Can rice make cement greener?

The construction industry accounts for a significant share of global CO2 emissions, mainly due to cement production. To lower the environmental impact, researchers have investigated rice husk ash (RHA), obtained by burning rice husks, as a partial cement replacement in Portland Pozzolana Cement (PPC) concrete, which already contains fly ash. In a recent study, mixes were prepared with RHA substituting 5 %, 7.5 %, 10 %, 12.5 % and 15 % of PPC by weight, and compressive and flexural strengths were measured at 3, 7, 28 and 56 days. The mix corresponded to an M20 grade, complying with Saudi Building Code provisions, ensuring practical applicability. Results showed that moderate RHA contents (5‑7.5 %) enhanced performance: the highest compressive strength (37.6 MPa) and flexural strength (5.47 MPa) were obtained with 7.5 % RHA after 56 days, representing improvements of 3.5 % and 9.6 % over the control mix. Early‑age strength was slightly reduced because of the slower pozzolanic reaction of RHA, whereas replacements above 12.5 % caused strength loss due to excessive cement dilution. The work highlights the synergistic effect of RHA in a dual‑pozzolan system with PPC, still underexplored, and identifies optimal replacement levels for greener construction, cutting clinker use and CO2 emissions while maintaining—or even improving—long‑term mechanical properties.

The study is based on the idea that burning rice husks under controlled conditions produces highly reactive amorphous silica, which can participate in the pozzolanic reaction with calcium hydroxide released during cement hydration. In Portland Pozzolana Cement (PPC) there is already fly ash, a class F pozzolan that reduces free calcium hydroxide and densifies the matrix. Adding RHA introduces a second source of silica, creating a dual‑pozzolan system where the two components can react complementarily: fly ash, more reactive at early ages, accelerates C‑S‑H formation initially, whereas RHA, with a higher specific surface area and finer particle size distribution, continues to react beyond 28 days, enhancing long‑term strength. Mechanical tests showed that the peak compressive and flexural strength occurs at 7.5 % RHA, reflecting an optimal balance between cement dilution and pozzolanic contribution. X‑ray diffraction (XRD) revealed a decrease in the portlandite peak and an increase in the amorphous C‑S‑H peak, confirming calcium hydroxide consumption. Scanning electron microscopy (SEM) displayed a more homogeneous microstructure, with fewer capillary pores and better aggregate distribution. Thermodynamic modelling predicted that beyond 12.5 % replacement the dilution effect outweighs the pozzolanic benefit, leading to strength loss. Environmentally, each tonne of RHA used avoids about 0.8 t of CO2 equivalent by reducing clinker demand. These findings support adopting RHA as a supplementary cementitious material in PPC mixes, advancing circular‑economy goals and sustainable construction.