That number headlines the abstract, but it is not a comparison with ordinary concrete. The charts in the same paper tell a smaller story, though they still favor the sludge mixes in several tests.<\/p>\n\n\n\n
Civil engineer Raghuvesh Tiwari and Priyansha Mehra of Manipal are the corresponding authors. Their co-authors are Sanchit Anand, also of Manipal, and Shaik Hussain of Louisiana Tech University.<\/p>\n\n\n\n
From Warangal to the mixer<\/h2>\n\n\n\n
The sludge came from a treatment plant at the Site Sanitation Resource Park in Warangal, in the Indian state of Telangana. Plants of this kind treat human waste pumped from septic tanks. The study credits Tide Technocrats, a Telangana company, with supplying the biochar.<\/p>\n\n\n\n
The sludge was first dried at about 221\u00b0F (105\u00b0C) for 24 hours. It then spent two hours in a low-oxygen furnace at 662\u00b0F to 842\u00b0F (350\u00b0C to 450\u00b0C).<\/p>\n\n\n\n
This process, known as pyrolysis<\/a>, leaves a carbon-rich solid. Grinding and sieving turned it into a dark powder fine enough to stand in for part of the cement.<\/p>\n\n\n\n Using that powder, the team cast cylinders and beams at five water-to-binder ratios, from 0.40 to 0.60. Each batch was broken at 28, 56 and 91 days. Plain mixes with no biochar went through the same tests.<\/p>\n\n\n\n The abstract says concrete with 10 percent biochar “demonstrated a 21% increase in compressive strength and a 42% increase in flexural strength” after 91 days. Compressive strength is resistance to crushing. Flexural strength is resistance to bending.<\/p>\n\n\n\n The abstract does not say what those gains were measured against, and the figures do not match a comparison with plain concrete. If the baseline were plain concrete, the 10 percent mix should beat the 5 percent mix in bending. The authors, however, write that the 5 percent dosage “yielded the highest flexural strength.”<\/p>\n\n\n\n A better clue sits in the compression bars of Figure 15. From 28 days to 91 days, the 10 percent mixes grew by roughly 16 to 28 percent, close to the reported 21 percent on average. So the gains look like growth over time, although the flexural percentages are harder to reproduce from the chart.<\/p>\n\n\n\n Set beside the no-biochar beams at 91 days, the 10 percent mix looks far more modest. At the 0.40 ratio, Figure 17 shows about 770 pounds per square inch (5.3 newtons per square millimeter) in bending. The control reached about 710 psi (4.9), so the lead is around 8 percent.<\/p>\n\n\n\n Across all five ratios, that gap runs from about 1 percent weaker to about 15 percent stronger. In compression, the same mix sits about 1 to 14 percent above its control. At 28 days, the authors report, it trailed plain concrete in both tests.<\/p>\n\n\n\n The 5 percent recipe holds up better. Its compression bars stand roughly 9 to 16 percent above the controls at 91 days.<\/p>\n\n\n\n The authors also report that water absorption and porosity at 15 percent biochar were comparable to conventional control mixes at 28 days, and that drying shrinkage of a 10 percent mortar mix resembled ordinary Portland cement mortar at 120 days. These chart readings are estimates, since the text does not print the underlying values.<\/p>\n\n\n\n Biochar is full of tiny cavities that soak up mixing water and release it later. That reserve lets the surrounding cement keep reacting as it hardens. Silica in the powder also combines with calcium hydroxide, a cement byproduct, to form extra binding minerals.<\/p>\n\n\n\n At 15 percent, those benefits gave way. Electron microscope images showed a layered structure with voids and cracks. The authors blame a dilution effect, with too little cement left to form binding compounds.<\/p>\n\n\n\n A 2025 study<\/a> from the University of Saskatchewan saw a similar ceiling with waste biomass biochar. There, a 2 percent dose raised compressive strength by 18.95 percent, and higher doses hurt.<\/p>\n\n\n\n Other teams have chased the same idea with different feedstocks, including seaweed biochar in concrete<\/a> at the University of Miami. Each source behaves differently, so one good dose says little about another powder.<\/p>\n\n\n\n The authors put cement at 5 to 8 percent of global carbon dioxide emissions. The International Energy Agency<\/a> calls making clinker, cement’s main active ingredient, the most emissions-intensive step of production. Blending in alternative ingredients is one way to use less of it.<\/p>\n\n\n\n Sludge biochar would add to a growing list of substitutes, from graphene additives that cut cement use<\/a> to basalt in place of limestone<\/a>. The paper offers no life cycle tally, however. Energy spent drying, heating, grinding and hauling the sludge has not been weighed against the cement saved.<\/p>\n\n\n\n Heavy metals are the sharpest open question for anything made from sewage. The authors conclude that more biochar reduced heavy metal levels in concrete.<\/p>\n\n\n\n Their Table 7, however, shows mercury rising with the dose: 5.09 parts per billion at 5 percent, 24.9 at 10 percent and 35.2 at 15 percent. Raw biochar held 45.4.<\/p>\n\n\n\n Those readings show what the samples contain once dissolved in acid, not what might escape from hardened concrete. The team says a standard toxicity leaching test will come in future studies “to ensure the environmental safety of material.”<\/p>\n\n\n\n Durability is also unfinished, since “comprehensive durability investigations could not be incorporated” in this paper. Freeze-thaw cycles, salty coastal air and extreme temperature swings are still untested. The sludge also came from a specific subset of plants, and the authors warn that scaling up could prove costly in some developing regions.<\/p>\n\n\n\n![\"Raw](\"https:\/\/www.ecoticias.com\/en\/wp-content\/uploads\/2026\/11\/fecal-sludge-biochar-powder.jpg\")
Tracing the 42 percent claim<\/h2>\n\n\n\n
Against plain concrete, a narrower lead<\/h2>\n\n\n\n
Why 5 percent beat 15<\/h2>\n\n\n\n
Cement’s carbon bill<\/h2>\n\n\n\n
Mercury leaching remains untested<\/h2>\n\n\n\n