Explained

Why a New Slab Does Not Match the Sample

Concrete varies in colour. It is not a manufactured product and it never comes out uniform. Here is what causes it, what settles down on its own, and what a sample board can honestly promise you.

Updated October 2026 · Antioch Concrete

This is the page we would most like people to read before a decorative pour rather than after one. Almost every unhappy conversation about finished concrete is about colour, almost none of it is about a defect, and nearly all of it could have been avoided by saying in advance what this material actually does.

Concrete is not manufactured to a colour

A paint has a code. A tile has a batch. Concrete has cement, sand, stone and water, mixed at a plant, delivered by road and finished by hand in whatever weather turned up.

Every one of those inputs moves. Cement shade varies between production runs. Sand and aggregate come from a working pit and change gradually as the face is worked. The water content that reaches your garden depends on the journey. None of that is sloppiness; it is what a site-mixed structural material is.

The four things that make one slab look like two

Moisture. The largest cause by a distance, and the most temporary. A curing slab holds water unevenly, and damp concrete reads darker. Patches that move and change over the first weeks are moisture doing exactly what it should.

Timing across the pour. A patio is not placed in one instant. The first barrow and the last can be separated by hours, during which the sun moves, the wind gets up and the first section starts setting. Finishing work done at different stages of stiffness leaves slightly different surfaces, and a different surface reflects light differently.

What is underneath. A slab losing water downward into dry ground behaves differently from one over damp ground or over a membrane. Where the sub-base is not uniform, the surface above it can map that difference.

Batches. A large pour can take more than one load, and two loads are never quite identical. Good practice keeps the joint between loads at a control joint so the change lands on a line, but the change itself is not avoidable.

The white bloom, which is not a fault

Three weeks after a pour, a white powdery film often appears. This is efflorescence, and it arrives more often on coloured work because it shows up against the pigment.

Water working its way out of the slab dissolves salts on the way. At the surface the water evaporates and the salts stay, as a pale deposit. It is a sign the concrete is still giving up moisture, which is normal, and it typically weathers away across the first season.

The useful advice is to leave it alone. Aggressive cleaning of a young surface does more harm than the deposit, and sealing over it locks it in where nobody can get at it.

What a sample board can honestly tell you

It can tell you a direction: warm or cool, light or dark, which family of colour you are in. That is worth having and we will always put one in front of you.

What it cannot be is a target. A sample is a small piece, cured in different conditions, at a different time, usually sealed and usually indoors under different light. A large slab outdoors will not reproduce it, and any contractor who promises it will has promised something the material cannot do.

Our habit is to put it plainly at quoting stage and to ask you to look at finished work a few years old rather than at a fresh sample, because that is a far better guide to what you are actually buying.

What a good contractor does to limit it

Variation cannot be eliminated and it can be substantially reduced, which is worth knowing so you can tell effort from indifference.

Ordering the whole pour from one supplier on one day, rather than splitting it, keeps the inputs as consistent as they can be. Planning where a load change lands, so the join falls on a control joint rather than in the middle of an open area, puts any difference on a line that was going to be there anyway.

Keeping the water content consistent between barrows matters more than almost anything else, and it is the discipline that slips when a pour is running late. Finishing the whole area with the same method at the same stage of stiffness does the same job for the surface texture. And curing the slab evenly, rather than covering half of it and leaving the rest to the sun, avoids building a pattern into the concrete that then takes a season to fade.

None of that is expensive. All of it is a crew paying attention, and it is reasonable to ask how a pour will be sequenced before the day.

What to do when the slab is new

Wait. Genuinely. The version you are looking at in the first month is the worst it will ever be, and the improvement over a season is substantial as moisture leaves and rain works on it.

Decide about sealing later rather than sooner, both because the surface has to be properly dry and because you will be making the decision about the real colour rather than a temporary one.

And if something still looks wrong once the slab is thoroughly dry, a fixed patch that does not move with the weather, tell us and we will come and look at it. That is a different thing from the variation described above, and it is worth separating the two rather than worrying about both.

The decorative choices themselves, and what each adds, are in stamped or coloured. The work sits on concrete patio.