Feasibility Assessment for Use of Rice Straw Ash in Concrete, 2019

 

Sabbie Miller, assistant professor, Dept. of Civil and Environmental Engineering, UC Davis

A previously conducted literature review showed that rice-based ashes have potential for use in concrete to fill a growing need in the construction industry and to help meet carbon emission reduction requirements.

Concrete mixtures formed with rice ash were examined to determine mechanical and durability properties.
Demand for cement to produce concrete in California is predicted to grow by 65% beyond 2015 levels. And with it, the demand for mineral mixtures like fly ash to improve constructability, performance, and sustainability of concrete will also grow. However, fly ash is a byproduct of coal-fired plants and must be imported from other states, raising environmental issues and concerns about continued supply.

Under proper combustion methods, rice hull ash can contribute desirable qualities to concrete. Rice straw ash is more constrained by its chemical composition. The potential availability of ash produced from rice hulls and rice straw is approximately 400,000 tons annually. Rice ash has the potential to be economically, logistically, and environmentally viable as an additive in concrete, but research must verify the feasibility of this product for the concrete industry to accept it.

Research findings

Concrete mixtures formed with rice ash from current bioenergy production methods were examined to determine mechanical and durability properties. Experimental findings showed that the use of rice hull ash could contribute to some changes in compressive and flexural strength, but varying changes in durability. In some cases, there were limited or improved changes to performance, but without any treatments to the rice-ash some expansive products were formed in alkali- and sulfate-reactivity tests.

One of the potential limitations of rice straw ash is the presence of high potassium levels, which leads to undesirable properties in concrete. Initial studies have shown that biomass treatments, such as leaching in water, can lead to a reduction in alkali metals, particularly potassium. Two leaching methods were tested to determine their success at removing unwanted alkali metals from rice-based waste. The first method used tap water and the other approach was with an acid solution. Leachate was analyzed for soluble elements. Acid leaching did not improve the removal rate of potassium from rice straw, although in most cases it did remove more soluble elements than water leaching.

Industry survey

To determine the benefits and constraints that could affect the economic feasibility of using rice-based ashes in concrete, surveys were given to bioenergy facilities, hullers, hull management representatives, straw balers, and concrete suppliers. The survey also sought to determine the potential for the leaching considered in this work.

Students working on this project batched concrete mixtures. In some cases, there were limited or improved changes to concrete performance.
Several key findings were noted. For energy producers, leaching with acid may not be feasible at any of the stages in ash production. They also noted that storage of ash is difficult. For millers and those involved in hull management, the storage of raw material was seen as difficult for some. Further, these groups mentioned that several other products using straw, hulls, and ash are either dwindling or are obtaining biomass at low prices. For straw balers, water leaching was seen as too difficult to do in the fields. Balers also noted that other products using biomass were obtaining low prices. For concrete suppliers, the key issue was obtaining ash with consistent properties and having consistent access to ash.

An economic feasibility assessment showed there is economic potential for using rice as an energy and ash resource in California. Assessments using current technologies for rice ash combustion show there is a margin for transport and limited processing that could still lead to an economically favorable energy resource from rice residues. Environmental impact assessments suggest that the production of rice-based energy is favorable compared to fossil fuel-derived energy. If both electricity and rice ash for concrete are products from rice residues, there would be an anticipated reduction in environmental impacts relative to conventional cementitious binders.

Thus, findings to date suggest there is potential for rice-based ashes to be used effectively in concrete production. Continuing experiments will assess the effects of different ashes formed with each leaching condition and biomass combustion temperature on cement-based mortar strength. Further study should also be considered in a few areas that could support feasibility of using rice-based ashes in concrete, such as material properties and an exploration of other, potentially beneficial products that could improve value and/or mitigate costs and environmental impacts.