January 28, 2020

What is The Real Colour Of Concrete?

You might think that concrete is always grey, but it rarely is. Join us as we breakdown what gives concrete its inherent colouring in this great little article.

A pink concrete overlay floor in a commercial property

What colour is concrete? Aren’t concrete floors always grey?

Now that’s a good question and it is a very common belief that concrete is grey; sometimes light, sometimes dark but certainly a variation of grey. It is definitely the most requested colour for concrete at the moment in the UK and that doesn’t matter whether it is for concrete flooring, concrete walls or furniture items such as concrete worktops, concrete countertops or tables.

Grey is the sought after colour, grey envy we call it. A lot of this envy was developed from the publication of certain photos of heavily power trowelled polished concrete floors in architects’ brochures and lifestyle magazines. Floors that look truly stunning when surrounded by contemporary furnishings but is concrete actually grey?

Well, in all the years CARRcrete have been polishing concrete surfaces, we have come across very few genuinely grey floors. Certain parts of Wales use a fine grey aggregate made of crushed slate that produces a nice grey tone but the rest of the country varies massively and results are different from county to county.

By and large, concrete is not grey past the surface paste material which is usually around one millimetre in thickness. Once this fine surface layer is removed the true colour of concrete is usually exposed and most often can actually be a beige or oatmeal colour. In areas of the country where the sand is red or tinted with red tones this tends to product a concrete that is tan or brown in colour. In the UK this is quite an unpopular choice.

Let’s have a look at what external influences may alter the colouring of a concrete floor slab. To answer this question competently we need to look at what ingredients are used within the concrete mixture. As we have mentioned, the majority of raw materials used are not grey in colour.

Here’s A Few Regions We Have Worked
But please do not forget we undertake project throughout the UK
Birmingham - Cardiff - Derby - Exeter - Leicester - Lincoln - Nottingham - Sheffield - York

Things Which Affect The Colour Of Concrete

  • Was a pigment added?
  • What is the colour of sand
  • How much cement was used?
  • Has the correct amount of water been added?
  • Was the slab cured correctly?
  • Tamped, pan or trowel finished?
  • What type of floor sealer was used after polishing?
  • Type and frequency of foot traffic

There are still a lot of people around the globe that still think of concrete as primarily cement, which it is not. Portland cement is just one component in the design matrix for any concrete slab, wall or piece of furniture. As we have mentioned, the mix of concrete can differ from region to region but will pretty much always consist of a binder such as cement powder, fine aggregates usually sand, a variety of coarse aggregates and water, as well as cement.

There now follows a highly simplistic explanation of a concrete design.

When all of these ingredients are combined together and mixed thoroughly, a process of hydration occurs whereby the cement particles begin to react with the water present in the mix. Initially a gel like substance is created; this cement gel readily binds to the sand and aggregates and where possible coats them. This phase usually gives the mix a greenish hue. A green slab or mix is widely understood as being a freshly laid slab or mix.

Think tossing salad in dressing, where the aim is to coat the salad as evenly as possible with the dressing to help ensure a consistent tasting experience. It is a similar thing with concrete, the aim is to coat all of those millions of grains of sand and piles of coarse aggregates with the gelled cement solution to help ensure a good strong mixture. The gel essentially acts like a glue.

After a short amount of time this gel will be fixed in place around the other ingredients as the whole mixture begins to set. During the setting phase the gel begins to turn into spiky shards of a crystalline material. For those of us who enjoy the technical bit this is referred to as calcium silicate hydrate. It is these crystalline spikes that bind the mix together to form a rigid material. Other materials are also produced during the setting of the concrete but I’ve purposely kept things as simple as possible for this post.

One of the most important elements of a quality mix design for polished concrete flooring is the amount of fluid used. Too much liquid will weaken the concrete, making for a slab which will often shrink and crack, be increasingly more porous than a similar slab with less water and too much water will also affect the finished colour of the concrete surface. It is important that liquid addition should be watched closely we always recommend a water:cement ratio of less than 0.40 for polished concrete floors.

There is always a chance concrete will crack as it is simply one huge rigid sponge, but there are many ways we can reduce cracking down to a bare minimum. The correct fluid addition is one of the most important methods of crack reduction.

One analogy I like to use is of coffee. When you go for an overly expensive coffee and order an espresso, you already know what to expect from within your cup. It should be strong and full of flavour. But what if it was dropped into a much larger cup and double, or triple the amount of liquid added? You wouldn’t expect the same taste, colour or aroma would you? No! The same goes with concrete, too much water alters the qualities of the mix design massively; if anyone says different then we advise to stay well clear.

So by looking at the above explanation of the mix you can tell that most of the cement is hydrated into a crystalline structure, which is largely invisible to the naked eye. This leaves the majority of the colouring to come from the sand used. Sand can be a variety of different colours, each region has a different profile with colours ranging from light beige to brown through orange; the colour of sand is highly variable.

Ingredients In A Concrete Design Mix

  • Large aggregates
  • Sand
  • Cement
  • Water
  • Admixtures

There are ways to colour concrete

The most common way of altering the colour of a concrete mix prior to installation and to have a greater idea of exactly what you’re going to end up with, is to include a pigment within the mix design. The pigment may be from a variety of sources but usually revolves around iron oxide colourants which are very popular with decorative concrete, have been around for years and are well proven. Iron oxide pigments come in wide range of colours, the intensity of the colour is determined by the percentage of pigment added and also whether the supplier uses white or grey cement within the mix.

Note that the pigment predominantly affects the colour of the cement element of the mix and thus the gel and crystalline structure that is then produced during hydration. If it is a strong black pigment used then there will be some colour staining to the sand and aggregate surfaces too. Overall this makes for a more complete colour change.

White cement is always more expensive than grey and less readily available in large amounts. But that should not be a major concern as grey coloured concrete can easily be achieved with grey cement and a low percentage of black iron oxide pigment.

If you do order a pigment with your concrete, please be sure to see the suppliers colour chart and bear in mind that there are always variations in colour when using organic materials. I know that may seem odd and even frustrating, but it is a valid consideration. You should also expect to receive a price increase over a standard concrete mix for any pigment used and also for cleaning out the mixing drum of the delivery vehicle.

It is also possible to use a dye on the surface to create a stained concrete surface and display a subtle tone. Dyes usually have to be mixed with water or acetone to dissolve the colourant before it is sprayed and worked into the surface. How well the dye works depends on the porosity of the surface and how much free lime or calcium hydroxide is available, therefore results are always different and questionable.

It is important to realise that dyed concrete or an acid stain does not give a uniform coloured finish, it simply is not possible. I know how us Brits love a uniform finish but that can’t be achieved using this method. A dyed floor will have what is best described as a cloudy or mottled appearance where the dye has soaked into some areas more than others. This type of colouring has its place and is incredibly popular over in America but is not a service we choose to offer as opinions of it are so varied and our clients usually want a more uniform finish than is offered by this medium.

Another often overlooked but very important factor to consider that alters the tone of concrete surfaces is the type of surface hardener or densifier and sealer used. Both products are widely used within the polished concrete industry and seen by many as necessary parts of the polishing process; this includes our own InfinityFloor system. Without a quality concrete hardener and sealer combination, a polished surface simply will not last.

You may be asking why these two products could affect the tone as much as they do. Well, some densifier products include ingredients that actively enhance the colour of the surface to show a richer tone and in the case of some brands, also to give the impression that more things are actually happening than just hardening the surface.

This enhancement can seriously affect the finished colour as the polymers used in these products really bring out the colours of the sand in the mix used. Note that I said the sand colour is enhanced not the cement colour, so if you’re looking for the elusive grey concrete floor then this is the complete opposite of what you want.

We have concrete densifiers which fall into both camps enhanced and non enhanced; both excellent densification products but look very different. We select the product to be used based on a clients brief to ensure we get as close to their vision as possible.

A tinted sealer also affects the colouring and tone of concrete surfaces. In the marketplace there are an even wider range of sealers available than there are densifier products. Once again there are products which either enhance colours or attempt to invisibly seal the concrete. Guard coat products that aim to offer stain and chemical resistance by placing a very fine coating over the mechanically polished surface, tend to leave a more varnish-like appearance.

Fully impregnating sealers like our own PROseal soak into the surface and leave very little on the surface, so the floor looks more like its mechanically polished state. Impregnating sealers often offer a decent level of protection to water and oil based stains, but generally fall short of chemical resistance. Acids are very often poorly resisted on concrete surfaces.

Would using a concrete overlay help with colour?

So, given that standard concrete mixes are problematic when choosing colours, are prone to cracking and come with the added complication of getting the product installed correctly, what would CARRcrete recommend for getting the right concrete flooring colour?

As seasoned experts in polishing concrete for many years, we would always recommend using one of our PROtop or MicroFloor concrete overlay products to get your ideal colour, especially if you think that ending up with concrete of a different colour would spoil your experience with concrete altogether. Whilst I know these products may appear at first glance to be more expensive than a regular ready mix product, in reality there is very little difference when you consider you a putting down a 50mm screed opposed to a 100mm concrete slab. Plus there are a lot of reasons for the cost difference; you do get what you pay for.

When I originally designed the PROtop and MicroFloor products, it was to overcome the whole slew of problems associated with concrete surfaces, one of which was the difficulty in getting decent or accurate colours out of a supply of ready mix concrete. So many times our clients were left frustrated by the concrete they or their contractors has purchased; I needed a way to reduce this scenario from happening and to give our clients the choice they deserved.

On top of the frustration with the issue of colour, we often experience problems where the tradesmen laying the concrete slab either do not fully understand the medium or show any regard or concern for the end result being a finished surface.

So when I brought CARRcrete PROtop to market, I had spent months developing the product in-house, testing the overlay over hundreds of samples and countless times in our showroom, where I would mix a small batch and install the concrete overlay over a thirty square metre floor and see how it performed. My team and I would abuse the samples with chemicals, heat and run furniture over the area to see what damage could be done. We went through this process for months before we were fully satisfied and sent the finalised product for lab testing.

CARRcrete PROtop overlays are for many clients an absolute revelation as it allows them much more control over the look of their finished floor. Control whilst still being completely unique to their installation and project; no two applications are ever the same.

We manufacture bespoke samples for all clients that book in with us. Our PROtop product is incredibly advanced with recycled glass aggregates, shrinkage compensation and low CO2 cements all while being incredibly flexible, so it is at home on heated floors. CARRcrete PROtop is the ultimate concrete product line, designed and manufactured by ourselves in the UK.