Rice Husk Ash (RHA) Effect on High Performance Concrete

Introduction

Rice husk is a by-product resulted from the rice paddy milling industries. Since sustainability and environmental pollution have become a common necessity in all industries to mitigate the climate change effects, concrete industry developers were the pioneers to propose effective re-use of rice husk ash as an alternative for cement.

Firstly, rice husk has to be burned to obtain rice husk ash which is going to be used as a replacement of cement to produce high performance concrete depending on its reactivity due to the fact of being silica-rich material. Since the burning process of rice husk determines the chemical composition and reactivity of the produced ash, it should be well-controlled in industrial furnaces with controlled temperature rate so that a complete removal of carbon occurs leaving a reactive amorphous pozzolan with SiO2 content ranging from 85% to 95%.

Chemical Composition

Rice husk-which constitutes about 20% of rice-composition is as follows: cellulose (50%), lignin (25%-30%), silica (15%-20%) and moisture (10%-15%) with low bulk density ranges from 90 to 150 kg/m3. The characteristics of the ash depend upon the composition of the rice husk, burning temperature and burning time. Under controlled burning conditions, the volatile organic matter consisting of cellulose and lignin will be removed and the remaining ash will be predominantly amorphous silica with a cellular structure and a specific surface as high as 50,000 m2/kg and particle size in the range of 10-75 μm.

The average composition of Controlled-burnt RHA is 90% amorphous silica, 5% carbon, and 2% K2O.

Physical properties

RHA is a grayish-black highly porous powder with specific gravity varies from 2.11 to 2.27 and a very high specific surface area.

Effect of incorporating RHA on concrete properties

Since RHA contains SiO2 in the range of 85%-95%, it possesses pozzolanic properties and will have a similar effect of other active pozzolans such as silica fume on concrete. If used as a supplementary cementitious material in concrete, the amorphous silica in the RHA will react with Ca(OH)2 in the secondary hydration reaction forming additional C-S-H gel which contributes to both strength development and durability enhancement through producing a strong transition zone and decreasing the permeability in hardened concrete.

Finally, it is advised to replace cement by RHA with a percentage ranging from 5% to 10%. Replacement rates beyond 10% will yield a stick concrete and may have adverse effects on concrete strength.