Ahmed et al. (2024) — Employing Limestone and Calcined Clay for Preserving the Strain-Hardening Response of PET Fiber-Reinforced Cementitious Composites

Citation

Ahmed, A. H., Signorini, C., Chikhradze, M., Liebscher, M., Butler, M., & Mechtcherine, V. (2024). Employing limestone and calcined clay for preserving the strain-hardening response of PET fiber-reinforced cementitious composites. Construction and Building Materials, 438, 137166.

DOI: 10.1016/j.conbuildmat.2024.137166
Atlas layer: extension
Related Victor Li book chapter: Chapter 7: Alternative Reinforcing Fibers (PET Fibers & Hydrophobic Interfaces) & Chapter 9: Green ECC (Limestone Calcined Clay Cement - LC3) & Chapter 10: Durability and Long-Term Aging
Source PDF: ahmed-2024-employing-limestone-and-calcined-clay.pdf
Extracted text: full_text/ahmed-2024-employing-limestone-and-calcined-clay_full_text.md
Source note: source_notes/ahmed-2024-employing-limestone-and-calcined-clay_source_note.md

Why this paper matters

Solves the long-standing degradation problem of low-cost polyethylene terephthalate (PET) fibers in cementitious environments by coupling them with a high-volume Limestone Calcined Clay (LC3) matrix (75 wt% PC replacement). Demonstrates that lowering pore solution alkalinity preserves PET fiber integrity and achieves high direct tensile strain capacity (> 4 %) without expensive PE or PVA fibers.

Main contribution

Develops an eco-efficient SHCC/ECC system utilizing 2.0 vol. % virgin PET fibers in an LC3 binder matrix (75 % Portland cement replaced by calcined clay, limestone, and gypsum).
Solves alkaline hydrolysis of PET fibers: The pozzolanic reaction of calcined clay consumes portlandite ($Ca(OH)_2$) and binds alkalis, decreasing pore solution pH to ~12.5, preventing fiber ester group saponification and surface pitting.
Confirms robust pseudo strain-hardening (PSH): Directly measures uniaxial tensile strain capacity exceeding 4.0 % (4.2–4.5 %) and tensile strength of ~3.5–4.2 MPa with multiple micro-cracks across 7 to 60 days of aging.
Provides micromechanical validation showing that LC3's reduced matrix fracture toughness ($K_m$) and moderate friction bond with hydrophobic PET fibers satisfy the steady-state cracking energy criterion ($J_b'/J_{tip} \ge 3$).

Evidence summary

Binder Composition: LC3 binder with 25 wt% Ordinary Portland Cement (OPC), 50 wt% calcined clay (metakaolin/calcined illite-smectite), 15 wt% limestone powder, and 10 wt% gypsum/anhydrite ($w/b \approx 0.30\text{--}0.35$, quartz sand aggregates).
Direct Tensile Performance:
Uniaxial tensile strain capacity: > 4.0 % at 28 days and maintained at 60 days.
First-crack strength: 2.5–3.0 MPa; Ultimate tensile strength: 3.5–4.2 MPa.
Compressive Strength: 42–52 MPa at 28 days.
PET Fiber Characteristics: Diameter $d_f = 40\ \mu\text{m}$, length $l_f = 12\text{ mm}$, tensile strength $\approx 700\text{ MPa}$, failure elongation $\approx 34\%$, Young's modulus $\approx 10\text{ GPa}$.
Aging & Hydrolysis Mechanism: In standalone saturated $Ca(OH)_2$ pore solution (pH 12.5+), unprotected PET fibers lose load capacity rapidly. However, within the dense LC3 matrix with reduced alkalinity and lower free water, fibers retain load-bearing capacity without surface embrittlement.

Linked Atlas nodes

02_concepts/strain_hardening_criteria.md
02_concepts/interface_properties.md
04_material_systems/green_ecc.md
04_material_systems/pet_fiber_ecc.md
05_experiments/direct_tensile_test.md
04_material_systems/low_carbon_binders.md

Relationship to Victor Li book

Extends Victor Li (2019) Chapter 7 (Alternative Polymeric Fibers) by demonstrating that high-elongation, compliant PET fibers (34 % elongation) can produce high-performance ECC when paired with tailored low-toughness, low-alkalinity LC3 binders.
Extends Chapter 9 (Green ECC) by replacing 75 wt% of OPC with limestone and calcined clay, cutting composite embodied carbon by > 50 % while achieving $\epsilon_u > 4\%$.
Directly addresses Chapter 10 (Long-Term Durability) regarding alkali-induced fiber degradation mechanisms.

Claim-evidence rows to add

Atlas node	Claim	Evidence summary	Page/Figure/Table	Status
`04_material_systems/pet_fiber_ecc.md`	LC3 binder enables ductile strain-hardening in PET fiber-reinforced composites with tensile strain capacity exceeding 4 %	Uniaxial tensile tests show $\epsilon_u > 4.0\%$ and $\sigma_u = 3.5\text{--}4.2\text{ MPa}$ at 28 and 60 days	Section 3.3, Fig. 8, Table 4	verified_from_pdf
`04_material_systems/low_carbon_binders.md`	Replacing 75 wt% OPC with limestone and calcined clay reduces pore solution alkalinity, preventing PET fiber alkaline hydrolysis	Consumption of $Ca(OH)_2$ by calcined clay pozzolanic reaction suppresses fiber surface pitting and maintains in-matrix fiber tensile capacity	Section 3.1 & 3.2, Fig. 5-7	verified_from_pdf
`02_concepts/strain_hardening_criteria.md`	Low matrix fracture toughness of LC3 allows compliant PET fibers ($E_f \approx 10\text{ GPa}$) to satisfy the energy criterion ($J_b'/J_{tip} \ge 3$)	Steady-state flat crack propagation achieved with multiple micro-cracks without premature localized rupture	Section 3.3.3, Fig. 9	verified_from_pdf

Verification status

PDF preserved: yes (ahmed-2024-employing-limestone-and-calcined-clay.pdf)
Text extracted: yes (full_text/ahmed-2024-employing-limestone-and-calcined-clay_full_text.md)
DOI verified: yes (10.1016/j.conbuildmat.2024.137166)
Metadata verified: yes (CBM, Vol. 438, 137166, 2024)
Claim-evidence matrix ready: yes

Cautions

PET fibers are hydrophobic and compliant; standard OPC matrices with high fracture toughness fail to show strain-hardening with virgin PET. Matrix toughness reduction via LC3 is mandatory.
Virgin PET fibers degrade if exposed to high alkaline pore fluids (pH > 13.0); maintaining low alkalinity via pozzolanic SCMs is essential for long-term durability.