Low-Dose Polymer Additives Reshape Mortar Strength in Surprising Ways

Polymer-modified mortars are everywhere in modern construction, hiding inside cementitious tile adhesives, repair mortars, and waterproofing systems. A new open-access study published on 5 October 2026 in a Springer Nature concrete and cement journal now offers one of the most systematic looks yet at what happens when these polymers are added at the low end of the industrially relevant dosage range.

The team, led by Aleksandar Jagličić and Torben Gädt, mixed each dispersion into mortars made with two commercial Portland cements, a rapidly hardening CEM I 42.5R designated cement A and a finer, more reactive CEM I 52.5N designated cement B, at a water-to-cement ratio of 0.47. They then tracked the mortars through an unusually broad battery of measurements: spread flow tests for rheology, compressive and flexural strength at 1, 7, and 28 days, isothermal heat flow calorimetry for hydration kinetics, and ultrasonic pulse velocity for stiffness evolution. Prior studies, the authors note, have typically focused on high polymer dosages of 5 to 10 weight percent or more, leaving a genuine knowledge gap at low dosages, where the interplay between air entrainment and polymer film formation is expected to differ fundamentally.

The headline finding is that with few exceptions, the polymers produced a small reduction in both compressive and bending strength at all tested ages. But that average masks striking polymer-to-polymer variation. All thirteen dispersions entrained air into the fresh mortar, lowering densities from roughly 2300 down to about 1700 kilograms per cubic meter, which alone would depress strength. To separate the chemistry from the air, the researchers built a clever normalization scheme: they prepared polymer-free reference mortars with controlled porosities by adding increasing doses of the air-entraining surfactant sodium dodecyl sulfate, then fitted a linear strength-density regression through those references. The fits yielded coefficients of determination above 0.98 across the density interval from 1800 to 2300 kilograms per cubic meter, allowing any polymer mortar to be compared against a polymer-free reference of identical density.

When the strengths were normalized this way, one dispersion stood out dramatically. Polymer 12 was the only formulation that exceeded the reference regression, delivering visibly higher flexural and compressive strength at matched density, especially at early ages. In fact, polymer 12 achieved higher absolute strengths than the unmodified reference mortar, a remarkable result at such a low dosage. The enhancement was strongest at one day and diminished as the cement hydrate matrix matured and increasingly dominated load-bearing capacity. Polymers 1, 3, and 10 stayed closest to the reference overall, while polymers 2, 4, and 6 showed the largest and most persistent strength deficits, and polymer 11 behaved anisotropically, combining a flexural gain with a compressive loss.

Why would one latex strengthen mortar while its close chemical cousins weaken it? The authors advance a film-formation hypothesis. According to a percolation mechanism described in the polymer-modified cement literature, a polymer network or film that forms throughout the cement matrix can bridge micro-cracks and absorb stress, increasing ductility and tensile strength. Whether such a film forms depends on the polymer’s glass transition temperature and its compatibility with the hydrating cement: particles that coalesce readily at ambient curing temperature can build a continuous, mechanically effective film within the first hours of hydration, before the fragile hydrate skeleton has fully developed. The team suggests polymer 12’s exceptional early performance stems from favorable film-forming characteristics combined with specific polymer-cement surface interactions, and they are careful to flag this as a hypothesis supported by converging indirect evidence rather than direct imaging, since no electron microscopy of the polymer film was performed.

Two further experiments strengthened that interpretation. First, when mortars were cured at an elevated 40 degrees Celsius instead of 20, the overall pattern of strength changes was largely confirmed, and polymer 12 continued to outperform the others, consistent with a coalescence-controlled mechanism since higher temperatures promote particle coalescence once curing exceeds the polymer’s film-forming temperature. Second, and more tellingly, the strength benefit of polymer 12 vanished when a silicone-based defoamer was added to remove entrained air, suggesting the defoamer interferes with film formation itself, perhaps by disrupting latex particle coalescence or introducing hydrophobic discontinuities into the nascent film. The authors also acknowledge a limitation here: no defoamer-only control was run, so the defoamer’s intrinsic contribution cannot be fully separated from its interaction with the polymer.

The defoamer series also clarified where the strength losses come from. With excess air removed, all defoamed mortars exceeded densities of 2150 kilograms per cubic meter, though they remained below the reference density of 2297. Several polymer systems then recovered compressive strengths at or even above the reference level, showing that entrained air voids account for much of the compressive deficit. Flexural strengths, however, stayed below the reference in every defoamed case, indicating that residual flexural behavior and the strength gains are governed by polymer film formation rather than porosity alone. Notably, isothermal calorimetry showed that the polymers altered cement hydration only marginally, so the strength differences among dispersions are not of hydration-kinetic origin.

On the hydration front, the calorimetry data told a consistent but modest story. All polymer emulsions shifted the characteristic time at which half of the maximum heat flow is reached, determined according to ASTM C1679, to later values, demonstrating a slight but clear retardation of the main silicate reaction of Portland cement. Cement B was retarded more strongly than cement A. When the researchers separated the serum fraction of the dispersions by centrifugation and tested it alone, the serum barely influenced the silicate reaction, except for one polymer, pointing to the latex particles themselves, likely adsorbing on anhydrous cement surfaces, as the retarding agents. Ultrasonic pulse velocity measurements echoed this: polymer-modified pastes showed delayed onset of stiffening and generally lower velocities after 100 hours, effects more pronounced in cement A, suggesting increased porosity and lower elastic moduli in the final matrix.

Rheology, by contrast, was affected only mildly. Spread flow values ranged from 11 to 22 centimeters, with the polymer-free references sitting in between at 16 centimeters for cement A and 19 for cement B. Some dispersions slightly dispersed the mix and others slightly reduced flow, but the relative behavior of each polymer was consistent across both cements, and the small total spread indicates the polymers exert neither strong dispersing nor flocculating properties at this dosage.

The study’s real value lies in its systematic breadth. Where previous work mostly examined a single polymer and cement at high dosage, this dataset screens thirteen dispersions across two cements, three curing ages, two temperatures, and four complementary measurement techniques at an industrially relevant low dosage, providing a self-consistent benchmark for linking polymer behavior to mortar performance. The authors identify the natural next step as connecting performance differences to the film-forming characteristics of individual dispersions, including glass transition and minimum film-formation temperatures, particle size, and surface chemistry, particularly to explain polymer 12’s exceptional behavior. For formulators of dry-mix mortars, the message is that at low dosage, polymer chemistry matters in sharply polymer-specific ways, and that air management and film formation, not hydration kinetics, are the levers that control the outcome.

Subject of Research: Effects of low-dosage styrene-acrylate polymer dispersions on the hydration, rheology, and strength of cement mortars

Article Title: The influence of low-dosage styrene-acrylate dispersions on mortar strength

Article References: Jagličić, A., Jahns, E., Dietzsch, M., & Gädt, T. (2026). The influence of low-dosage styrene-acrylate dispersions on mortar strength. Discover Concrete and Cement, 2(1), Article 80. https://doi.org/10.1007/s44416-026-00115-y

Image Credits: AI Generated

DOI: 10.1007/s44416-026-00115-y

Keywords: polymer-modified mortars, styrene-acrylate dispersions, cement hydration, compressive strength, flexural strength, air entrainment, isothermal calorimetry, ultrasonic pulse velocity, film formation, defoamer, mortar rheology, construction materials