Kan et al. (2019) — Self-Healing of Engineered Geopolymer Composites Prepared by Fly Ash and Metakaolin

Citation

Kan, L.-L., Lv, J.-W., Duan, B.-B., & Wu, M. (2019). Self-healing of Engineered Geopolymer Composites prepared by fly ash and metakaolin. Cement and Concrete Research, 125, 105895.

DOI: 10.1016/j.cemconres.2019.105895
Atlas layer: core
Related Victor Li book chapter: Chapter 9: Green ECC & Chapter 10: Long-Term Durability and Self-Healing Mechanisms
Source PDF: kan-2019-self-healing-of-engineered.pdf
Extracted text: full_text/kan-2019-self-healing-of-engineered_full_text.md
Source note: source_notes/kan-2019-self-healing-of-engineered_source_note.md

Why this paper matters

The landmark Cement and Concrete Research study uncovering the chemical and physical mechanism of autogenous self-healing in cementless Engineered Geopolymer Composites (MFA-EGC). Disproves the traditional assumption that self-healing requires calcium carbonate ($\text{CaCO}_3$) precipitation, proving that low-calcium geopolymers heal autonomously through secondary geopolymerization forming amorphous sodium aluminosilicate hydrate (N-A-S-H) gel.

Main contribution

Develops PVA-reinforced fly ash/metakaolin engineered geopolymer composites (MFA-EGC, 2.0 vol. % PVA) exhibiting initial tensile strain capacities of 3.5 % to 5.2 % and compressive strengths of 32–45 MPa.
Preloads specimens to controlled tensile damage strains ($\epsilon_{pre} = 1.0\%, 2.0\%, 3.0\%$) and compares self-healing efficacy under laboratory air exposure versus wet-dry cycling.
Quantifies tensile recovery: pre-damaged specimens recover up to 100 % of their initial tensile strain capacity and ultimate tensile strength, with cracks narrower than 30 $\mu\text{m}$ completely closing.
Unveils that air exposure provides superior healing over wet-dry cycles because excessive water immersion leaches unreacted soluble alkali silicates away from crack faces.
Utilizes SEM-EDS, XRD, and FT-IR to establish that the healing precipitate is amorphous N-A-S-H aluminosilicate gel rather than crystalline calcite.

Evidence summary

Material Matrix: Class F Fly Ash + Metakaolin precursors activated by liquid sodium silicate ($\text{Na}_2\text{SiO}_3$) and $\text{NaOH}$ solution ($M_s = 1.2\text{--}1.5$).
Fiber Reinforcement: 2.0 vol. % Kuraray PVA fibers ($l_f = 12\text{ mm}, d_f = 39\ \mu\text{m}, \sigma_f = 1600\text{ MPa}$).
Pre-Damaged Strain Levels: $\epsilon_{pre} = 1.0\%, 2.0\%, 3.0\%$.
Self-Healing Regimes:
Exposure A: Laboratory air (20 °C, 50 % RH) for 28 days.
Exposure B: Wet-dry cycling (24 h water immersion at 20 °C + 24 h air drying) for 28 days.
Tensile Recovery Metrics:
Crack closure: Saturated microcracks ($w < 30\ \mu\text{m}$) showed 90–100 % visual closure.
Tensile strain capacity recovery: Tensile ductility after air-healing recovered to 3.8–5.5 % (recovery ratio $\ge 100\%$).
Resonant frequency & stiffness recovery: Initial elastic modulus recovered by 75–92 %.
Chemical Characterization:
SEM-EDS: High Si/Al ratio precipitates bridging opposite crack surfaces.
XRD & FT-IR: Broad amorphous hump with Si-O-Al / Si-O-Si stretching vibrations around $1000\text{ cm}^{-1}$, verifying N-A-S-H gel synthesis.

Linked Atlas nodes

02_concepts/strain_hardening_criteria.md
04_material_systems/geopolymer_ecc.md
04_material_systems/green_ecc.md
04_material_systems/pva_ecc.md
04_material_systems/self_healing_ecc.md
05_experiments/direct_tensile_test.md
05_experiments/self_healing_evaluation.md

Relationship to Victor Li book

Extends Victor Li (2019) Chapter 10 (Long-Term Durability and Self-Healing, pp. 267–305).
Expands self-healing science beyond Portland cement: shows that the micromechanical tight-crack control of ECC ($w < 50\ \mu\text{m}$) enables zero-cement geopolymer matrices to undergo secondary condensation polymerization, autonomously restoring mechanical integrity without requiring calcium or external repair agents.

Claim-evidence rows to add

Atlas node	Claim	Evidence summary	Page/Figure/Table	Status
`04_material_systems/self_healing_ecc.md`	Fly ash/metakaolin EGC autogenously heals tensile microcracks through amorphous N-A-S-H geopolymer gel formation	SEM-EDS, XRD, and FT-IR chemical characterization of crack-bridging precipitates	Section 3.3 & 4, Fig. 10-14	verified_from_pdf
`04_material_systems/self_healing_ecc.md`	Preloaded MFA-EGC achieves $\ge 100\%$ tensile strain capacity recovery after 28 days of air exposure	Uniaxial tensile reloading tests of specimens preloaded to 1.0–3.0 % tensile strain	Section 3.2, Fig. 6-9, Table 3	verified_from_pdf
`05_experiments/self_healing_evaluation.md`	Air exposure is more favorable than wet-dry cycling for geopolymer self-healing by preventing alkali leaching	Comparative mechanical recovery and microscopic crack closure tracking under air vs wet-dry cycles	Section 3.1 & 4.2, Fig. 4 & 5	verified_from_pdf

Verification status

PDF preserved: yes (kan-2019-self-healing-of-engineered.pdf)
Text extracted: yes (full_text/kan-2019-self-healing-of-engineered_full_text.md)
DOI verified: yes (10.1016/j.cemconres.2019.105895)
Metadata verified: yes (Cement and Concrete Research, Vol. 125, 105895, 2019)
Claim-evidence matrix ready: yes

Cautions

Cracks wider than 50 $\mu\text{m}$ do not achieve full closure; self-healing relies strictly on ECC's intrinsic tight crack width capability.
Wet-dry cycles with high water exchange volume cause alkali cation leaching ($\text{Na}^+$), slowing down secondary geopolymerization.