Researchers have converted processed human waste into biochar that can be used as a partial replacement for cement in concrete, in an experiment that showed that adding specific proportions of it may enhance the strength of concrete and reduce its porosity and shrinkage.
Concrete is one of the most widely used building materials in the world, but the production of one of its basic components, cement, is a major source of carbon dioxide emissions, due to the energy and heat required to process the limestone.
Therefore, researchers are searching for materials that can replace some of the cement without compromising the strength and durability of concrete. In this context, a team of researchers led by civil engineer Ragovich Tiwari from Manipal University in Jaipur, India, tested the use of biochar made from sewage sludge in concrete production.
The results, presented in a research paper accepted for publication in the journal Scientific Reports, showed that adding specific amounts of this biochar improved a number of concrete properties.
Biochar is produced by heating organic materials in a low-oxygen environment. It can be made from various sources, such as sawdust and rice husks, but this time the researchers tested an unusual source: sewage sludge resulting from the treatment of human waste.
The team obtained the sludge from a treatment plant in Warangal, India, then dried and heated it to temperatures between 350 and 450 degrees Celsius in a low-oxygen environment. Afterward, the material was ground and sieved to obtain a fine powder of biochar.
The researchers used this powder to replace 5%, 10% and 15% of the cement in concrete mixes, then compared its performance to traditional concrete by measuring compressive and flexural strength, as well as water absorption, porosity and shrinkage.
Tests showed that the best results were achieved when part of the cement was replaced with biochar at a rate of 5% or 10%.
After 91 days of treatment, the compressive strength of the mixture containing 5% biochar increased by about 20% on average, while the flexural strength increased by about 36%.
When the replacement ratio was increased to 10%, the increase was approximately 21% in compressive strength and 42% in bending strength.
Concrete containing 5% biochar also showed lower water absorption, lower porosity, and less shrinkage during drying compared to conventional concrete. At 10%, its performance remained close to that of ordinary concrete in these properties.
However, increasing the proportion of biochar to 15% resulted in lower quality results, as the concrete continued to gain strength during curing, but its overall strength decreased compared to mixtures containing 5% and 10%.
Researchers link these results to several properties of biochar. Its high porosity allows it to absorb and retain water, then gradually release it as the concrete hardens, providing the water necessary for the chemical reactions that help the cement gain strength.
Biochar also contains silica, which can react with compounds produced during cement hardening to form more calcium silicates, materials that contribute to the strength of concrete. These reactions are known as pozzolanic reactions.
In addition, fine particles of biochar can fill some of the voids within concrete and improve the compaction and bonding of its components.
Microscopic examination confirmed this idea, as samples containing 5% biochar showed a denser and more cohesive structure. In contrast, samples with 15% biochar exhibited weaker pores, cracks, and bonding areas, which may explain their reduced strength.
Despite the encouraging results, this technology is still in the research phase and cannot be considered ready for widespread use in the construction sector.
Researchers need to test the performance of concrete under realistic conditions, such as freeze-thaw cycles, exposure to salinity, and extreme temperatures.
Sewage sludge also raises the issue of heavy metals, which may remain trapped within the concrete after use. Researchers need to determine the long-term stability of this trapping and whether these metals can leach into the environment over time.
Furthermore, the study did not assess the impact of using biochar made from sludge on total carbon emissions, which is a key factor in determining the actual environmental benefit of this method.
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