Red Pigment Doubles as Strength Booster in Colored Mortar, Study Finds

Concrete has long been the workhorse of modern construction, prized for its compressive strength, durability, and low cost. Yet its dull gray appearance has always been an aesthetic liability, forcing architects to disguise it behind ceramic tiles or surface coatings that can crack, detach, or release volatile organic compounds over time. A new open-access study in Case Studies in Construction Materials offers a compelling alternative: coloring the mortar itself from within using iron oxide red pigment, and doing so in a way that may actually make the material stronger rather than weaker.

The research team, led by Xiaojun Zhou, Chenxi Wang, and Chang Cai, systematically tested how dosages of iron oxide red (Fe₂O₃) ranging from 0 to 10 percent by mass of the cementitious materials affect the workability, mechanical performance, color, and long-term stability of cement mortar. Unlike many pigments that interfere with cement chemistry, iron oxide red turned out to be a largely inert guest in the system, and its physical presence produced a surprising cascade of beneficial effects at the right dose.

The workability results followed a predictable but important pattern. Without pigment, the mortar achieved a flow spread of 315 millimeters at a fixed superplasticizer dosage. Up to 4 percent pigment, the flow barely changed. Beyond that, however, flow declined linearly, dropping to 260 millimeters at 8 percent and 240 millimeters at 10 percent, the point at which casting became impractical. The culprit is geometry: the pigment particles are significantly finer than cement grains, so each added dose raises the total specific surface area of the powder blend, adsorbing more mixing water and leaving less free water to lubricate the paste.

Strength told a more interesting story. Both compressive and flexural strengths at 7 and 28 days rose and then fell with increasing pigment content, peaking at 8 percent. At that optimum, the 28-day compressive strength reached 71.9 megapascals, an 11.1 percent gain over the pigment-free reference, while flexural strength hit 12.3 megapascals, a 16.0 percent improvement. Earlier studies on iron oxide colorants had reported strength losses of up to 30 percent, making this finding notable. The team attributes the gains not to any chemical transformation but to particle packing: the micron-scale pigment grains fill the voids between cement particles and hydration products, densifying the matrix.

Microstructural analysis backed this interpretation. X-ray diffraction showed no new crystalline phases forming at any dosage, confirming that the pigment does not participate in hydration reactions. Scanning electron microscopy revealed progressively denser paste microstructures with increasing pigment content, and quantitative air-void analysis using the ASTM C457 method showed total air content falling by 46 percent at the 8 percent dosage. Most strikingly, the proportion of harmful pores larger than 200 micrometers dropped from 68 percent to 52 percent, while fine pores below 100 micrometers increased from 15 percent to 22 percent, a wholesale shift of the pore network toward smaller, better-connected scales.

Color performance was tracked using the CIELAB system, the international standard for quantifying perceived color. As dosage climbed from 0 to 10 percent, the redness coordinate a surged from 0.23 to 23.54 at 7 days, while lightness L fell as the pigment’s strong absorption of visible light reduced surface reflectance. The total color difference ΔE reached 20.1 at 8 percent, far beyond the threshold of 3 at which the human eye clearly distinguishes two colors. Importantly, the gains saturated above 6 percent dosage, as higher paste viscosity caused pigment particles to agglomerate and stop contributing to light reflection, meaning more pigment eventually buys diminishing chromatic returns.

Two mix-design variables proved surprisingly irrelevant to color. Varying the water-to-binder ratio from 0.32 to 0.40 produced a maximum ΔE of just 1.84, and sweeping the binder-to-sand ratio from 0.54 to 0.94 kept total color differences below 3.0. The decisive factor was instead the color of the cementitious matrix itself. When ordinary gray Portland cement and dark fly ash were replaced with white cement and pale yellow phosphorus slag, redness values jumped dramatically, and even at half the pigment dosage the light-colored system outperformed the dark one. A bright background simply lets the pigment’s selective reflection of red wavelengths shine through unmasked by gray hydration products.

Durability questions were addressed through a 56-day natural exposure test conducted through summer conditions, with 26 rainy days and mean temperatures rising from 22 to nearly 27 degrees Celsius. After an initial week in which surface lightness rose about 30 percent as free water evaporated and particles packed more tightly, both lightness and redness stabilized, with subsequent changes never exceeding one unit. The pigment’s photochemical inorganic structure resists degradation and fading. Shrinkage data added a bonus: at 56 days, pigmented mortar showed 790 microstrain versus 880 in the reference, a 10.2 percent reduction, again credited to the micro-filling effect that lowers capillary porosity and the capillary tension driving shrinkage.

The environmental accounting complicates the picture in an instructive way. A cradle-to-gate life cycle assessment following ISO 14040 and 14044 found that cementitious materials dominate the carbon footprint, contributing 82.7 to 87.1 percent of total emissions, while the pigment, despite representing only 8 percent of the binder mass, accounted for 12.5 to 16.7 percent because its emission factor of 1.6 kilograms of CO2-equivalent per kilogram vastly exceeds that of sand or industrial by-products. Sensitivity analysis showed that even a ±50 percent swing in the pigment’s emission factor moved total emissions by only ±8.34 percent, keeping the conclusions robust. The clearest decarbonization lever remains substituting clinker with fly ash and yellow phosphorus slag, which simultaneously lowers emissions and, by brightening the matrix, enhances the pigment’s coloring efficiency.

Taken together, the study reframes iron oxide red from a mere cosmetic additive into a multifunctional mineral admixture. Within the tested range, 8 percent dosage emerges as the sweet spot, delivering saturated red color, double-digit strength gains, reduced shrinkage, refined pore structure, and stable appearance under weathering, all without altering the fundamental hydration chemistry of cement. For architects and engineers seeking facades that are colored to their core, the message is that the pigment in the mix may be doing far more work than its hue suggests, provided the dosage, the binder brightness, and the clinker content are chosen with care.

Subject of Research: Effects of iron oxide red pigment on the color, mechanical performance, microstructure, and carbon footprint of colored cement mortar

Article Title: Study on the influence of iron oxide red colorant on the color and performance of colored mortar

Article References: Zhou, X., Wang, C., Cai, C., Jiang, J., Zhang, Y., & Luo, H. (2026). Study on the influence of iron oxide red colorant on the color and performance of colored mortar. Case Studies in Construction Materials, 25, Article e06596. https://doi.org/10.1016/j.cscm.2026.e06596

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06596

Keywords: iron oxide red pigment, colored mortar, cement-based materials, compressive strength, CIELAB color system, pore structure, shrinkage, life cycle assessment, carbon emissions, white cement, fly ash, construction materials