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A slag-and-silica-fume geopolymer ultra-high-performance concrete (UHPC) formulation retained reported strength after 12 months in the Cantabrian Sea, with no detected carbonation or chloride penetration front in the examined specimens. That is encouraging evidence for this particular mix—not proof that cement-free concrete generally can withstand decades of offshore service. Laboratory tests also revealed a notable weakness in magnesium-bearing solutions.
What the study tested
A study by Y. Sleiman, B. El Oifi, C. Martin, N. Saiyouri and Z. M. Sbartaï, published in Case Studies in Construction Materials in 2026, compared a cement-free geopolymer UHPC with a Portland-cement UHPC reference in laboratory durability tests. Its title is “Durability of a cement-free ultra-high performance geopolymer concrete: A laboratory comparison with Portland-cement UHPC validated by real marine exposure” (volume 25, article e06583; DOI: 10.1016/j.cscm.2026.e06583). The results described here are reported in a secondary account of that paper, rather than verified against the full journal article. Read the study record.
The tested GEO binder used ground granulated blast-furnace slag and silica fume, activated with sodium silicate and potassium hydroxide. It included steel fibers at 1.5% by volume. The reported compressive strengths were 152 MPa for GEO and 165 MPa for the Portland-cement reference, labeled REF. These are results for the mixes in this study, not representative values for all geopolymer or Portland-cement concretes.
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Geopolymer 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; others had project-developed bio-based coatings. The secondary 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 fronts in the examined specimens, based on colorimetric checks.
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This is a real marine exposure result, but its scope is bounded: the field campaign tested GEO, not the Portland-cement reference, and the study used one production batch with limited specimen counts in some groups. Twelve months is also far shorter than the service life expected of offshore infrastructure. The observations support further evaluation of this formulation; they do not establish how it will perform over decades or in other marine sites.
Laboratory durability was mixed
The reported laboratory program exposed specimens for a year to distilled water, 5% sodium sulfate, 5% magnesium sulfate, nitric acid maintained at pH 3, and a combined magnesium/sodium sulfate solution intended to represent seawater cation pairing at accelerated concentrations. The results depended on the solution, so “chemical resistance” is not a single pass-or-fail property.
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| Exposure | Reported result | How to read it |
|---|---|---|
| 5% sodium sulfate | REF expanded by as much as 0.16%; GEO contracted slightly, to around −0.05%. | Favors GEO for dimensional change under this test’s conditions. |
| Nitric acid at pH 3 | GEO showed negligible dimensional change; REF peaked above 0.2% expansion. | Favors GEO for the reported acid-exposure measurement. |
| Magnesium-bearing solutions | GEO expanded in both; the combined solution reached 0.29% expansion. | A material-specific concern in these tests, despite the other favorable chemical results. |
The authors reportedly proposed that magnesium interacted with the calcium-rich gel and that brucite precipitation contributed to expansion. That is the explanation reported for these results, not a universal rule established for every geopolymer.
Chloride and heat results need careful interpretation
Electrical chloride test
The secondary report gives average rapid chloride penetration charges of 254 coulombs for GEO and 43 coulombs for REF. These readings should not be treated as a direct head-to-head measure of chloride diffusion: potassium-rich geopolymer pore solution can increase electrical current and complicate interpretation. The report describes both mixes as having low measured penetrability relative to conventional concrete, but the charge values alone do not establish that GEO transports chloride more readily in service.
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Heating to 600°C
At 600°C, the reported strength retention was 36% for GEO and 90% for REF. The comparison has an important specimen limitation: only one REF specimen remained in testable condition at each of the higher temperature conditions. At 900°C, the report describes severe spalling of REF, while all GEO specimens remained intact. Survival without breaking apart and retention of strength are different outcomes; the reported 600°C retention figures show GEO performed less well on the latter measure.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What this does—and does not—show
- Promising: One slag-and-silica-fume geopolymer UHPC formulation maintained reported mechanical performance through a year of exposure across three marine zones, with no detected carbonation or chloride front in examined specimens.
- Not a general verdict: The results cannot be transferred automatically to other geopolymer binders, activators, fiber contents, production batches or exposure conditions.
- A clear durability caveat: Magnesium-bearing solutions caused expansion in laboratory testing, and GEO retained less strength than REF after heating to 600°C.
- Not a long-term service-life demonstration: A one-year field campaign, small specimen numbers in some groups and no Portland-cement field control cannot show multi-decade performance.
For engineers considering this material, the practical next question is whether the same formulation performs under the project’s specific seawater chemistry, temperature, detailing, curing and loading conditions—and whether longer exposure and larger, controlled field comparisons confirm the early results.
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