Self‑healing concrete for autonomous crack repair

A review of crystalline, microbiological, microencapsulated, and natural-agent healing pathways, including reaction mechanisms and characterization evidence for crack sealing.

The review synthesizes how self‑healing concepts can extend service life by sealing microcracks through precipitation, polymerization, or biomineralization, while emphasizing that feasibility depends on trigger conditions, transport pathways, and compatibility with cement chemistry.

Crystalline admixtures (CA)

Bacteria‑mediated CaCO₃ precipitation

Microcapsule‑triggered healing agents

Scope and classification of healing strategies

The article frames self‑healing concrete as a class of technologies designed to reduce permeability and mitigate durability loss by sealing cracks, thereby reducing maintenance burdens. Healing approaches are categorized into autogenous healing (continuing hydration and precipitation from existing cementitious constituents) and autonomous healing (externally introduced agents that activate upon a trigger such as water ingress or mechanical damage). The review further emphasizes that microcracks can be sealed more reliably than large cracks and that many strategies are optimized for early-stage crack development.

Crystalline admixtures: pore blocking and crack filling

Crystalline admixtures are described as cementitious additives that react with water and cement hydrates to form insoluble crystalline products. These products can deposit within pores and crack voids, thereby restricting capillary transport and limiting leakage pathways. The review presents two related conceptual modes: (i) complexation/precipitation reactions in the presence of a moisture gradient and (ii) precipitation reactions involving reactions with calcium-bearing phases and silicate-bearing species, producing pore-blocking solids.

Autogenous versus autonomous healing

Autogenous Continued hydration/precipitation from the cementitious matrix

Autonomous Henabled by introduced agents requiring triggers (e.g., water)

Core function: Provides the conceptual framework for mechanism selection and expected healing capacity.

CA composition contrast (oxide-level anchor)

CA vs cement (reported table) CA shows markedly different oxide fractions versus cement (see numeric anchors below)

Design implication Chemistry differences enable altered precipitation pathways

Core function: Supports the claim that CA introduces distinct reactive capacity relative to base cement.

Complexation–precipitation mode (moisture-gradient driven)

Mechanistic framing Active species move with water toward Ca²⁺-rich regions, forming precipitates

Expected outcome Enhanced crack filling via crystalline deposition

Core function: Explains why moisture ingress is treated as a trigger for CA operation.

Pore-blocking precipitation products

Outcome intent: Insoluble precipitates progressively occlude pores/crack paths

Performance aim Reduced leakage and reduced permeability through capillary interruption

Core function Links reaction product insolubility to transport property improvement.

Microbiological activity and CaCO₃ biomineralization

The review presents microbial self‑healing as incorporating alkaliphilic microorganisms that can precipitate calcium carbonate in situ. In the described pathway, bacteria (examples include Bacillus strains) produce enzymes that convert organic precursors into carbonate species; in the presence of Ca²⁺, these species precipitate CaCO₃ (calcite), which fills cracks and voids. The article highlights that the success of microbial healing depends on bacterial survival and activity in a high-alkalinity cementitious environment, nutrient supply, and crack transport conditions.

Microencapsulation: rupture‑triggered release of healing agents

Microcapsule-based systems are described as embedding capsules containing healing agents within concrete. Under mechanical stress or cracking, capsules rupture and release their cores into the crack plane, where the released agents react (or polymerize) to seal the crack and recover partial mechanical function. A representative example described involves sodium silicate-containing capsules that release silicate upon cracking; the released silicate can form binding products and fill voids, often discussed in relation to C–S–H type gel formation.

Microbial ureolysis pathway (reaction framing)

Mechanism described Enzyme-mediated conversion to carbonate species → CaCO₃ precipitation

Outcome Calcite deposition seals microcracks and reduces permeability

Core function: Links biological activity to mineral crack filling.

Reported strength improvement with microbial addition

Reported anchor Compressive strength after 28 days increases substantially in a cited bacterial system (numeric below)

Core function: Provides performance evidence in terms of a mechanical endpoint.

Microcapsule design: agent release upon rupture

Trigger Mechanical stress/crack formation ruptures capsules

Agent role Released core initiates chemical sealing reaction in the crack zone

Core function: Formalizes microencapsulation as a triggerable delivery mechanism.

Sodium silicate capsule pathway (binding gel formation)

Mechanistic framing Released silicate reacts with cementitious species; partial crack filling and strength recovery are described

Core function Connects capsule release to a plausible cement-chemistry-consistent sealing route.

Sodium alginate as a hydrogel-forming healing agent

Sodium alginate (described as derived from brown algae) is presented as a natural polymer capable of forming hydrogels through crosslinking with divalent cations, especially Ca²⁺. In a concrete environment, the release of Ca²⁺ (and the presence of calcium-bearing phases) can promote calcium-alginate formation, yielding a swollen hydrogel network that may occupy microcracks and reduce transport. The review frames the approach as relying on polymer swelling and ion-mediated crosslink formation to achieve crack filling.

Eggshell powder as calcium carbonate source and microstructure modifier

Eggshell is described as naturally impermeable and rich in calcium carbonate. When processed into powder and incorporated into concrete, it is presented as contributing CaCO₃ and interacting with cement hydration pathways. The article highlights composition characteristics (dominant CaCO₃ with minor organic and phosphate fractions) and proposes that reaction products can fill pores and improve durability-related properties by reducing permeability and enhancing matrix stability.

Sodium alginate: hydrogel swelling and crack filling

Mechanism Ca²⁺ crosslinking creates a three-dimensional hydrogel network

Functional behavior Swelling under water contact supports void filling

Core function: Provides a non-mineral, physically driven sealing route.

Chemical sequence for alginate in concrete (as presented)

Step logic Dissolution/ion exchange → calcium alginate formation

Core function: Links polymer chemistry to concrete calcium availability and sealing outcome.

Eggshell powder composition anchor

Reported composition ~94% CaCO₃; ~4% organic content; ~1% calcium phosphate (minor)

Core function: Positions eggshell as a calcium carbonate donor and microstructure modifier.

Eggshell-mediated microstructure refinement

Mechanistic framing Pore filling and hydration-pathway influence (e.g., monocarboaluminate formation described)

Core function Connects natural mineral addition to permeability and durability improvements.

Reaction pathways, characterization evidence, and design constraints

Crystalline admixture healing is presented as driven by water-mediated transport of reactive species and precipitation of insoluble products. The review implies that effectiveness depends on the availability of calcium-bearing phases, water ingress as a trigger, and the capacity for crystalline products to form within crack geometries rather than only in bulk pore space.

Microbiological healing relies on microbial viability, nutrient availability, and a pathway to generate carbonate species in situ. The review notes that healing effectiveness is coupled to the cementitious chemical environment (alkalinity, availability of Ca²⁺) and transport (water access to activate bacterial metabolism).

Biomineralization conditions for bacterial healing

Capsule-based strategies require that capsules survive mixing and placement yet rupture under crack-induced stress. The review presents rupture-triggered release as the enabling event and implicitly treats capsule wall design and distribution as governing parameters.

Microcapsule payload and rupture mechanics

The sodium silicate capsule pathway is framed through the formation of binding gel products (C–S–H–type) that can partially fill cracks. The review includes reaction descriptions consistent with silicate reacting with calcium hydroxide and subsequent formation of calcium carbonate and silica-related products under CO₂ exposure.

Sodium silicate reactions and binding product formation

Water‑retaining and water‑exposed infrastructure

Crack sealing to limit leakage and transport-driven degradation.

Durability‑critical concrete elements

Permeability reduction to slow ingress of aggressive species.

Maintenance‑limited structures

Systems where autonomous crack sealing can reduce inspection/repair frequency.

Protective structural systems

Microcrack sealing as part of resilience design in exposure-intensive environments.

Quantitative anchors reported in the review

Compressive strength (28 d)

A bacterial system is reported to reach 52.0 MPa after 28 days, compared with 37.25 MPa for a conventional reference (context-specific).

EDS calcium increase

EDS analysis reports an increase in calcium content from 12.03 to 40.41 (weight %) alongside evidence consistent with calcite formation (context-specific).

Bacterial dosage anchor

A bacterial additive dosage is reported as 18.18 kg/m³ with a liquid-culture introduction described at 20 mL/L (context-specific).

Search

Trending Queries

Quick Links

Actions

Best Results