An iron oxide red pigment did more than color mortar in a 2026 experiment: a secondary report says the mix with pigment at 8% of cementitious-material mass delivered the study’s highest compressive and flexural strengths. But flow declined as pigment increased, and the 10% mix was reportedly impractical to cast. These are results from one laboratory system, not a universal dosage recommendation.
What the reported experiment found
The study, “Study on the influence of iron oxide red colorant on the color and performance of colored mortar,” is identified as a 2026 article by X. Zhou, C. Wang, C. Cai, J. Jiang, Y. Zhang, and H. Luo in Case Studies in Construction Materials, volume 25, article e06596 (DOI: 10.1016/j.cscm.2026.e06596). The DOI page was not accessible for independent verification; the figures below are reported by Bioengineer.org’s October 4, 2026 summary.
Across mixes with iron oxide red (Fe₂O₃) pigment from 0% to 10% by mass of cementitious materials, the summary says both strength measures peaked at 8%. At that dosage, the reported 28-day compressive strength was 71.9 MPa, 11.1% above the pigment-free reference. Flexural strength was 12.3 MPa, a reported 16.0% increase over the reference.
| Pigment dosage by mass of cementitious materials | Reported flow spread | Strength result reported | Color result reported |
|---|---|---|---|
| 0% | 315 mm | Reference mix | 7-day redness coordinate a*: 0.23 |
| 8% | 260 mm | 28-day compressive strength 71.9 MPa; flexural strength 12.3 MPa | Reported total color difference ΔE: 20.1 |
| 10% | 240 mm; casting reportedly became impractical | Below the reported 8% strength peak | 7-day redness coordinate a*: 23.54 |
All table values are reproduced from the secondary report, not checked against the journal article. The report also gives a 46% reduction in total air content at 8% relative to the reference.
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Why more pigment may help strength—and hinder placement
The secondary report attributes the strength increase to physical particle packing, not a newly formed chemical reaction. It says pigment particles were finer than cement grains and that more pigment raised the powder blend’s specific surface area and water demand. Fine particles may fill spaces among cement particles and hydration products, producing a denser paste; microscopy was described as showing denser paste, while X-ray diffraction reportedly detected no new crystalline phases. This is the report’s account of the proposed mechanism, not independent confirmation.
The same fineness and added surface area help explain the fresh-mix trade-off. The reported flow spread decreased from 315 mm without pigment to 260 mm at 8% and 240 mm at 10%. A mix that is harder to flow or place can undermine practical performance even if hardened test specimens show higher strength. The report says the 10% mixture became impractical to cast.
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Color depends on dosage and the mortar matrix
The summary reports that the 7-day redness coordinate a* rose from 0.23 with no pigment to 23.54 at 10%, but says color gains began to saturate above 6%. In other words, adding pigment continued to increase redness in the tested range, while each additional increment reportedly offered less color benefit beyond that point.
The background cementitious materials also affected appearance. According to the report, white cement and pale yellow phosphorus slag gave a brighter background than ordinary gray cement and dark fly ash. Thus, a target red appearance cannot be inferred from pigment percentage alone; the base materials matter too.
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What the short-term durability and emissions results do—and do not—show
The report describes 56 days of natural exposure. After an initial shift associated with drying, later changes in lightness and redness were reported as under one unit. That is a short observation period, not proof of long-term outdoor color stability or service-life durability.
At 56 days, reported shrinkage was 790 microstrain for pigmented mortar versus 880 microstrain for the reference, a stated reduction of 10.2%. The summary does not establish that this difference will recur in other formulations, under other curing and exposure conditions, or over longer periods.
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The study’s cradle-to-gate assessment, which the report says followed ISO 14040 and 14044, attributed 82.7–87.1% of assessed emissions to cementitious materials and 12.5–16.7% to pigment. It identifies replacing some clinker with fly ash and yellow phosphorus slag as a possible emissions lever that can also lighten the matrix. These are shares for the mixes assessed, not general carbon factors for mortar or a complete account of impacts over a structure’s life.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What a contractor should take from the result
The reported 8% result is a useful laboratory finding, not a ready-to-use field specification. The summary does not provide independently verifiable exact mix proportions, protocols, statistical uncertainty, or enough information to transfer the result to a particular job. Before specifying a colored mortar, evaluate the actual materials and intended placement method in a representative trial mix.
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- Confirm the required strength and color with the project specification; do not treat the reported peak as a guaranteed result for a different mortar.
- Check flow and placement at the intended pigment dosage. The study summary reports a workability penalty as dosage rose.
- Assess the cementitious matrix and its effect on color, especially if the mix uses gray cement or dark supplementary materials.
- Use project-specific testing and applicable standards to establish fresh and hardened performance, curing behavior, and exposure suitability.
The article is identified as Zhou et al., 2026, but its primary text was not accessible for this account. Consequently, the detailed values and mechanisms here should be understood as secondary-reported findings rather than independently verified mix-design guidance.
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