Frequently Asked Questions

Welcome to the Blue Circle Cement products FAQ page.

What is the difference between Blue Circle General Purpose Cement and Blue Circle Mastercrete?

General Purpose Cement is a CEM II Portland-composite suitable for most general purpose applications. ‘Mastercrete’ cement is an enhanced CEM II cement; blending Portland cement and finely ground limestone with specially selected additives. Mastercrete has improved workability and is more resistant to freeze thaw attack and has the added benefit of being packed in our unique plastic packaging, making it 38% better for the environment than cement packed in paper sacks.

What is the difference between cement and concrete?

A good way to think of this is cement is to concrete as flour is to a cake. Cement is only one of the constituents of concrete and mortar. The various basic mixes can be defined as:

  • Mortar – cement, fine aggregate (sand) and water.
  • Concrete – cement, course aggregate (generally 10-20mm rocks), fine aggregate (sand) and water.

These can also contain various additives (such as lime) and admixtures (such as a waterproofer).

What are admixtures and how do I use them?

Admixtures are used generally by addition to the mixing water to alter the fresh and long term properties of cement based mixtures for a specific purpose. The most common admixtures are:

  • Plasticisers: Increase the workability of fresh mixes for a given amount of water allowing less water to be used and thus greater long term strength.
  • Waterproofers: Reduce the permeability of the mix.

It is essential that the manufacturer’s instructions are followed precisely as overdosing can have serious effects

Can my cement still be used once it has passed its use by date?
We strongly recommend that you do not use any cement that has exceeded its use by date. The use by date on cement is related to health and safety regulations about ‘chromium VI’ which can cause allergic dermatitis. After the use by date on the bag we are unable to guarantee that ‘chromium VI’ is below the required legal limit, which could increase the risk of skin irritation. See the material safety data sheet for the product you have purchased for more details.
Does Blue Circle Snowcrete have a shelf life?

Snowcrete is naturally low in Chromium V1 which means there is no H&S requirement for a shelf life. Most cement contains an agent to reduce the levels of Chromium V1.

Does Blue Circle Hydralime have a shelf life?
Hydralime does not have a shelf life, it is not cement and does not contain chromium VI.
Can I still use my cement if some has gone hard or there are lumps in the bag?
No, we would not recommend that any cement containing lumps is used. Lumpy cement will not create a homogeneous mix. There is also a good chance it may not gain as much strength as you would expect.
Can my cement bags be stored outside?
All of our plastic packed products can be stored outside, however we would advise that our paper packed products are stored inside.
How should I dispose of cement I haven't used?

Wet the cement and leave it to go hard, then dispose of the product with your other building waste.

Can Mastercrete be used underground?
Yes it can, unless the ground contains high levels of sulfates. Mastercrete can be used in DC -1, DC-2 & DC-2z ground conditions. In ground containing higher sulfate levels use Blue Circle‘Sulfacrete’. Refer to your soil survey for this information.
I have been asked for a C30 mix. What is it?

C30 refers to a characteristic 28 day compressive strength of 30 N/mm². It depends on a number of factors including, among others, water:cement ratio, workability requirements, aggregate properties, specified minimum cement content, use of admixtures and additives. Nominal mix proportions for various purposes and requirements can be sourced through the Tarmac Blue Circle Cement Technical Helpdesk. If a specific strength requirement is specified trial mixes are strongly recommended.

I want to make a mix that is easy to place. Should I add lots of water to make the mix sloppy?

No! The addition of extra water to a mix will indeed make a mix that is far easier to place due to the higher workability. However, major problems can occur if this is carried out. Cement based mixes are generally stronger in the long term with the least amount of water needed to give the desired workability/consistency or a low water to cement ratio. With a high water to cement ratio, the strength of the mix will be reduced. Also, segregation (aggregate falling to the bottom of the mix) and bleeding (water rising to the surface) may occur, thus further reducing the mix strength and durability performance. In general terms use the minimum water content that allows for placement and finishing.

Is it true that concrete can be overworked when finishing off to a smooth surface?
Concrete can be overworked when smoothing a surface finish. If overworking occurs (especially in high water content mixes), water is likely to rise to the surface causing a weaker layer to develop which reduces strength and durability. It may also cause flaking of the uppermost surface layer. It is thus important to stop smoothing a surface at the earliest time consistent with a suitable finish and use relatively low water content in the original mix.
How long will it take for the Blue Circle High Strength Concrete (40N) to gain maximum strength?
High strength concrete will gain full strength at 28 days. For best practice we recommend the concrete is properly cured and covered for 5-7 days. If mixed in accordance with the instructions, we guarantee a strength of more than 40 Newton’s per square millimetre at 28 days.
When using Snowcrete my mortar has turned red. Why is this?

All of the solid constituents in a mortar (cement, lime, fine sand) will contribute to the final colour of a mortar. The use of a light, clean sand is thus essential if a white final mortar is required. Trial mixes should be carried out in small disposable cups and an exact note of the various proportions and sand types marked on the cup. This will allow consistency of colour throughout the job.

Can I add extra water to mortar if it starts to harden on the spot board, to make it more workable again, or should I throw it away?

Once the mortar has begun to set, the addition of extra water will not make any difference as the mortar is now non-workable. Even with or without the addition of water, if the set mix is reworked, this will cause major damage to the structure of the mortar. This will in turn seriously affect both the strength and durability properties and so it is thus vital that the mortar is fully placed prior to the setting time.

What colour will the quality assured mortar dry to?
The finished product will dry to an off grey.
What is the difference between hydraulic and non-hydraulic lime?
Hydraulic lime, when mixed with water, gains strength in a similar manner to cement (chemically reacts with the water). Non-hydraulic lime (or hydrated lime), when mixed with water, forms a putty that can only slowly gain strength from exposure to air. Hydraulic limes are typically used in soft stonework or historical structural renovations. Hydrated lime is always mixed with cement in a render.
What is the difference between Hydraulic lime and Hydrated lime?

Hydraulic lime is used for the restoration and repair of historic buildings which enables building movement and breathability allowing any damp to evaporate.

Hydrated lime (Blue Circle Hydralime) is used as a workability aid with cement and sand to make a more cohesive mix. This is especially good if you are in an area of the country which has poor sand quality. This can also be used to make lime putty or a lime wash.

What is the most general rule for renders?

The most general rule for renders is that each coat is thinner and weaker than the coat preceding it. Curing is important for effective rendering and it is recommended that each coat is covered for a minimum of 3-5 days before applying next coat.

If I use Blue Circle Snowcrete for rendering will it be white?

Sourcing a good light sand will give the closest possible colour to white. If you used a darker sand this would most likely give a more orange effect to the render. The colour of the sand used alongside the Snowcrete would have an impact on the overall colour of the end result.

How thickly or thinly should screeds be applied?

A floor screed is a layer of mortar consisting of cement and clean, sharp sand. Traditional screeds are NOT intended as a wearing surface and as such tiles, carpets etc are needed to cover the screed.

  • Bonded, to a concrete base, 20-40mm thick.
  • Unbonded, over a damp proof course, minimum 50mm thick.
  • Floating, over insulating material, minimum 65mm thick.
A screed has recently been placed, so when can I lay carpet on it?
Traditional cement and sand based screeds need to dry out slowly (after sufficient strength has been achieved) prior to the application of the surface finish. Failure to do this may result in bonding and durability problems. A rough guide is to allow one day drying time for every millimetre depth of screed. Attempts to reduce this time by prematurely drying out the screed (e.g. heating it) are not recommended as this can cause cracking, a loss of ultimate strength and possible durability problems.
How much of my fencepost should be under the ground?
As a general rule we would recommend that around 25% of your post should be buried below ground.
How much water should I use for Blue Circle Postcrete and can I put the Postcrete in before the water?

Blue Circle Postcrete should be used as per the method on the bag. Fill the hole one third with clean water and add the Postcrete on top of the water. Allow 5-10 minutes for the initial set. You will be able to hang a fence panel in as little as 10 minutes!

What is lime bloom and how can I prevent/remove it?

Lime Bloom is a crystalline or powdery deposit that forms on the surface of cement based products and brickwork. It is normally white in colour. Efflorescence occurs when soluble salts in the concrete, mortar, render or brickwork come into contact with the atmosphere. As the construction dries out, water dissolves the soluble salts, carrying them in solution to the surface where they are deposited when the water evaporates. It tends to form predominantly in cold, damp conditions.

Although efflorescence is unsightly, it is not usually harmful to the concrete or masonry and can normally be removed by stiff brushing or in severe cases a mild brick cleaning acid.

Concrete or mortar is dry so why is it not strong?

The cement in concrete and mortar hardens by a chemical reaction with water so if there is no water present then essentially no further strength development will occur. It is thus vital that the mix is kept in a moist condition for the first few days after it has set.

Common causes of moisture loss from exposed concrete or mortar surfaces are: high ambient temperatures, exposure to direct sunlight, drying winds and excessive suction of moisture by surrounding porous materials.

Common prevention methods are to cover with damp hessian and/or plastic sheeting (being sure that this does not mark the concrete) to prevent evaporation, prewetting of surrounding porous materials, misting (for use especially with mortars and renders) with light water spray at regular intervals and pooling water on horizontal surfaces.

Removal of stains from slabs?
Obviously, it is better not to stain slabs in the first place, but if staining occurs then there are several solutions. Ideally the stain should be removed as soon as possible by extensive flushing and scrubbing of the affected area. If this is not possible to remove immediately, then physical abrasion or a mild brick cleaning acid may be required to remove. These can sometimes do more harm than good to the slab as they may have to be excessively strong to remove the stain and hence a small trial area is recommended. In general, the longer the stain is left before treatment, the harder it will be to remove.
What cement should I use for concrete directly exposed to sea water?

A Portland-fly ash cement such as Blue Circle Sulfacrete is a good choice for concrete in these circumstances (look for the designation CEM II/B-V +SR) as it get progressively more impermeable with time and has a greater chemical resistance to the chlorides present in sea water. Sea water does not significantly attack good quality concrete, but it can cause rusting of any embedded metal which may eventually crack the concrete. Concrete exposed to sea water should always have a low water/cement ratio (below 0.55), high strength (C30 or above) and must be properly cured in order to develop its full resistance to seawater penetration. For reinforced structural concrete seek specialist advice.

What cement should I use to level the floor under my garage door?
You want a high strength cement that will resist abrasion and trafficking. The best option would be a Portland cement (CEM I) such as Blue Circle Procem. Make sure that you keep it moist for a few days after placing so that it develops its full potential strength.