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One reported cement-free geopolymer ultra-high-performance concrete (UHPC) formulation retained mechanical performance after 12 months at a marine test site in Spain. That is encouraging evidence for this particular slag-and-silica-fume mix, not proof that geopolymer concrete broadly can withstand decades of offshore service. Laboratory results were also mixed: the mix performed favorably in some chemical exposures but expanded in magnesium-bearing solutions and retained less strength than the Portland-cement reference after heating to 600°C.
What was tested
A secondary report published October 3, 2026, describes a study by Y. Sleiman, B. El Oifi, C. Martin, N. Saiyouri and Z. M. Sbartaï, “Durability of a cement-free ultra-high performance geopolymer concrete: A laboratory comparison with Portland-cement UHPC validated by real marine exposure,” in Case Studies in Construction Materials, volume 25, article e06583 (2026), DOI 10.1016/j.cscm.2026.e06583. The reported results concern one formulation, referred to as GEO, and a Portland-cement UHPC reference, REF.
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GEO used ground granulated blast-furnace slag and silica fume, activated with sodium silicate and potassium hydroxide, with steel fibers at 1.5% by volume. Its reported compressive strength was 152 MPa. The REF mix had a reported compressive strength of 165 MPa. Those starting strengths describe the tested mixes; they do not by themselves establish comparative service life.
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What happened during the year at sea
GEO prisms were deployed at HarshLab in the Cantabrian Sea for 12 months. The campaign included atmospheric, splash and fully immersed zones. Some specimens were uncoated and others had project-developed bio-based coatings. The report gives post-exposure flexural strengths of 14–17 MPa and compressive strengths of 150–167 MPa. It also reports no indication of carbonation or chloride penetration front in the examined specimens using colorimetric checks.
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This is a real marine exposure result, but the field campaign tested GEO only: it did not place the Portland-cement reference alongside it at the site. The reported observations therefore show how this GEO formulation fared during the stated exposure, not that it outlasted REF in the sea.
How the mixes compared in laboratory tests
The laboratory program exposed specimens for a year to distilled water, 5% sodium sulfate, 5% magnesium sulfate, nitric acid held at pH 3, and a combined magnesium/sodium sulfate solution intended to approximate seawater cation pairing at accelerated concentrations. These are controlled exposures, distinct from the Cantabrian Sea deployment.
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| Exposure or measure | Reported result | How to interpret it |
|---|---|---|
| Compressive strength before exposure | GEO: 152 MPa; REF: 165 MPa, as reported for the tested mixes. | Gives the reported starting strength, not a durability ranking. |
| 5% sodium sulfate, one-year laboratory exposure | REF expanded as much as 0.16%; GEO contracted slightly, to around −0.05%. | Favors GEO on dimensional change in this specified test. |
| Nitric acid at pH 3, laboratory exposure | GEO had negligible dimensional change; REF had peak expansion above 0.2%. | Favors GEO on the reported dimensional measure under this protocol. |
| Magnesium-bearing solutions, laboratory exposure | GEO expanded in both; expansion reached 0.29% in the combined magnesium/sodium sulfate solution. | Identifies magnesium-bearing exposure as a concern for this formulation. |
| Rapid chloride penetration test | Average measured charge: GEO 254 coulombs; REF 43 coulombs. | The report cautions that potassium-rich geopolymer pore solution can raise electrical current, so these charge values are not a clean head-to-head measurement of chloride transport. |
| Heating to 600°C | Reported strength retention: GEO 36%; REF 90%. | The REF comparison is uncertain because only one reference specimen remained testable at each of the higher temperature conditions. |
Why magnesium is a notable limitation
The secondary report says the authors proposed that magnesium interacts with the calcium-rich gel in GEO and that brucite precipitation contributes to expansion. That is the authors’ explanation for the observed behavior, not a universal rule established for all geopolymer binders. For this mix, however, the measured expansion in both magnesium-bearing solutions is a meaningful counterweight to the favorable sodium-sulfate and acid results.
What the heat results do—and do not—show
At 600°C, the reported strength-retention figures were 36% for GEO and 90% for REF. The comparison has a serious specimen-count limitation: only one REF specimen survived in testable condition at each of 600°C and 900°C, while all GEO specimens remained intact. The report also describes severe spalling of REF at 900°C. These observations are not enough to establish a broad thermal-performance ranking.
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How strong is the evidence for long-term marine use?
The evidence is promising but bounded. The reported field exposure lasted one year, used one production batch, included limited specimen counts in some groups, and covered GEO rather than both mixes. The laboratory results add useful information about specific chemical and thermal conditions, but accelerated or controlled exposures do not on their own establish long-term performance in a working structure.
- Supported by the reported study: this GEO formulation retained mechanical performance during the stated 12-month HarshLab exposure, with no carbonation indication or chloride penetration front detected in the examined specimens.
- Not established by that result: multi-decade service life, performance across all geopolymer formulations, or superiority to Portland-cement UHPC in a direct field comparison.
- Important formulation-specific cautions: expansion in magnesium-bearing solutions and lower reported strength retention than REF after heating to 600°C.
The appropriate conclusion is that this material merits further evaluation for marine applications—not that cement-free geopolymer concrete as a category is already validated for offshore infrastructure.
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