Coloured, not natural, slabs are generally pigmented throughout. A fresh break will reveal this, the colour being ´stronger´ through the depth of the slab. The surface colour varies for many reasons and tends to fade with age.
During the manufacturing process the slabs are ´cured´. This is to say they are stored and allowed to harden, sometimes in a damp warm atmosphere. Where the slabs are in close proximity or air flow is less, the concrete can be darker due to the moisture in the slab being retained longer. These darker areas generally occur in the centre of the slabs – picture framing – the edges being lighter as they dry more rapidly. This is noticeable at the point of delivery or laying the slabs.
High spots on profiled or textured slabs will wear preferentially. Where this happens the coarser fraction in the sand becomes exposed as the thin covering of cement and pigment is worn away. This is to be expected when slabs are trafficked. If the sand fraction is a darker material than the pigmented cement paste, it will show as darker areas.
The surface will carbonate (natural process of weathering) and as a result the original colour fades. Additionally, the pigmentation may be leached from the surface, poorer quality pigments being more prone to colour fade. A surface sealer may minimise this.
Generally, the use of an acid etch (path cleaner) helps lift the colour as it removes the original surface and exposes the pigmented cement paste. However, over application may expose the darker sand fraction and open the surface texture making it more likely to retain dirt and lichen etc. A surface sealer may help prevent this.