The Effect of Utilizing Rice Husk Ash on Some Properties of Concrete – A Review

World environmental pollution is having direct and visible influence of construction industry. Relatively, 10 quintal of CO2 emitted by the manufacturing of 10 quintal of ordinary portland cement (OPC), which accounts almost seven percent of the global CO2 emissions. Averagely, ten thousand million cubic meter of concrete is produced yearly. Byproducts like rice husk ash (RHA), fly ash, blast-furnace slag, metakaolin and silica fume will be able to utilize as supplementary cementitious material because of their pozzolanic behavior. Rice husk cannot be used as animal’s feed due to the low nutritional value. Neither can it be discarded as landfill nor by burning because it arise a great environment challenge. By converting rice husk into rice husk ash (RHA), it can be used in concrete as a fractional substitution of cement because of high silica quantity in RHA. In this present paper, the result of some authors with the use RHA as a fractional substitution of cement on the properties of concrete like workability, permeability, compressive and tensile strength are reviewed. Current World Environment Journal Website: www.cwejournal.org ISSN: 0973-4929, Vol. 13, No. (2) 2018, Pg. 224-231

of which India is one of the major rice producing country.Main problems tackled by these countries are in the disposal of husk because it cannot be utilized as animal's feed due to the low nutritional value.Also, neither can it be discarded as landfill nor by burning because it arise a great environment challenge.Therefore, rice husk is generally utilized as a fuel in the boilers for processing paddy either by gasification or by direct combustion which create pollution [12][13][14][15][16] .Rice husk ash (RHA) is a pozzolanic material consists of more than 75% by weight as silica 17 .It was found that by burning at a temperature lesser than 800 0 C, rice husk ash formed consist of nearly 90% silica, however crystalline silica is obtained beyond this temperature [18][19][20][21][22] .It was also observed by Nair et al. that by burning for more than 12 hours, at 500-700 0 C produced remarkably reactivity rice husk ash, whereas high carbon content rice husk ash was obtained for short period of burning i.e. for 15-360 min 23 .Size, fineness of the particle, surface area, silica crystallization phase, silica content, time required for chilling process, temperature of burning process etc. are the key factors the pozzolanic behavior of RHA and the fineness of RHA falls between that of silica fumes and cement 24 .It was found from the study that the mechanical strength of the mortar increases with 10% by weight RHA replacement with cement 25,26 .From the previous study, it was concluded that fractional substitution of cement by RHA reduces the heat of hydration, improves the strength which is making concrete more economical and also environmental friendly.
In the present review paper, the results and discussion of some researchers on using rice husk ash (RHA) as additional cementitious material on the properties of concrete like compressive strength, workability, permeability and tensile strength are reviewed.

Properties of Rice Husk Ash
The color of rice husk ash depends upon the source of raw material, process, time, duration and temperature of burning and varies from whitish grey to black.According to the code IS 456, RHA is categorized as N type pozzolana 27 .Table 1 enlists the chemical properties and table 2 depicts the physical properties of RHA respectively.
Table1.enlists the components of RHA and cement.It is well known fact that the main binding component of cement is CaO, which is also a constituent of RHA although lower in percentage that cement.It can be observed from the table that all other components are nearly similar in both RHA and cement..Due to huge surface area and carbon content in RHA, a large quantity of water reducing admixture to be used for maintaining the constant workability than that of the controlled concrete 43 .The workability of concrete containing two different RHAs, one controlled combustion rice husk ash from USA while the other uncontrolled combustion rice husk ash from Uruguay were explored and it was found that uncontrolled combustion rice husk ash concrete require less superplasticizer as compared controlled combustion rice husk ash concrete due to low unconsumed carbon content 44 .
Safiuddin et al., studied that the concrete made with super plasticizer (3.5 to 4.5%), RHA (5 to 30%) and water cement ratio (0.30 to 0.40) exhibited 265-280 mm slump which is same as of 0% RHA concrete, however the deformability of concrete enhanced significantly 17 .Sensale performed slump test and found that normal concrete exhibit medium workability (52mm), whereas for RHA concrete possesses low workability (45-48mm).Nagrale et al., stated that with increase in RHA in concrete the water required for workable mix increases due to increase of the fineness material in the concrete 45 .It was revealed from the literature that specific surface area, void size scattering, and water demand of RHA were the key factor controlling the flowability of mortar 46 .Moulick stated the reason for the low slump that with the increase in the RHA quantity by weight increase the concrete volume, as density of cement higher that the RHA i.e. 3.11g/cm 3 as compare to 2.1 g/cm 3 in RHA 47 .

Compressive Strength
From the previous study it had been found that the strength increases with partial substitution RHA in concrete 29,48,49 22 .kulkarni et al., found that by varying RHA percentage from 0 to 30 % there was slight increase in the strength (0.75%-4%) at 7 th day, whereas there was significant increase in the strength (7.5%-15.5%).This can be attributed to the fact that with the increase in the percentage of RHA, the concrete become lighter and stiffer as compared to control concrete hence show more cracks 54 .

Permeability
The resistance from chloride is a significant factor for the concrete toughness because chlorides will initiate and accelerate the corrosion process of reinforcement in concrete.Madandoust et al., observed that a pozzolanic material enhances the protection from chloride entrance and diminishes the corrosion start time of steel reinforcement 25 .Le et al., stated that by increasing RHA percentage diminishes permeability of concrete and the lowest permeability value was obtained with 20 % RHA replacement 11 .Sensale studied that with 15% RHA replacement in concrete, tolerance to chloride ion infiltration was the best because of voids refining ability of RHA 44 .Ganesan et al., concluded from his trial that upto 30% RHA replacement the total charge passing through concrete containing RHA consistently decreases 48 .
Another researcher Ramasamy studied that with the use of RHA in concrete, chloride ion diffusion increases but yet remain in the ''very low category".The minimum charge passed was obtained in 15% RHA motar because voids ratio reductions which led to low permeability 55 .Hesami et al., conducted a test with varying w/c ratio (0.27, 0.33 and 0.44) on pervious concrete pavement and found that permeability vary with w/c ratio (0.33 < 0.27 < 0.44) and ranges between 0.08-0.48cm/s which is very high to be used as a drain layer 56 .
industrial and Practical use of RHA RHA has been used in various industries of cement, steel, rubber, cosmetic, food etc. Presence of Silica and calcium oxide content govern the suitability of RHA for various applications.Rice husk ash was found to be better supplementary material than silica fumes, fly ash, furnace slag etc. for industrial use.In steel industry, RHA has been used as an excellent insulator due to its low thermal conductivity, low bulk density etc. RHA is also used for coating molten metal because it does not allow rapid cooling of metal.In cement industry, upto 10% RHA replacement with cement showed remarkable results in the strength for the mixture.Due to high presence of silica in RHA, extraction of silica is quiet economical.This silica can be used as reinforcing agent in rubber industry, it can also be used as cleaning agent in cosmetic industry and anti cracking agent in food industry 57,58 .RHA mixed with cement/lime can be used as a stabilizing agent for soil as it improve the index properties and particle size distribution of soil 59,60 .

Conclusion
As per the literature review, the following closures are initiated: • As the percentage of rice husk ash (RHA) increases in the mix, there is considerable decrease in the workability due to which good superplasticizer is mandatory to achieve the required workability.

•
The compressive and tensile strength of the mix increase with the addition in the RHA percentage, whereas 20% RHA replacement could be considered optimal.

Table 2 : Physical Properties of rice husk ash given by various authors 33-40 are: Property Specific Specific Pozzolanic Bulk median nitrogen Surface gravity surface activity density particle adsorption area, area index(%) (kg/m 3 ) size(μm) (m 2 /kg) (cm 2 /g) (m 2 /kg)
determined that with 15 or 20% RHA, the mix becomes stiffer therefore extra super plasticizer has been required to keep up the vital workability41.Suaiam and Makul stated that by replacing RHA up to 25% gives the adequate results whereas if fine aggregates are partially replaced by the large quantity of RHA, workability is going to decrease 42 .
Le et al., find out the consequence of various percentage of RHA on high performance fine grained concrete and found that as the RHA content amplify workability decline because of huge specific surface area of RHA 48,49,50uitable RHA replacement varies from 5% to 30% by weight of cement.Specific surface area and bulk density of RHA are the important parameters affecting cement properties.As the specific surface area increase, it displays a remarkable pozzolanicity by consuming calcium hydroxide present in concrete, whereas the bulk density escalates the mechanical strength due to less void fraction.Replacing cement with 10% RHA increase the compressive strength irrespectively to ages due to high pozzolanicity and silica content23,30,50.Lung et al., examined RHA and normal concrete properties and concluded that at 7, 14 and 28 days strength are 47MPa,52 MPa, 61MPa and 50MPa, 54MPa, 56 MPa respectively30.Muthadhi et al., and Ganesan et al., detected that the existence of amorphous silica and fine particle size of RHA, 20%RHA concrete exhibits the maximum compressive strength48,49.Figure1depicts the SEM of ordinary Portland cement and rice husk ash 32 .