Modified mortar could rise from ashes

Corn straw ash might become part of more sustainable building mortars

Ash produced by burning crop waste could become an environmentally friendly replacement for part of the cement used to make construction mortar. Researchers in Brazil report the results of their investigation into the suitability of corn straw for this purpose in the journal Cement.

“This is a promising alternative thatx could reduce the use of natural raw materials, energy consumption, and carbon dioxide emissions caused by cement production,” says first author Charles Prado Ferreira de Lima from the Universidade Estadual do Norte Fluminense Darcy Ribeiro. He undertook the study as part of his Masters degree under the supervision of Guilherme Cordeiro.

The authors explain that the cement industry faces a daunting challenge in reconciling the ever-increasing demand for cement with the damaging environmental impact of its production. Heating calcium carbonate to make calcium oxide for cement is an energy-intensive process that also releases the greenhouse gas carbon dioxide.

Silica-rich ash from agricultural waste, including corn straw, rice husk, sugar cane bagasse and elephant grass, could be added to generate more environmentally friendly forms of effective mortar for cement. Corn ash holds particular potential as it is the most abundant form of waste biomass generated worldwide, at around 1.4 billion metric tons per year.

The ability of silicon- and aluminum-containing materials such as ash to be converted into cements is described as their pozzolanic potential. The term is derived from the natural volcanic ash called pozzolana, a substance that reacts with water and alkali to form natural cement.

Ferreira de Lima developed a method for producing highly reactive pozzolanic ash from corn straw through a process of treating with citric acid, two-step burning and ultrafine grinding. “This produced better results than expected,” he says.

The key to the success was careful analysis of the effects of different pretreatments of the straw and different subsequent burning and processing steps. Pretreatment with acid proved particularly effective in generating ash with high pozzolanic activity. The researchers were also able to produce ash with suitably low levels of unwanted chemical contaminants.

The compressive strength of mortar made with the most suitable corn straw ash was shown to be greater than that of traditional cement-based mortar. These impressive results were obtained when the ash replaced cement at between 20 and 30 percent of total mass. At that level the ash clearly has potential to offer a significant reduction in the more environmentally damaging aspects of cement production needed to make effective mortar.

The team hope that the research will now continue on several fronts. Ferreira de Lima says the production of the corn straw ash needs to be assessed on an industrial scale to verify its full commercial potential. Work is also needed to better characterize the influence of ash supplementation on concretes in both their fresh and hardened states, and to evaluate the durability of these concretes. Any significant change in the material used to hold buildings together must obviously be undertaken with great care.

Article details: Ferreira de Lima, C, et al.: “Evaluation of corn straw ash as supplementary cementitious material: Effect of acid leaching on its pozzolanic activity,” Cement (2021)