Rice husk ash

Ahmad Fuad, Z. Ismail, Z.A. Mohd Ishak and A.K. Mohd Omar 

The properties of plastics can be significantly modified by the incorporation of fillers. Rice husk ash (RHA) fillers are derived from rice husks, which are usually regarded as agricultural waste and an environmental hazard. Rice husk, when burnt in open air outside the rice mill, yields two types of ash that can serve as fillers in plastics materials. The upper layer of the RHA mound is subjected to open burning in air and yields black carbonized ash. The inner layer of the mound being subjected to a higher temperature profile results in the oxidation of the carbonized ash to yield white ash that consists predominantly of silica. 

WRHA has been analysed and found to have about 95% silica content. BRHA on the other hand has typically only about 54% and a substantial carbon content, i.e. about 44% (Table 1). 

It is a well known fact that incorporation of fillers into thermoplastics increases the flexural modulus, i.e. the stiffness of the material [5]. Likewise addition of both the BRHA and WRHA fillers into polypropylene 

Morphological studies have indicated that failures of all BRHA composites are brittle in nature while WRHA composites experience ductile failiu-es [2]. It appears that the incorporation of the WRHA does not alter the ductile mode of failure of the polypropylene matrix. On the other hand, slight addition of the BRHA results in a marked transition of the failure mode (to brittle fracture). 

The rate of reaction is increased by the addition of NaCl and NaBr in the 2-8 hour period. By the addition of these compounds, an increase in strength results at all ages up to 91 days. 

Rice husks, also called rice hulls, are the shells produced by the dehusking operation of paddy rice. Each ton of paddy produces about 400 pounds of husk which on combustion produces about 90 pounds of rice husk ash (RHA). Amorphous silica may be obtained by maintaining the combustion temperature of RHA below 500°C under oxidizing conditions for prolonged periods or up to 680°C, provided the total time is less than a minute.t l 

Thermal analysis techniques have been applied to determine the decomposition of RHA and also to investigate the hydration characteristics of RHA-cement pastes exposed to different conditions. The reported results ofDTA, DSC, and TG of RHA-cement pastes show variations. The thermal curves of RHA from different sources are not comparable because of variation in their physico-chemical characteristics, the conditions under which the husk is heated, and other factors.Thermograms generally show endothermal and exothermal peaks. Those ashes formed at lower temperatures show an exothermal effect for the oxidation of the unbumed carbon. Endothermal effects at about lOO C denote the expulsion of water from the adsorbed surface. The oxidation reactions correspond to the loss of weight in TG. Some DSC results have been obtained for the ash obtained at 1200°C.[ The peaks in DSC (Fig. 22) were interpreted from XRD studies. An exothermic peak at 135°C is attributed to the transformation of the try dimite phase (T toT ) and four endothermal effects at 190,220,235, and 250°C represent the transformation of the low form of disordered cristabolite. The endothermic peak at 250°C is caused by a transformation of the well-crystallized form. 

Because RHA has a high surface area, it acts as a good pozzolan. A mixture of CH and RHA promotes the formation of a C-S-H product of composition Ca 5 Si03 5 XH20J The TG/DTA curves for the system RHA-CH mixtures are shown in Fig. 23. The evaporation of water is denoted by an endotherm at about 140°C. Thereafter, there is a gradual loss of water and at 827°C crystallization to wollastonite is indicated by an exothermic peak. 

The rate of heat development in RHA cement pastes containing from 5 to 20% RHA has been followed by Hwang and The shapes 

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Rice husk ash has characteristics broadly similar to those of microsilica but can be even more strongly pozzolanic. A study of pastes with C3S (K48) showed the CH content to pass through a maximum of 3% at 7 days, referred to the ignited weight by 28 days, it had fallen to 1%. The hydration product was C-S-H with a Ca/Si ratio estimated at about 1.3 by analytical electron microscopy, or 0.9-1.2 from the contents of CH and unreacted starting materials. There were indications that an initial product with a Ca/ Si ratio of 0.1-0.2 was formed. 

Clean flat glass and waste carpet fibers were used in this study. The chemical composition of the glass was analyzed using an X-ray microprobe analyzer and listed in Table 2 together with that of silica fume and rice husk ash for comparison. 

Table 2. Chemical composition of glass, silica fume and rice husk ash used in this study (by...

Particle size (pm) Cement Glass 1 Glass 11 Silica fume Rice husk ash... 

Figure 2. Particle size and shape of ground waste glass type I, type II, silica fume, rice husk ash and ordinary Portland cement.

Result of all tests in Figure 2 shows that inereasing the amount of replacement form 10% to 40%, decrease the compressive strength of composite cements. However, the highest compressive strength has seen in rice husk ash with 10% replacement but by increasing the amount of replacement its strength decreases sharply in comparison with other composite cements. This principle is also seen in silica fume specimens. 

Mansaray, K.G., and Ghaly, A.E., Agglomeration characteristics of alumina sand-rice husk ash mixtures at elevated temperatures , Energy Sources 19 (1997) 1005-... 

It has been demonstrated that rice husk ash (RHA) can be added to concrete mixtures as a substitute for the more expensive Portland cement to lower the constmction costs. Compared to other agricultural by-products, RHA is high in ash and similar to silica fiune - it contains considerable amounts of Si02- Thus, RHA is not just a cheap alternative but a well ground rice husk with most of its silica in an amorphous form and with enough specific surface so that it is very active and considerably improves... 

TABLE 5.1 Chemical composition of cement and rice husk ash. 

Srivastava, V. C., Mall, I. D., and Mishra, I. M. 2009. Competitive adsorption of cadmium (II) and nickel(II) metal ions from aqueous solution onto rice husk ash. Chem. Eng. Process. 48 370-379. 

In some countries in Easter Asia the rice husk ash, a very good pozzolanic material, is used [16,17]. 

Total replacement of the raw feed silica with rice husk ash (Ghosh, Mohan, and Gandhi, 1992) resulted in more uniformity in the clinker silicate distribution, larger alite and smaller belite crystal sizes, and an approximately 11% higher 28-day strength (41.2 MPa), compared to a standard mix with 7% sand as the silica source. 

Microscopical examination should start in the quarry where samples of each of the varieties of limestone, sandstone, shale, etc., are collected, layer by layer, by a geologist or someone with an adequate knowledge of the quarry. An assumption of the mineralogy of most quarried materials, as well as many of the industrial byproducts, is commonly questionable. Representative portions of each rock variety are sent off, if necessary, for thin sectioning (see partial list of professional companies in Table 11-3). Some of these companies will also stain the thin sections as directed. Another portion of the rock is crushed in the plant laboratory with a mortar and pestle or other suitable crushing device, and sieved to produce a 45- to 75- im fraction for examination in a powder mount, using, at first, a liquid with a refractive index of approximately 1.542. Samples of nondeposit materials, such as slag, fly ash, bottom ash, rice husk ash, clay catalyst, etc., are examined similarly. 

Compared with the limited PPy/inorganic composites, a variety of organic materials were combined with PPy, which included sawdust, polysaccharides, carbon-related nanomaterials, rice husk ash, etc. This can be attributed to the natural compatibility and the easy covalent bonding with PPy. PPy/sawdust composites were synthesized by in situ FeCl oxidation and had applied to remove Cr(VI), [13] Zn(Il), [14] phosphate, [12] nitrate. 

PPy/rice husk ash composites were fabricated by Eisazadeh group to remove a variety of inorganic ions from aquatic environments, such as Zn, Fe, Cu, Mn, GT,, SO , and so on [78]. Electropolymerized PPy/SDBS composite was applied to remove bovine serum albumin and fibrinogen... 

Our current work has so far focused on two silica-containing plants horse tail Equisetum arvense complete plants without roots) and rice husks (Oryz saliva). The latter can be of commercial interest, because they are cheap and available in huge amounts as agricultural waste [4-6]. Rice husk ash is already used as raw material in some ceramic processes [6]. 

It is necessary to be able to identify and quantify the additives in polymers and vibrational spectroscopy is a particularly useful approach to this problem. Compared with traditional chemical analyses, vibrational methods are nondestructive and are time-and cost-effective as well as more precise. A large number of examples exist in the literature. For example, antistatic agents (polyethylene glycol (PEG) in polyethylene (PE)) can be detected directly using FTIR sampling (367). An IR spectroscopic technique for the analysis of stabilisers (2, 6-di-tert-butyM-methylphenol) in PE and ethylene-vinyl acetate (EVA) copolymer has been described (368). It is possible to quantify the amount of external and internal lubricants (stearic acid in polystyrene (PS)) (371). Fillers in polymers can also be analysed (white rice husk ash (predominantly silica in polypropylene (PP)) (268). Raman spectroscopy has been used to detect residual monomer in solid polymethyl methacrylate (PMMA) samples (326). 

White rice husk ash (predominantly silica) was incorporated as a filler (10, 20, 30 and 40% by weight) into PP homopolymer. Absorption peaks at 480,621 and 790/cm gave good linearity with increasing filler content. 

Recently, Jang s group reported a time-saving synthesis toward ordered mesoporous carbon supported MgO (Mg-OMC) materials, which were fabricated by the carbonization of sulfuric acid-treated silica/triblock copolymer/suCTose/Mg(N03)2 composites. In the current approach, triblock copolymer P123 and sucrose were employed as both structure-directing agents for the self-assembly of rice husk ash silica solution and carbon precursor. Sulfuric acid was used to cross-Unk P123 and sucrose in the as-synthesized composites in order to improve the carbon yield. The CO2 adsorption capacity of Mg-OMC-1 was observed to be 92 mg g which is comparable with that of the well-established CO2 sorbents [113]. 

Rice husk ash is a product that may be obtained by a controlled ashing of rice husk, a material widely available in rice-producing countries. 

Because of the presence of a highly reactive form of Si02 and its high specific surface area, rice husk ash reacts readily with calcium hydroxide in the presence of water, yielding an amorphous C-S-H phase (Zhang et al., 1996 Lin, 1997). In suspensions of rice husk ash in saturated calcium hydroxide solutions at 40 °C the formation of a semicrystalline calcium silicate hydrate phase of the composition Caj j.SiOj 5.XH2O has also been reported (Yu eta/., 1999). 

If blended or ground together with quicklime or hydrated lime (in ratios of 70-80 wt% ash to 20-30 wt% lime), rice husk ash yields a binder suitable for the production of rapidhardening, water-resistant mortar mixes. 

In addition to an increase of strength, the presence of rice husk ash also improves the resistance of concrete to acid attack. The expansion due to alkali-silica reaction and sulfate attack is reduced considerably. Also, the frost resistance has been foimd to be significantly improved by adding rice husk ash to the concrete mix (Mehta and Folhard, 1995). 

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