One of the most widely cultivated cereal crops in the world is rice. Rice grains (paddy) are enclosed in a natural protective shell known as rice husk (RiH). Due to its high silica content, this husk is resistant to natural degradation and can create a significant environmental burden if not properly managed. At the same time, the production of one ton of ordinary portland cement (OPC) releases nearly one ton of CO₂, raising concerns about its environmental impact [1]. In this context, pozzolanic materials derived from industrial and agricultural by-products including rice husk ash (RHA) are gaining attention, as they can improve concrete properties while potentially reducing cost and environmental impact. However, these depend on factors such as processing methods, differences between laboratory and field conditions, and the presence of alternative utilization pathways.
Globally, rice husk constitutes about 20% of the roughly 500 million tons of paddy produced (Bhanumathidas & Mehta, 2004). When rice husk is burned, the resulting ash can contain either amorphous or crystalline silica, depending on the combustion temperature and duration. Controlled burning at around 700 °C produces amorphous RHA, which is reactive and suitable for use as a pozzolan in cementitious systems. Several studies report that partial replacement of cement with RHA improves durability, enhances homogeneity, and increases compressive strength at later ages. Microstructural analyses show that RHA contributes to pore refinement, which helps explain these improvements. However, beyond a certain content level, compressive strength tends to decrease.
Despite these promising findings, several practical challenges remain. Much of the existing research is based on controlled laboratory conditions. This results in limited insight into long-term field performance. In addition, the properties of RHA are highly sensitive to processing methods. Rice husk is typically burned through open-field burning, fluidized-bed furnaces, or industrial furnaces. Open-field burning produces low-quality ash and causes significant pollution. Industrial furnaces can produce high-quality, silica-rich ash under controlled conditions, but such facilities may not be readily available in many regions, particularly in developing countries.
Another important aspect is the range of alternative uses for rice husk, especially for energy generation. While such applications are documented in the literature, the majority of research remains focused on its role in cementitious systems, with comparatively fewer studies regarding these alternative uses. The combustion conditions in energy generation determine the type of the resulting ash. Amorphous form is suitable for cement applications while crystalline form can be used in manufacture of ceramics and bricks.
Considering these factors, the use of agricultural by-products like rice husk in a stepwise manner, first, for energy generation and the resulting ash to material industry may offer a more balanced and practical pathway toward sustainability, rather than direct and uniform adoption in concrete. Such a strategy, however, requires careful economic and environmental evaluation, including sourcing, transportation, combustion methods, ash quality control, and end-use allocation.
References:
- Amran, M., Fediuk, R., Murali, G., Vatin, N., Karelina, M., Ozbakkaloglu, T., Krishna, R. S., Sahoo, A. K., Das, S. K., & Mishra, J. (2021). Rice Husk Ash-Based Concrete Composites: A Critical Review of Their Properties and Applications. Crystals.
- Bhanumathidas, N., & Mehta, P. K. (2004). Concrete mixtures made with ternary blended cements containing fly ash and rice husk ash. V. M. Malhotra (Ed.), International conference proceeding seventh CANMET Chennai, India.
