Concrete discoloration
Blotchy dark and light patches across a slab that should be one tone, or a distinct colour difference between areas placed from different trucks. Sometimes appears as dark spots that never fade.
VerdictCosmetic — does not compromise the slab
What causes it
Several unrelated causes produce the same symptom. Calcium chloride accelerator is the classic one — it darkens concrete and is rarely dosed identically across loads. Variation in cement source, water-cement ratio between trucks, or finishing timing across a slab all shift colour. Uneven curing leaves darker patches where plastic sheeting touched the surface. Hard-troweled areas are darker than floated ones because burnishing densifies the paste.
How it is repaired
Discoloration often fades over months as the slab weathers and carbonates, so patience is a legitimate first step on exterior work. Repeated washing with clean water helps. Where it does not fade, the options are all cosmetic: a stain or dye to unify the tone, a tinted sealer, or grinding and polishing to a uniform surface.
How to prevent it
One cement source, one mix design, and consistent water across every truck on the pour. Avoid calcium chloride where appearance matters. Cure uniformly — either fully covered or fully exposed, not a mix of both — and finish the whole slab with the same technique and timing.