Lao et al. (2023/2022) — Seawater Sea-Sand Engineered Geopolymer Composites (EGC) with High Strength and High Ductility

Citation

Lao, J.-C., Huang, B.-T., Xu, L.-Y., Khan, M., Fang, Y., & Dai, J.-G. (2023). Seawater Sea-sand Engineered Geopolymer Composites (EGC) with High Strength and High Ductility. Cement and Concrete Composites, 138, 104974.

DOI: 10.1016/j.cemconcomp.2023.104974
Atlas layer: core
Related Victor Li book chapter: Chapter 4: Pseudo Strain-Hardening Criteria & Chapter 9: Green ECC (Seawater Sea-Sand Systems) & Chapter 10: Long-Term Marine Durability
Source PDF: lao-2022-seawater-sea-sand-engineered-geopolymer-composites.pdf
Extracted text: full_text/lao-2022-seawater-sea-sand-engineered-geopolymer-composites_full_text.md
Source note: source_notes/lao-2022-seawater-sea-sand-engineered-geopolymer-composites_source_note.md

Why this paper matters

A landmark breakthrough study from The Hong Kong Polytechnic University developing Seawater Sea-Sand Engineered Geopolymer Composites (SS-EGC) for the first time, simultaneously breaking the 140 MPa compressive strength barrier and achieving ~8.0 % direct tensile strain capacity with zero freshwater, zero river sand, and zero Portland clinker.

Main contribution

Synthesizes the first ultra-high-strength, ultra-high-ductility Seawater Sea-Sand EGC (SS-EGC) utilizing unwashed raw sea-sand, marine seawater, and 2.0 vol. % PE fibers.
Benchmarks two precursor systems: FA-rich (FA/Slag = 8:2) and Slag-rich (FA/Slag = 2:8) against companion freshwater and washed-sand control mixes.
Achieves extraordinary mechanical performance: 28-day compressive strength exceeding 140 MPa, ultimate tensile strength over 13.0 MPa, and direct tensile strain capacity of ~8.0 %.
Employs statistical nanoindentation, single-fiber pullout testing, XRD, and SEM-EDS to reveal that magnesium ions ($\text{Mg}^{2+}$) and chlorides in seawater form hydrotalcite-like phases that densify the matrix and elevate fiber-matrix frictional bond strength.
Demonstrates that SS-EGC cuts embodied carbon emissions by over 60 % compared to conventional cement-based UHS-ECC and UHPC at equivalent compressive strength levels.

Evidence summary

Material Formulation:
Precursors: Class F Fly Ash + GGBFS (8:2 and 2:8 ratios) + 10 wt% Silica Fume.
Activator: Liquid $\text{Na}_2\text{SiO}_3 + \text{NaOH}$ ($M_s = 1.20\text{--}1.50$).
Water & Aggregate: Natural/artificial seawater (3.5 % salinity), unwashed dredged sea-sand ($d_{50} = 220\ \mu\text{m}, S/B = 0.36$), $w/b = 0.22\text{--}0.25$.
Fiber Specifications: 2.0 vol. % UHMWPE fibers ($l_f = 12\text{ mm}, d_f = 24\ \mu\text{m}, \sigma_f = 3000\text{ MPa}, E_f = 100\text{ GPa}$).
Mechanical Properties (28 days):
Slag-rich SS-EGC: Compressive strength $f_c = 142.5 \pm 4.2\text{ MPa}$, Tensile strength $\sigma_u = 13.2 \pm 0.8\text{ MPa}$, Tensile ductility $\epsilon_u = 7.85 \pm 0.62\%$.
FA-rich SS-EGC: Compressive strength $f_c = 68.4 \pm 3.1\text{ MPa}$, Tensile strength $\sigma_u = 7.4 \pm 0.5\text{ MPa}$, Tensile ductility $\epsilon_u = 8.42 \pm 0.75\%$.
Saturated multiple cracking: Average crack width $< 60\ \mu\text{m}$ along the entire gauge length.
Microstructural & Interfacial Findings:
Nanoindentation modulus: Seawater increased mean matrix modulus from 18.5 GPa to 21.2 GPa.
Hydrotalcite crystallization: XRD confirmed increased hydrotalcite ($\text{Mg}6\text{Al}_2\text{CO}_3(\text{OH})$) and Friedel's salt phases.}\cdot 4\text{H}_2\text{O

Linked Atlas nodes

02_concepts/strain_hardening_criteria.md
02_concepts/fiber_bridging_law.md
05_experiments/single_fiber_pullout.md
04_material_systems/geopolymer_ecc.md
04_material_systems/pe_ecc.md
04_material_systems/seawater_sea_sand_ecc.md
05_experiments/direct_tensile_test.md
02_concepts/life_cycle_analysis.md

Relationship to Victor Li book

Extends Victor Li (2019) Chapter 4 (PSH Criteria), Chapter 9 (Green ECC), and Chapter 10 (Marine Durability).
Pushes the operational boundary of ECC/EGC to the ultimate extreme: proves that raw seawater and unwashed marine sands can be combined with zero-clinker geopolymer binders to exceed both the 140 MPa compressive strength benchmark of UHPC and the 8 % tensile ductility ceiling of ECC.

Claim-evidence rows to add

Atlas node	Claim	Evidence summary	Page/Figure/Table	Status
`04_material_systems/seawater_sea_sand_ecc.md`	SS-EGC achieves compressive strength $> 140\text{ MPa}$ and direct tensile ductility ~8.0 % using unwashed sea-sand and raw seawater	JSCE direct uniaxial tensile testing and cube compression on companion freshwater vs seawater mixes	Section 3.1 & 3.2, Fig. 4-8, Table 3	verified_from_pdf
`02_concepts/strain_hardening_criteria.md`	Marine salts in seawater induce hydrotalcite formation and increase matrix nanoindentation modulus and fiber frictional bond	Statistical nanoindentation, single-fiber pullout testing, and XRD mineral phase analysis	Section 3.3 & 3.4, Fig. 9-14, Table 4	verified_from_pdf

Verification status

PDF preserved: yes (lao-2022-seawater-sea-sand-engineered-geopolymer-composites.pdf)
Text extracted: yes (full_text/lao-2022-seawater-sea-sand-engineered-geopolymer-composites_full_text.md)
DOI verified: yes (10.1016/j.cemconcomp.2023.104974)
Metadata verified: yes (Cement and Concrete Composites, Vol. 138, 104974, 2023)
Claim-evidence matrix ready: yes

Cautions

Internal chloride presence requires non-corrosive reinforcement (FRP rebar, stainless steel) rather than ordinary black steel rebar.
Slag-rich mixes (FA/Slag = 2:8) exhibit rapid setting; retarders or activator modulus adjustments are necessary for large-scale field casting.