A client once described a finished concrete terrace as “the colour of a wet cardboard box,” and she was right. The pour had been placed on a hot afternoon, watered on the surface by a crew trying to keep it workable, and then left to dry in the sun with no cure at all. The pigment was fine. The mix design was fine. Everything that went wrong happened in the last six hours of a three-week job, and it went wrong because concrete was being treated as a structural material that happened to be visible rather than as a finish with its own rules.
Where colour actually comes from
Four sources, in descending order of reliability. Cement colour sets the baseline — white cement costs more and produces a genuinely different palette from grey, particularly with pale pigments. Integral iron-oxide pigment, batched at the plant by weight of cement, goes all the way through and cannot wear off; dosages above roughly 5 to 6 per cent give diminishing colour return. Dry-shake colour hardener, broadcast onto the fresh surface and floated in, gives stronger colour and a harder wearing surface but lives only in the top 1/8 in, so any grinding or spalling exposes grey underneath. Acid and water-based stains react with or sit on the cured surface and are the least predictable because they read whatever the slab already is — including every trowel mark and every place the cure was uneven.
Aggregate is a design decision
In a troweled finish the aggregate is invisible and irrelevant to the look. The moment the surface is ground, polished, sandblasted or washed, it becomes most of what you see. Exposed aggregate finishes read almost entirely as stone with cement paste between, so the stone choice — river gravel, crushed granite, a recycled glass or a local quarried aggregate — decides the colour and scale of the pattern far more than pigment does. Polished concrete is the extreme case: a shallow cut exposes only sand, a deeper cut of about 1/8 to 1/4 in exposes full aggregate, and those two are different floors from the same slab. This is the specification item most often left off a drawing, and it cannot be resolved after the pour.
Placement and finishing are part of the finish
Everything a trowel does shows up in a decorative slab. Overworking the surface brings fines and water to the top and creates a weak, blotchy skin that stains unevenly later. Adding water at the truck to improve workability raises the water-cement ratio, lightens the colour and reduces strength — using a plasticiser instead achieves the workability without either penalty. Joints have to be planned on the drawing: saw-cut control joints at roughly 24 to 36 times the slab thickness in feet, cut within the first 6 to 12 hours, placed to align with columns, corners and door openings rather than wherever the saw lands. A well-placed joint is part of the design; an unplanned crack never is. Decorative slabs are also where a decorative concrete contractor earns the premium over a general placing crew, because the sequence and timing are unforgiving.
Cure is not optional and it is not waiting
Curing means keeping water in the concrete so the cement can hydrate, and it is the single most abused step on decorative work. Seven days of moist curing is the classic benchmark, and the difference between that and no cure is on the order of 30 to 50 per cent of ultimate surface strength — which shows up as dusting, crazing and a slab that will not take a polish. On coloured work the aesthetic consequence is just as blunt: uneven cure produces uneven colour, and plastic sheeting laid in wrinkles prints those wrinkles into the surface as permanent mottling. Curing compounds work if they are compatible with the intended finish; many of them interfere with stains and with subsequent sealers, so the compound has to be chosen against the finish schedule, not grabbed off the truck. In hot, humid or windy conditions, evaporation retarders and early cover matter more than any other decision on the day.
Sealing and living with it
Decorative concrete is porous and it will record the life of the household unless it is sealed. Penetrating sealers change the look least and need renewal every five to ten years. Film-forming sealers deepen colour and add gloss, but they wear in traffic lanes, can trap moisture in an exterior slab, and need stripping and reapplication every two to four years. Outdoors in a hot, wet climate, film sealers can also go cloudy. Expect a plain broom-finished slab to sit at the bottom of the cost range, integral colour to add modestly, and a polished or exposed-aggregate finish with planned joints and a proper cure regime to land well above it — the extra is almost entirely labour and time, which is exactly the part that gets cut when a schedule slips.
Frequently asked questions
Will two pours of the same mix match in colour? Rarely exactly. Batch variation, placement conditions and cure differences all shift the tone, which is why a visible surface should be poured in one operation or deliberately divided by a joint.
Can a grey slab be coloured after the fact? Stains and dyes can change its appearance substantially, but they work with whatever the surface already is. A blotchy, poorly cured slab stains blotchily.
How long before a decorative slab can be sealed? Commonly 28 days for a water-based penetrating sealer, and longer for some film-forming products, because residual moisture and high early alkalinity interfere with adhesion.