Waste from our toilets could be used to make stronger concrete while replacing part of the cement.
The researchers used sludge from a treatment facility in India. It was dried, then heated to between 350 and 450°C with very little oxygen. This treatment produces a carbon-rich powder called biochar.
Cylindrical, cubic and prismatic concrete specimens containing biochar, prepared for mechanical tests. Credit: study authors, Scientific Reports, CC BY 4.0.
This powder then replaced part of the cement in several mixtures. The researchers tested proportions of 5, 10 and 15%. They measured their strength, ability to absorb water, porosity and shrinkage during drying. Conventional concrete served as a reference.
The results become particularly interesting after 91 days of curing. With 10% of the cement replaced, compressive strength increases by 21% compared with the reference concrete. Flexural strength rises by 42%. The mixture containing 5% biochar also achieves good results, with respective increases of 20 and 36%.
To understand this improvement, we need to look more closely at the powder. Biochar contains many tiny pores capable of retaining water. This water remains available during cement curing. Its very fine particles can also fill small spaces between the other components, making the material denser.
Biochar also contains silica. This participates in chemical reactions that produce more compounds contributing to the concrete's strength. Under a microscope, the 5% mixture therefore has a denser, better-bonded structure than conventional concrete.
Adding more biochar does not improve the material indefinitely, however. With 15% replacement, the researchers observe more pores and areas that are less well bonded. Performance therefore remains highly dependent on the dosage. The tests also show that the 5 and 10% mixtures retain interesting properties regarding water absorption and shrinkage.
Before considering buildings made with this material, further testing will be necessary. In particular, the study did not evaluate its behavior under freeze-thaw cycles, exposure to salt or extreme temperatures. Nor did it directly measure the carbon footprint resulting from replacing part of the cement.