Feraidon Ataie, program coordinator, Concrete Indistry Management Program, CSU Chico

From top to bottom: Rinsing out rice straw after 24 hours of soaking (top): washed rice straw in a stainless steel cage prior to burning (middle) ; Rice straw after burning - rice straw ash powder (bottom)

Researchers at California State University, Chico, continued an investigation into the use of rice straw ash in alkali-activated slag cement.

Rice straw ash was produced by burning washed rice straw in a kiln at 1,112 degrees Fahrenheit for one hour. The ash was then ground into a powder. The chemical composition of this substance was about 90% silica, which is beneficial to concrete. Slag and sand were mixed with an activator solution (sodium hydroxide) to make mortar samples.

Two methods were used to add rice straw ash to the mix. The first used rice straw ash without any further processing after grinding. In the second method, rice straw ash was presoaked in the activator solution for 24 hours and then mixed into the samples.

Compressive strength of mortar samples containing 5%, 10%, and 15% rice straw ash was measured at three, seven, and 28 days. Samples were compared with those that did not include rice straw ash.

Results indicate that adding rice straw ash in alkali-activated slag significantly increases compressive strength. For instance, samples containing 10% treated rice straw ash had more than twice the strength of those without rice straw ash.  Treated rice straw ash also performed better than the untreated rice straw ash. Treated rice straw ash gained higher strength when cured at higher temperatures.

Based on the results of this research, two conclusions can be drawn. First, rice straw ash can be a potential candidate to replace sodium silicate in alkali-activated systems as a strength booster. Second, addition of rice straw ash in these systems will allow concrete producers to reduce activator concentrations to achieve a given strength. This could help reduce the cost and carbon footprint of alkali-activated concrete production.

This research provides preliminary, fundamental evidence that utilization of rice straw ash in alkali-activated systems enhances properties of these systems. The project leader presented the results of this study at the American Concrete Institute convention in October 2018.