Biochar in concrete and asphalt: what the evidence shows

In cement, the best evidence is a meta-analysis of 606 paired observations, where the subgroup dosed below 2.5% of binder weight raised compressive strengthby 6% at 7 days and 7% at 28 days, while higher dosages reduced it — and the whole effect fades to nonsignificance once sand and gravel are added. In asphalt, biochar improves high-temperature performance and tends to worsen low-temperature performance, on a much thinner evidence base.

Zhao and colleagues (2024) pooled 606 paired observations on biochar in Portland cement composites, split across three dosage bands — below 2.5%, 2.5–5% and above 5% — and two test ages, 7 and 28 days. None of the figures below comes from all 606 observations: each one belongs to a subgroup. Their findings: biochars from plant-based feedstocks, with the exception of rice and hardwood, improved 28-day compressive strength by 3–13%. Small particle sizes added 2–7%. Pyrolysis above 450 °C at a heating rate near 10 °C per minute was more effective. And dosage below 2.5% of binder weight — binder meaning cement plus biochar, not cement alone — raised compressive strength by 6% at 7 days and 7% at 28 days. Above that level strength falls away: the authors state that higher biochar doses negatively impacted compressive strength. For that statement they point to a figure, not to a number: the size of the decline appears nowhere in the text, so no percentage should be quoted for it. The mechanisms the authors offer — biochar's porous structure, dilution of the cement hydration products, and agglomeration through van der Waals forces above 5% of cement weight — are cited from earlier studies, not findings of this meta-analysis.

There is one place where the paper contradicts itself, and it is worth saying out loud because it changes how everything above reads. On page 12, reporting results, the authors write that higher biochar dosages negatively impacted compressive strength. In the conclusions they write the opposite: "Overall, we concluded that adding pyrolytic biochars did not decrease the compressive strength of Portland cement composites." Both sentences are in the same paper. We do not average them and we do not pick the convenient one — both stay here, and the rest of this article follows the first, which is the more conservative of the two.

What the paper does not cover is as important. It analyses compressive strength only. It does not analyse flexural strength, tensile strength or permeability, so it cannot be cited for any of them. It also does not quantify carbon storage in the resulting composite, so it cannot be cited for that. CO₂ does appear as a treatment rather than only as motivation: carbonation curing raised compressive strength by 30% at 7 days and 21% at 28 days.

The paper reports that adding fine and coarse aggregates — sand and gravel — diminished the biochar effect and rendered it nonsignificant.

That single sentence reframes the whole result. The strength gains are clearest in cement pastes and mortars. In concrete, which is what most people mean when they ask about this, the signal largely washes out. A supplier quoting "3 to 13% stronger concrete" from this paper has dropped the condition that the paper itself attaches.

This does not make biochar in cementitious materials pointless. It means the honest case rests on the applications where the effect holds, on what happens to the carbon, and on displacing a share of a very emissions-intensive binder — not on a blanket strength claim. 

The asphalt evidence is over a decade of laboratory work — the earliest studies date from 2014 — gathered in several narrative reviews, with no meta-analysis and essentially no verifiable field trials. Ke and colleagues (2026) state it directly: within the literature they review, "field evidence on the cracking and fatigue performance of biochar-modified asphalt remains scarce, with most findings derived from binder- and laboratory mixture-scale tests." Rondón-Quintana and colleagues (2022) summarise the direction: biochar increases the performance of asphalt binders in high-temperature climates and tends to reduce it in low-temperature ones. The same review notes that "few studies have evaluated the performance of BC on asphalt mixes and the long-term properties associated with durability."

Wani and Garg (2024) report improved complex modulus and rutting resistance at 10% biochar, and fatigue resistance temperature raised by 4.6 °C at up to 20%, while stating that efficiency at low temperatures "has not yet been satisfactory" and that aging behaviour remains a noticeable limitation. On dose the reviews do not agree: one recommends 5–10% by weight of binder with particle size under 75 µm, another discusses studies at 10% and up to 20%.

For a hot tropical or high-altitude climate this asymmetry is favourable — the improvement is on the side that matters and the penalty is on the side that rarely binds. But the honest framing is that this is a promising laboratory result awaiting field validation, not a proven pavement technology.

The construction route for biochar is real but narrower than it is usually sold. In cement the pooled evidence supports modest strength gains at low dosages, clearest in pastes and mortars and fading in concrete. In asphalt it supports better rutting resistance and worse low-temperature behaviour, from a laboratory literature that has not yet been tested in the field. Both are worth pursuing. Neither yet supports a performance guarantee, and the useful question for a buyer is which specific application, at which dosage, tested where. 

· Zhao, Z., El-Naggar, A., Kau, J., Olson, C., Tomlinson, D. & Chang, S. X. (2024). "Biochar affects compressive strength of Portland cement composites: a meta-analysis." Biochar 6(1): 21. DOI 10.1007/s42773-024-00309-2. Published 6 Mar 2024 (consulted 30 Sep 2026)· Rondón-Quintana et al. (2022). "Use of Biochar in Asphalts: Review." Sustainability 14(8): 4745 (consulted 30 Sep 2026)· Yaro et al. (2023). "A Comprehensive Review of Biochar Utilization for Low-Carbon Flexible Asphalt Pavements." Sustainability 15(8): 6729 (consulted 30 Sep 2026)· Wani, S. & Garg, N. (2024). "Critical review on use of biochar as a modifier in asphaltic binders for pavement construction." Environment, Development and Sustainability. DOI 10.1007/s10668-024-05784-y. Published online 7 Dec 2024 (consulted 30 Sep 2026)· Zhou (2024). "A Review of Biomass-Derived Biochar and Its Potential in Asphalt Pavement Engineering." Materials Science-Poland 42(2): 81–99. DOI 10.2478/msp-2024-0022 (consulted 30 Sep 2026)· Ke, Y., Pang, E., Gustave, W., Gu, B., Chen, H., Song, Y., Lin, W., Zhang, X. & He, F. (2026). "Performance Tailoring and Environmental Implications of Biochar-Modified Asphalt Materials: Toward Sustainable Road Design." Infrastructures 11(9): 305. DOI 10.3390/infrastructures11090305. Published 28 Aug 2026 (consulted 2 Oct 2026)

Methodology: conditions attached to a finding in its source are reproduced here rather than dropped. Where the reviews disagree on dose, both figures are given. No figure is applied to any specific mix design or project.