Unprocessed Rice Husk Ash As A Partial Replacement Of Cement For Low-cost Concrete

1.1 Background/ Context

In many countries around the world, people live in substandard housing. Due to insufficient incomes, some are forced to use makeshift housing from scrap materials such as metal, glass or cardboard, which creates unsafe and inhumane living conditions. If there were a way to create lower- cost construction materials, adequate housing would become more accessible to even impoverished populations.

Concrete is one of the most widely used construction products in the world (Yakut 2004).

Concrete construction does not require highly skilled labor and uses mostly locally sourced materials— sand, gravel and water—as additions to purchased Portland cement powder, which is part of the reason for its wide adoption worldwide. Several studies in developing countries, including Guyana, Uruguay, Thailand, Pakistan and Brazil, have shown that rice husk ash (RHA) can be used as a partial replacement for cement in concrete (Boateng and Skeete 1990; Rodri’guez de Sensale 2006; Sujivorakul et al. 2011; Memon et al. 2011; Zerbino et al. 2011). This ability to use an agricultural waste product to substitute a percentage of Portland cement would not only reduce the cost of concrete construction in these countries, but would also provide a means of disposing of this ash, which has little alternative uses.

Additionally, cement manufacturing is an energy-intensive process, so in addition to reducing the cost of concrete construction and providing a means for disposing of an agricultural waste product, incorporating RHA into concrete as a partial substitute for Portland cement would also stand to reduce the amount of energy associated with concrete construction.

l.l.l Rice Husk Ash

Rice husk ash, a by-product of rice processing, is produced in large quantities globally every year and due to the difficulty involved in its disposal, can lead to RHA becoming an environmental hazard in rice producing countries, potentially adding to air and water pollution. Rice husk ash is a natural pozzolan, which is a material that when used in conjunction with lime, has cementitious properties (Mehta and Pitt 1976; Cook 1986). Several studies have shown that due to its high content of amorphous silica, rice husk ash can be successfully used as a supplementary cementitious material in combination with cement to make concrete products (Barkakati et al. 1994; Ganesan et al. 2008; Abu Bakar et at. 2011; etc.). RHA can be carbon neutral, have little or no crystalline SiO or no toxic materials, as in the case of off-white rice husk ash. This off-white RHA has been shown to be advantageously blended with white Portland cement up to a 15% replacement to enhance the performance of the concrete without sacrificing the aesthetics of the final product and even shows higher compressive and splitting flexural strengths as compared to a control sample (Ferraro et al.

2010).

In rice producing countries like Cambodia, RHA-concrete has the potential to provide a low-cost construction material produced from locally sourced, abundant materials while having the added benefit of providing a means to dispose of an agricultural waste product. Cambodia provides an ideal location to explore the potential for RHA in low-cost concrete.

1.2 Potential for use of Rice Husk Ash in Cambodia

In Cambodia, 92% of the country’s poor lives in the rural countryside (World Bank 2012). In addition to the need for providing adequate housing for the entire population, Cambodia has taken on the challenge of improving the national standard of education by constructing more schools after the Khmer Rouge killed most of the country’s educated class. In this country inexpensive construction materials would not only provide better housing, but would also allow for the construction of more schools for a given budget. Luckily, Cambodia has an abundance of rice husk (a waste product from rice processing), which is often burned to produce rice husk ash.

In a country that produces approximately 6 million tonnes of rice annually (from 2000 to 2010), rice husk is a locally abundant material and can be found in large mounds (Figure 1.1) in rural Cambodia, produced from small-scale rice producing operations (FAOSTAT 2011b). In many of the studies conducted on the use of rice husk ash to replace cement in concrete, the ash used was produced under highly controlled laboratory settings to produce the optimal ash for use in concrete. These ashes are high in silica content produced by burning the rice husks at high temperatures (between 500 and 700°C) and are mechanically ground to have a mean particle size around 9 pm (Zhang and Malhotra 1996; Tashima et al. 2004). However, in rural Cambodia, where the use of rice husk ash concrete could improve quality of life, the equipment and electricity necessary for the level of processing employed in these studies is unavailable. Therefore there is a need to determine whether this degree of processing of rice husk is necessary for producing an ash suitable for creating concrete of appropriate strength for the types of structures that are typically built in the Cambodian countryside.