Same Batch of Pigment-Why Do Paving Bricks Still Show Color Differences?
In the production of paving bricks, permeable pavers, and various landscape bricks, color difference is one of the most common causes of customer complaints. When factories discover color differences, their first reaction is often to look for the pigment supplier-is this batch of iron oxide red unstable? But even when using the same batch, the same manufacturer, or even the same bag of pigment, the paving bricks produced can still show visible differences in depth and brightness.
The root cause often lies not in the pigment itself, but in three easily overlooked variables in the process from pigment entering the concrete to brick forming.
1. The "Micro-Metering" of Pigment: The Contradiction Between Large Scales and Inaccurate Small Scales
The addition ratio of pigment in paving bricks is usually very low, often only 1%–3% of the total material. If the factory's batching system only has high-capacity scales for weighing aggregates and cement (such as PL1200 or PL1600 batching stations), using these large scales to weigh tens of kilograms or even a few kilograms of pigment will not achieve the required accuracy. The residual amount in the weighing hopper of a large scale and the slight zero drift of the sensor, when allocated to a amount of pigment, will cause a huge proportional error.
High-end paving brick production lines usually need to be equipped with independent pigment micro-dosing scales, which have a smaller range and higher accuracy, and the discharging mechanism needs to adopt a precision anti-residue design to ensure that the pigment weight entering the mixer in each mold cycle is exactly consistent. Without independent micro-metering, color difference is "weighed out" from the very beginning.
2. The "Dead Corners" of Forced Mixing: Has the Pigment Really Been Dispersed?
After the pigment is added, if mixing is insufficient, local enrichment will occur. These concrete particles enriched with pigment become "color spots" or "dark patches" on the brick surface after forming. This requires the mixer to have not only sufficient power but also an excellent three-dimensional flow field design.
Twin-shaft forced mixers (such as the JS series supplied by UNIK) form strong convection and shear action in the mixing drum through two counter-rotating mixing shafts, and can uniformly disperse a微量 amount of pigment throughout the mix in a very short time. In addition, the feeding sequence is also crucial-some factories put pigment and aggregate in at the same time, causing the pigment to be wrapped by the aggregate; a more scientific approach is to first pre-mix the pigment with cement and part of the water (similar to a two-stage mixing process), and then add the aggregate, which significantly improves the dispersion uniformity of the pigment.
3. The "Surface Effect" of Forming Parameters: Density Determines Color Depth
Even if the mix ratio and mixing are perfect, fluctuations in forming machine parameters will still cause color differences on the brick surface. The color performance of paving bricks depends to a large extent on the surface density and reflectance. If the hydraulic pressure is low and the vibration time is short, there are more micro-pores on the brick surface, and light undergoes diffuse reflection on the rough surface, making the color look darker and drier; conversely, if the surface is pressed too densely or even shows slight "laitance," the surface reflectance increases, and the color appears deeper and brighter.
Therefore, maintaining absolute stability of the forming machine's vibration frequency, vibration time, and pressing pressure is the last gate for controlling color difference. If the equipment's hydraulic system has pressure fluctuations, or the vibrator's exciting force decays due to poor lubrication, the color of the same batch of bricks will be as unpredictable as a "blind box." In addition, uneven temperature and humidity in the curing kiln will also cause uneven distribution of hydration products on the brick surface, leading to efflorescence or inconsistent color.
Color difference is never caused by a single stage. From the accuracy of micro-metering, to the homogenization capability of the mixer, and to the stability of forming and curing, only when the entire production line achieves full-process controllability can the color of paving bricks truly achieve "batch-to-batch consistency."
