IMPACT OF RICE HUSK ASH ON CEMENT CONCRETE

CONCRETE

Concrete is a composite material composed mainly of water, aggregate, and cement. Often, additives and reinforcements (such as rebar) are included in the mixture to achieve the desired physical properties of the finished material. When these ingredients are mixed together, they form a fluid mass that is easily molded into shape. Over time, the cement forms a hard matrix which binds the rest of the ingredients together into a durable stone-like material with many uses.

RICE HUSK ASH
Workability, strength, and durability are three basics properties of concrete. Amount of useful internal work necessary to overcome the internal friction to produce full compaction is termed as Workability. Size, shape, surface texture and grading of aggregates, water-cement ratio, use of admixtures and mix proportion are important factors affecting workability. Strength is to bear the desired stresses within the permissible factor of safety in expected exposure condition. The factor influencing the strength are: quality of cement, water-cement ratio, grading of aggregates, degree of compaction, efficiency of curing, curing temperature, age at the time of testing, impact and fatigue. Durability is sustenance of shape, size and strength; resistance to exposure conditions, disintegration and wearing under adverse conditions.



RICE HUSK ASH


CONCRETE BLOCK MADE FROM RICE HUSK MIXED CONCRETE
RHA, produced after burning of Rice husks (RH) has high reactivity and pozzolanic property. Indian Standard code of practice for plain and reinforced concrete, IS 456- 2000, recommends use of RHA in concrete but does not specify quantities. Chemical compositions of RHA are affected due to burning process and temperature. Silica content in the ash increases with higher the burning temperature. As per study by Houston, D. F. (1972) RHA produced by burning rice husk between 600 and 700°C temperatures for 2 hours, contains 90-95% SiO2, 1-3% K2O and < 5% unburnt carbon. Under controlled burning condition in industrial furnace, conducted by Mehta, P. K. (1992), RHA contains silica in amorphous and highly cellular form, with 50-1000 m2/g surface area. So use of RHA with cement improves workability and stability, reduces heat evolution, thermal cracking and plastic shrinkage. This increases strength development, impermeability and durability by strengthening transition zone, modifying the pore-structure, blocking the large voids in the hydrated cement paste through pozzolanic reaction. RHA minimizes alkali-aggregate reaction, reduces expansion, refines pore structure and hinders diffusion of alkali ions to the surface of aggregate by micro porous structure. 

Portland cement contains 60 to 65% CaO and, upon hydration, a considerable portion of lime is released as free Ca(OH)2, which is primarily responsible for the poor performance of Portland cement concretes in acidic environments. Silica present in the RHA combines with the calcium hydroxide and results excellent resistance of the material to acidic environments. RHA replacing 10% Portland cement resists chloride penetration, improves capillary suction and accelerated chloride diffusivity.

Pozzolanic reaction of RHA consumes Ca(OH)2 present in a hydrated Portland cement paste, reduces susceptible to acid attack and improves resistance to chloride penetration. This reduces large pores and porosity resulting very low permeability. The pozzolanic and cementitious reaction associated with RHA reduces the free lime present in the cement paste, decreases the permeability of the system, improves overall resistance to CO2 attack and enhances resistance to corrosion of steel in concrete. Highly micro porous structure RHA mixed concrete provides escape paths for the freezing water inside the concrete, relieving internal stresses, reducing micro cracking and improving freeze-thaw resistance.

CONCLUSION
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In nominal mix M20 grade concrete and 1:4 cement mortar RHA was added as an admixture from 7.50% to 17.50% with an uniform variation of 2.5%. During destructive test, compressive strength of mortar cubes and rebound hammer strength of concrete samples found increased with maximum variation of 67.85% and 39.65% for 10% RHA. Maximum variations of elastic modulus were 55.87% followed by 27.94% for 12.50% and 10% RHA mixed samples. Compressive strength of concrete samples showed maximum increase 3.08% between RHA 7.50% to 10.00% which decreased further for higher percentage of RHA.

Reduction in water absorption, from results obtained from 6 tests concrete and 3 tests on mortar samples, it is observed that up to 10% RHA with concrete and mortar enhances all properties (Figures 2a to c) and it is observed that 12.5% of Rice Husk Ash by mass of cement as the optimum doses to be added in concrete production of M20 particularly when the husk is burnt under field condition to utilize the easily available and low cost resources for betterment of concrete structure with respect to economy, durability and strength. So best applicable percentage of rice husk ash as per field condition 10.00% for optimal strength and durability

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