Durability of a cement-free ultra-high performance geopolymer concrete: A laboratory comparison with Portland-cement UHPC validated by real marine exposure

This paper presents a comprehensive durability comparison of a 100% cement-free ultra-high performance geopolymer concrete (GEO), based on ground granulated blast-furnace slag and silica fume, against a Portland cement-based ultra-high performance concrete reference of equivalent strength class (REF). A laboratory program assessed both mixes under sulfate attack, nitric acid, elevated temperature up to 900 °C, rapid chloride penetration (ASTM C1202), and electrical impedance monitoring over 365 days. GEO remained dimensionally stable in sodium sulfate and under nitric acid, where REF expanded by up to 0.23%, but expanded in both magnesium-bearing solutions, exceeding the 0.10% criterion of ASTM C1012 and identifying magnesium as the principal chemical vulnerability of the calcium-rich binder. Both mixes exhibited very low chloride penetrability. At elevated temperature GEO retained strength less effectively than REF, holding 36% of its ambient compressive strength at 600 °C against 90% for REF, although it remained intact without explosive spalling. The laboratory findings were validated by a 12-month real marine exposure campaign at the HarshLab floating facility in the Cantabrian Sea, where GEO specimens were deployed in atmospheric, splash, and immersed zones. After one year of marine service, GEO retained its full ultra-high performance strength class in every zone, with residual compressive strength of 150–167 MPa and no carbonation or chloride penetration front detected. The combined laboratory and field evidence demonstrates that a cement-free geopolymer concrete can meet ultra-high performance durability requirements for marine service, with acid and chloride resistance offset by magnesium-sulfate sensitivity and lower high-temperature strength retention.