Concrete Art: FAQs

THIS PAGE answers some common basic introductory questions about using concrete in art projects. For more depth see other pages in this web site, particularly the technical page and the how-to pages.

What's the difference between concrete and cement?

Concrete is an artificially engineered material made from a mixture of Portland cement, aggregates (such as sand or gravel) and water. Cement (in this case Portland cement) is the binding agent that holds the aggregates together.

Why is it called Portland cement?

The early cements were similar in appearance to a limestone from the Isle of Portland in England. It gave the finished concrete the respectability of stone. Portland limestone was used to build St Paul's Cathedral in London.

What kind of sand should I use? Can I use beach sand or pit-run sand?

You want the sand grains to be "sharp" so the cement can grab them. Beach sand is usually too rounded for optimum strength in the final concrete. Look for "mortar sand", "sharp sand" or "brick sand."

What do plastic fibers do?

There are three kinds of plastic fibers, all available in different lengths and thicknesses: polypropylene, nylon and PVA or polyvinyl alcohol. When concrete first sets, it shrinks, sometimes causing shrinkage cracks. These cracks can later develop into bigger cracks. Fibers help prevent shrinkage cracks. In smaller work, and in particular with the new PVA, fibers can actually add tensile strength.

What about using glass fibers or fiberglass?

Glass can be broken down by the alkalinity of the cement reaction. Glass fibers must be labelled as "alkali resistant". This means they are coated, usually with an epoxy. These are available as chopped strands or mesh (scrim). Glass fibers or scrim add more tensile strength than plastic fibers.

What should I seal my concrete with?

The main purpose of sealing concrete is to prevent water or oil penetration. There are three main types of sealers:
1. acrylic-based sealers sit close to the surface, can yellow, and need reapplying
2. silane or siloxane sealers penetrate deeply
3. sodium or potassium water-based sealers (like waterglass) penetrate and combine with excess alkalinity to make denser concrete near the surface

What is ferrocement?

"Ferrocement is both a method and material used in building or sculpture with cement, sand, water and wire or mesh material - often called thin shell." (Wikipedia) The "ferro" part is the steel mesh or welded wire which is formed as an armature. The cement part is a mortar mix that is applied to the armature.

How can I colour concrete?

Pigments or dyes can be added to the mix, or applied to the surface of set concrete. Special dyes are made for this purpose because common pigments can be broken down by the alkalinity of the cement. The range of colours are in the earth tone range: black, grey, brown, ochre, dark red. Blues and greens are available but costly for large projects. Pigments are available as liquids, or as powders.
Concrete can also be surface coloured with acid stains which penetrate slightly, or opaque stains made for outdoor use. The latter has a wide range of colours, but some may not be long-lasting.

How can I make my concrete more frost resistant?

Make a high quality crack-free concrete. Cracks allow moisture penetration; the water can then freeze and crack the concrete even more.
The main thing is to use as little water as possible. In a wet mix, some of the water does not get chemically combined. This water can freeze and crack the concrete from inside. Also make the concrete as dense as possible (vibrate to get the large air bubbles out if it's a casting).
Also, use an air entrainer. When added to concrete, then mixed (either in a concrete mixer or with a power drill - hand mixing doesn't work), the air entrainer creates tiny air bubbles in the mix which allow expansion and contraction when freezing. Also, use a good quality sealer to prevent as much water penetration as possible.

How do I work with concrete at near freezing temperatures?

In the concrete trade (pouring driveways, foundations, sidewalks, etc), they heat the water in the mix. The mass of the concrete keeps that heat until it sets. They also use "accelerators" to increase the rate of the set. Calcium chloride has been used commonly as an accelerator, but recently it has been implicated in corrosion problems, attacking the steel reinforcing. There are chloride-free accelerators available.
With smaller objects such as sculptures, you have a problem below freezing. If the concrete freezes before it sets it will be ruined. And at temperatures close to freezing, the chemical reaction and the set really slows down. You'll either have to move the work indoors or wait for spring.

What are pozzolans and are they important? (Metakaolin, silica fume, fly ash?)

These are additives (silicates) used to make extra strong, extra dense concrete. They are not essential, but produce a higher quality concrete. These silicates combine with the excess alkalinity (calcium hydroxide) in the cement paste to produce more cementitious material, increasing the density and the impermeability of the concrete. Metakaolin, silica fume and fly ash are all pozzolans, and are grey or white powders added to the mix.

How much water should I use?

There is no hard and fast rule, because there are so many variables: the amount and dryness of the aggregate, the absorption of the aggregate, whether you are casting or building up a layer on an armature. The general rule, though, is to use as little as possible. Excess water in a mix remains as pockets of water after the cement sets, weakening the concrete. Wet mixes also shrink more when setting. Keep the mix just wet enough that it will fill a mold with the aid of vibration, or if it is to be applied in a layer, you should be able to make a ball with it without it falling apart or slumping very much. Using a water-reducer (plasticizer, super-plasticizer) in your mix allows you to use less water and still have a workable mix.

Can I use mortar mix in bags (or sand mix, or concrete mix) for making art?

Yes. To improve it's workability and final strength you might want to add a pozzolan and perhaps some plastic fibres. Usually sand mix in a bag is about 1 part cement to 3 parts sand. If you are doing a lot of work it is cheaper to buy the cement and sand separately.

Is there any type of "glue" that can help bond cement or concrete to already dry concrete or cement?

Yes, you can use either a latex or acrylic admixture. It is sometimes referred to as a concrete adhesive. You can paint this on the concrete surface, let it get tacky, then use the latex or acrylic liquid in place of water for the concrete top coat. In addition, the sooner you can add the new layer of concrete to the already set concrete, the better the bond. Try for two to three days. It may also be a good idea to roughen the surface of the set concrete to help make a good mechanical bond between the two.

A. Is Portland cement still usable once it gets "clumpy"?
B. Is cement usable for ever if it's kept away from moisture?

A. Yes, although the longer a bag stays open and absorbs moisture from the air, the weaker it gets. Cement gets its strength by "hydrating," that is, combining with water, so if some of it is already hydrated it will not be as strong. If the clumps will break apart in your hand easily, you can use it for some applications, but not where maximum strength is required.

B. No. Some people recommend not using cement one year after the bag has been opened. I drop an open bag into a plastic garbage bag, close that, then drop that into a plastic garbage pail with a tight lid.

~Andrew Goss