of all the residues of plants, the ash obtained from the incineration of RH has highest amount of silica content [7]. The rice crop normally ingest orthosilicic A 77-01 in large quantity from ground water and this acid is further polymerized in the husk, ultimately contribute to amorphous silica [18]. RHA can be used as a partial replacement of Portland cement in mortar and concrete due to its pozzolanic nature [11] and to lower the cost of construction and to make a sophisticated contribution to the construction industry by providing low cost building materials and subsequently an economical shelter [7], [19] RHA not only increase the basic strength properties but also enhance the durability of mortar and concrete [18] and to produce high performance cement [20]. Highly reactive RHA can be produced by burning the RH at a temperature of 500 °C or lower for comparatively protracted time under oxidizing environment or for a slighter instant at a temperature of up-to 680 °C [8]. The burning of RH beyond this temperature may cause the conversion of amorphous silica to crystalline silica [10], firstly to cristobalite and as the temperature increase, it converted to tridymite [21]. The relevant research work of other investigators also confirm that combustion procedure and temperature affects the surface area of RHA particles, so a critical care must be taken in the processing of RH so that maximum reactivity of the ash can be achieved as a supplementary cementitious materials [4]. Combustion of RH at a temperature of 500–600 °C produced amorphous silica. At a temperature of 800 °C, Cristobalite was noticed in the ash [10] and after burning to a temperature of 1150 °C, tridymite with a huge of cristobalite was detected in the ash [22]. Nehdi et al. [4] reported some work of researchers and it is stated that combustion process of RH up-to a temperature of 900 °C produce ash with silica in amorphous state and at 1000 °C, yield crystalline silica when the duration of combustion cross the limit of 5 min. Bui et al. [23] reported that RH yield crystalline silica at a low temperature of 600 °C and even at 350 °C for 15 h exposure. A systematic research work was performed on the processing and reactivity of RHA by James and Rao and concluded that isothermal heating of RH at a temperature of 402 °C (675 K) completely oxidize the organic matter and yield silica content[23]. Della et al. [24] reported that combustion of RH at a temperature of 700 °C for 6 h duration produce about 95% amorphous silica powder. Abu Bakar et al. [25] demonstrated that less effect was observed on the production of silica at a temperature between 500 °C and 900 °C. Omatola et al. [26] have worked on the combustion process of RHA and elemental analysis and declare that amorphous silica is produced at a temperature of 600 °C to 1000 °C and also time of combustion have significant impact the production of amorphous silica. An optimum content of amorphous silica was recorded as 97% at an incinerated temperature of 700 °C with a presoaking duration of 3 h. By using different procedure, it has been concluded that RHA shows positive pozzolonic behavior [3], [27]. Due to this nature, RHA was used as a blended component in cement up-to 30% replacement, to evaluate the consistency and permeability and hence better results achieved [28], [29]. The pozzolonic behavior, vast availability and due to low cost, the RHA has ignited the interest of researchers to be used for blending cement [26], [27]. The fineness of RHA particles, structure of silica particles, silica content, loss on ignition and surface area of the particles are the main factors affecting the reactivity of RHA [32]. In Pakistan, rice is considered as one of the cash crop and about 6 million tonnes of good quality rice produce every year [33]. From the report of researchers, it is concluded that 20% of rice paddy is husk, it means that Pakistan produces nearly 1.2 million tonnes of rice husk every year. Out of 6 million tonnes, Khyber Pakhtunkhwa (KPK) produce approximately 80,000 tonnes of rice per annum and rice husk of about 16,000 tonnes produced normally [34]. This rice husk is normally burnt in the open field or in the mills. If this rice husk is burnt under controlled condition, it can be used beneficially in the construction industry.