The material at the center of the study is called self-protected underwater mortar, or SPUM. Rather than loading the mortar with viscosity-modifying chemicals, the method relies on an underwater protective agent, a water-soluble polymer dissolved in the surrounding water at a concentration of 0.5 percent by mass. The polymer, a linear ionic macromolecule with a weight-average molecular weight of at least 7.5 million, accumulates at the boundary between the fresh paste and the water, forming a stable protective film. This layer suppresses the dispersion of cementitious material into the water without increasing the viscosity of the mortar itself, which means the mix can still flow freely into place. Earlier work has shown that this approach can retain around 90 percent of its compressive strength when cast in seawater compared with dry-land casting, an impressive figure for underwater construction.

The motivation for the new study comes from one of the most demanding environments on Earth: the Qinghai-Tibet Plateau. Hydraulic structures there are often cast underwater and then spend their service lives in water-level fluctuation zones, where seasonal freezing and thawing batter the material year after year. To make matters worse, the plateau is dotted with saline lakes whose surface and groundwater carry high concentrations of sulfate ions. Sulfate attack is a notorious durability problem because sulfate ions react with cement hydration products to form expansive compounds such as ettringite and gypsum, generating internal stresses that crack the material from within. When sulfate attack and freeze-thaw cycles act together, previous research has shown they can accelerate damage far faster than either agent alone.

To probe these effects, the team prepared three versions of the same mortar mix, which used ordinary Portland cement partially replaced with 10 percent fly ash and 30 percent tuff powder, a locally available mineral admixture, along with a polycarboxylate superplasticizer. The first batch, designated SPUM-LE, was cast and cured in a conventional land environment. The second, SPUM-TW, was cast and cured in water containing 0.5 percent of the protective polymer. The third, SPUM-SS, was cast and cured in water containing both the polymer and 5 percent sodium sulfate, and was later exposed to freeze-thaw cycling in the same sulfate solution. The researchers measured slump flow, compressive and flexural strength at 3, 7 and 28 days, and then subjected all specimens to 100 rapid freeze-thaw cycles spanning temperatures from minus 18 to 5 degrees Celsius, tracking mass loss and relative dynamic elastic modulus along the way.

The rheology results showed immediately how much the surrounding medium matters. The land-cast mortar spread the farthest and fastest, while the version cast in the polymer solution lost 11.6 percent of its slump flow and needed 66.8 percent longer to reach a 200-millimeter spread, largely because the viscous solution and added buoyancy resist the flow of fresh mortar. The sulfate-polymer bath was harsher still: slump flow dropped by 33.8 percent and the spread time ballooned by 772.5 percent, a consequence of the denser liquid further suppressing the driving force for spreading. These numbers matter because flowability is the entire selling point of the self-protection concept, and the study shows that the chemistry of the water being poured into can quietly erode that advantage.

Strength development told a more subtle story. At 3 days, the underwater specimens were dramatically weaker than their land-cast counterpart, with the polymer-only batch reaching just 8.73 megapascals against 19.22 megapascals on land. The sulfate batch fared slightly better early on, apparently because sulfate ions initially accelerated hydration. But that early boost did not last. By 28 days, the land mortar reached 34.50 megapascals, the polymer-only batch 27.00 megapascals, and the sulfate batch only 23.40 megapascals, giving underwater-to-land compressive strength ratios of 0.80 and 0.60 respectively. The explanation lies at the interface: the polymer film that protects the mortar from washout also slows the transport of water to cement particles, retarding hydration, while accumulating gypsum and other sulfate products coat unhydrated cement and further impede the reaction. Flexural strength proved even more sensitive, with the sulfate-exposed batch actually declining from 6.29 to 5.55 megapascals between 7 and 28 days.

Freeze-thaw performance split the specimens in an unexpected way. The land-cast mortar held up best, losing only 1.29 percent of its mass and retaining 84.61 percent of its dynamic elastic modulus after 100 cycles. The polymer-only underwater mortar suffered the worst surface damage, shedding 5.30 percent of its mass as scaling and particle detachment, while still retaining 76.09 percent of its modulus. The sulfate-exposed batch showed the opposite pattern: modest mass loss of 3.99 percent, partly masked by white sulfate deposits accumulating on the surface, but the steepest internal degradation, with its dynamic elastic modulus falling to just 67.92 percent. In other words, one material fell apart from the outside in, while the other crumbled from the inside out. Sulfate ions penetrating the specimen during cycling formed expansive products that drove internal microcracking even as deposited crystals compensated for surface mass loss.

Microscopy and porosity measurements explained why. Scanning electron microscopy revealed dense, continuous matrices in the land-cured specimens but looser structures with visible micropores in the underwater batches, and abundant needle-like sulfate products in the sulfate-exposed material at 28 days. Mercury intrusion porosimetry added a twist: after 100 freeze-thaw cycles, the underwater specimens actually showed lower total porosity and fewer large harmful pores than the land-cast material, because continued hydration filled their more open pore systems. Yet they still performed worse. The lesson is that pore structure measured after the fact can be misleading; what governs freeze-thaw durability is the initial pore structure before cycling and the cumulative damage that accumulates within it. A loose starting framework gives hydration products plenty of room to fill, but that same looseness lets ice crystals and expansive reactions inflict damage that no amount of later pore-filling can undo.

To turn these observations into something engineers can use, the team fitted their damage data to a Weibull statistical model, using both mass loss and dynamic elastic modulus as damage indicators. The fits were strong, with coefficients of determination reaching 0.99 for the modulus-based model. Converting laboratory cycles into field years using an acceleration factor of 12 and an estimated 100 natural freeze-thaw cycles per year in the Lhasa region, the model predicted service lives of roughly 23.7 years for the land-cast mortar, 10.9 years for the polymer-only underwater mortar and 13.3 years for the sulfate-exposed batch under the more conservative mass-loss criterion. The authors caution that these figures are engineering estimates, since laboratory acceleration can never perfectly reproduce field conditions, and they note that the coupled nature of casting, curing and freezing makes it difficult to isolate the contribution of each stage.

The broader significance of the work is twofold. For engineers planning hydropower and hydraulic projects in cold, chemically aggressive regions, it provides hard numbers on a construction method that could otherwise be adopted on faith, showing that self-protected underwater mortar can genuinely work but that its durability ceiling depends heavily on the water it is born into. For materials science more broadly, it is a vivid demonstration that durability cannot be judged from a single snapshot of microstructure. A material that looks denser after testing may in fact be more damaged than one that looks coarser, because the true story lives in the sequence of events, the initial defects, the migrating ions and the accumulating cracks, rather than in the final photograph.

Subject of Research: Freeze-thaw durability and sulfate resistance of self-protected underwater mortar cast with an underwater protective agent

Article Title: Effects of underwater protective agent and sulfate exposure on the freeze-thaw resistance of self-protected underwater mortar

Article References: Zhang, J., Liu, Z., Shen, D., Lang, T., Li, Y., Sui, H., & An, X. (2026). Effects of underwater protective agent and sulfate exposure on the freeze-thaw resistance of self-protected underwater mortar. Case Studies in Construction Materials, 25, Article e06601. https://doi.org/10.1016/j.cscm.2026.e06601

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06601

Keywords: self-protected underwater mortar, underwater protective agent, freeze-thaw cycles, sulfate attack, concrete durability, Qinghai-Tibet Plateau, cement hydration, pore structure, Weibull damage model, hydraulic engineering, anti-washout, service life prediction