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

Traditional concrete, with its monotonous and dull appearance, struggles to meet the ever-increasing demands of modern urban landscape construction and diverse architectural aesthetics. This study utilizes iron oxide red colorant to prepare colored mortar, aiming to enrich the surface finishing effects of concrete building structures. The effects of iron oxide red colorant on the workability, mechanical properties, hydration products, microstructure, and pore characteristics of mortar were investigated through fluidity and compressive strength tests, as well as XRD, SEM, and hardened pore structure analyses. Simultaneously, a colorimeter was employed to evaluate the appearance and color effects of the colored mortar, exploring the influencing factors of its color and the patterns of color variation under natural environmental conditions. The results indicate that incorporating an appropriate amount of powdered iron oxide red colorant does not adversely affect the workability of the mortar. However, when the dosage exceeds 8%, the fluidity of the paste becomes detrimental to casting and molding. The addition of iron oxide red pigment does not lead to the formation of new hydration products but primarily acts as a micro-filler. Compared to the mortar without colorant, incorporating 8% iron oxide red colorant reduced the air content from 9.71% to 5.24%, increased the compressive strength by 11.1%, and decreased the shrinkage rate by 10.2%. This improvement in pore structure characteristics enhances the density and mechanical properties of the mortar. The color performance of the colored mortar containing iron oxide red colorant is relatively stable. In the first 7 days, the color change is mainly reflected in an increase in brightness ( L value) by 30.6%, after which it stabilizes. The redness ( a value) remained within the range of 21.53 to 22.04 throughout the study period. Color differences resulting from changes in the water-to-binder ratio and binder-to-sand ratio were less than 3, indicating a minor influence on color. The coloring effectiveness of the mortar is related to the dosage of iron oxide red colorant and the color of the cementitious materials. Furthermore, the use of industrial solid wastes like fly ash and phosphorus slag powder to partially replace cement can effectively reduce the carbon dioxide emissions of colored mortar. The findings of this research can provide a reference and basis for the application of colored mortars with stable color and environmental friendliness.