Literature Review of using Rice Hull and Rice Straw Ash in Concrete, 2018



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

Demand for cement to produce concrete in California continues to grow. As in most regions, California uses mineral mixtures like fly ash to improve constructability, performance, and sustainability of concrete. Fly ash is a byproduct of coal-fired plants. It reacts with cement to produce desirable properties in concrete. California consumes about 1 million tons of fly ash annually and has to import all of this material from other states.

The potential availability of ash produced from rice hulls and rice straw is approximately 400,000 tons annually. Motivation for this project was to assess the current understanding of using rice-based ashes to reduce the quantity of cement needed for concrete production. Specific objectives were to conduct a literature review and to develop a roadmap for future research, development, and implementation of rice hull ash and rice straw ash for the concrete industry.

Literature review

Project researchers conducted a robust review of the scholarly and technical literature on a number of topics related to using rice hull and rice straw ash as a supplementary cementitious material. While there is a wealth of literature on the use of rice hull ash, there is still limited work and literature on the use of rice straw ash to draw substantive conclusions. The review focused on several key areas:

Biomass preparation, combustion, and ash properties—The biggest finding was that combustion and gasification of rice hulls and straw can be used successfully as sources of energy. Combustion also produces ash with properties similar to popular supplementary cementitious materials for concrete. Rice hull ash can exhibit similar properties to silica fume, a desirable attribute for concrete. However, specific preparation methods are necessary to maintain the glassy state of silica necessary for reactivity in concrete production.

Advantages and disadvantages of the use of rice hull ash in concrete—Findings from the literature suggest that rice hull ash may offer several advantages for use in concrete. Some of the potential advantages of using rice ash in concrete include that rice hulls are broadly available, benefits from energy production, increased long-term concrete strength and durability, and expected reductions to the carbon footprint of concrete. Some of the disadvantages include variability resulting from different cultivation practices, increased water requirements in concrete batching, reduced workability, increased setting time, and capital investment required for combustion facilities. The outlook is promising but the number of studies are few and more research is needed for implementation.

Market and environmental impacts associated with use of rice hull ash and rice straw ash in concrete—Literature on the environmental impacts of rice combustion and ash products for use in concrete is limited. A more thorough assessment of the environmental, social, and economic impacts on rice hull ash and rice straw ash is needed. If it can be shown that these alternative ash sources provide similar properties to conventional supplementary materials with lower environmental impact, they could command similar or higher prices than cement in regions with high demand for supplementary cementitious materials.

Cement producer survey

Surveys given to ready-mixed concrete producers revealed that cement users are hesitant about the incorporation of rice-based ashes into industry applications. Their hesitation is based on several factors—one of the biggest being the lack of understanding on the performance of rice ash in concrete. They also expressed concern about the continuous availability of high quality ash with minimal variation in properties.

Roadmap for future work

From the findings of the literature review, a roadmap was developed to guide future research, development, and implementation. Key areas to pursue include:

Research—Assess how rice hull and straw treatment methods, coupled with energy production methods, influence ash properties for use in concrete. Further evaluate the impact of rice hull ash on concrete durability. Further evaluate the impact of rice straw ash on concrete strength and durability. Determine how biomass treatment methods for energy production can change the properties of rice-based ashes.

Development—Determine how much ash could be produced annually. Assess feasibility to store ash at combustion sites, as well as how it can be transported.

Implementation—Estimate costs associated with producing, storing, and transporting ash. Determine thresholds for the cost of electricity or ash that would make these inputs reasonable. Assess the environmental impacts, considering different production methods, of the electricity and ash. Identify the extent to which environmental impacts might drive the market for these ashes.