“Improving Durability of Discrete Zinc Sacrificial Anodes for Steel Rebar Protection: Biochar Incorporation into the Encapsulation Matrix.”

Core Finding

The paper proposes adding biochar particles (BCPs) into the cementitious encapsulation matrix around discrete zinc-based sacrificial anodes (DZSAs) used to protect steel rebar in chloride-contaminated concrete. The central result is that biochar-modified anodes may perform worse at the very beginning, but they deliver  much better long-term cathodic protection during repeated wet–dry exposure.

Why this Matters

Traditional ways to improve zinc anode performance are to increase matrix alkalinity or increase porosity. The paper argues both can backfire: higher alkalinity can accelerate zinc consumption, while high porosity can weaken the matrix and increase anode depletion. Biochar is presented as a “third strategy” because it can store and gradually release electrolyte while also improving sustainability.

Main Performance Results

Biochar-modified DZSA systems reduced rebar corrosion much more effectively over time than unprotected steel or non-biochar controls. The best-performing system highlighted in the paper was F1R6-DZSA, which achieved corrosion current density values consistent with passive corrosion, below 0.1 μA/cm². Other biochar systems generally moved the rebar into a metastable corrosion state, while unprotected rebar remained actively corroding.

The paper also reports that non-biochar anodes showed a sharp decline in anodic charge output over five wet–dry cycles, while biochar-modified systems maintained much higher and more stable output, often exceeding non-biochar anodes by two to three orders of magnitude after the early stage.

Proposed mechanism

The authors’ explanation is that biochar acts like an internal electrolyte buffer. Its porous structure absorbs KOH-rich electrolyte at first, which can temporarily reduce early anode activity. Later, as the system dries or electrolyte is consumed, the biochar gradually releases stored electrolyte, helping maintain ionic conductivity and slowing zinc passivation.

EIS results support this: conventional anodes increasingly shifted toward passivation and diffusion-limited behavior, while biochar-modified anodes showed an initial blocking/passivation stage followed by partial recovery and a more stable long-term electrochemical state.

Microstructural Evidence

SEM and EDS observations showed that biochar helped influence the transport and distribution of zinc oxidation products. In conventional systems, zinc corrosion products can accumulate near the anode–matrix interface and form a barrier. Biochar-modified matrices appeared to reduce harmful buildup and, in some cases, helped disperse zinc products farther into the matrix. The paper specifically notes very thin anodic product layers, around 2 μm, at the interface.

Trade-offs

Biochar is not an unconditional improvement. Adding BCPs reduced compressive strength substantially, and the effect became stronger at higher dosages. The authors therefore recommend optimizing the mix rather than maximizing biochar content. Their practical recommendation is roughly 5–10 wt.% biochar combined with 0.5–1.0 wt.% foaming agent, balancing mechanical strength, pore structure, and long-term electrochemical performance.

Biochar type Matters

The two biochar behaved differently. OR-BCP had much higher water absorptivity, larger particle size, higher surface area, and much greater carbon content. RSD-BCP contained far more mineral ash and aluminosilicate phases. These differences affected water storage, pore structure, zinc distribution, and likely the electrochemical behavior of the anode systems.

Sustainability Angle

The paper positions biochar as both a performance additive and a decarbonization tool. Because biochar can sequester biogenic carbon and can be made from agricultural or forestry residues, the authors frame this approach as a potential low-carbon corrosion-protection strategy for reinforced concrete infrastructure.

Important Caveats

This is an Article in Press, so the manuscript is unedited and may still contain errors before final publication. The study is also laboratory-based, with accelerated wet–dry cycling, so field validation will be needed. The authors note that the datasets are not publicly available because of an ongoing patent application, and two authors have filed a patent related to the technology.

Bottom line: Biochar-modified zinc sacrificial anodes appear promising because they can extend long-term cathodic protection by buffering electrolyte and moderating zinc passivation, but the mix design must be carefully optimized to avoid mechanical weakening or early-stage underperformance.