Guide To Portland Cement: Your Questions Answered

What is Cement?

Cement is a binding material made from finely ground limestone, sand, clay, marl, bauxite, shale, chalk, and iron ore. It is the main ingredient in concrete and is one of the most important modern-day building materials. When it sets and hardens, it creates a strong bond.

The type of cement we use today is known as ordinary portland cement (OPC) and is used in practically all concrete products. It was invented in the 19th century and replaced hydraulic lime.

What is Cement Used For?

Cement is most commonly used to make concrete for construction and landscaping projects, including roads, housing, floors, driveways, and patios. It is also used to create mortar for plastering, bricklaying, and pointing. More specialist applications include fire, chemical, and frost proofing industrial structures.

How is Cement Made?

Cement is manufactured by combining limestone, sand, clay, marl, bauxite, shale, chalk, and iron ore. The raw materials are crushed into stones that measure approximately 7 cm before being placed into a kiln. When in the kiln, which reaches temperatures up to 1482°C, the elements merge to create stones called clinkers.

When they have cooled down, the clinkers are ground into a fine powder and mixed with gypsum and limestone to help regulate the setting time of cement. Without these additives, it would set instantly when mixed with water and would be impossible to work with.

How Does Cement Work?

When cement comes into contact with water it triggers a chemical reaction and starts to harden. The water slowly hydrates the dry mix, which creates heat and binds the powder into a paste. It will harden and set once the chemical process is complete.

How Long Does Cement Take to Set?

It takes up to 48 hours for cement to harden enough for you to walk or drive over its surface. However, the chemical reaction that turns it into concrete continues for around 28 days. This means it will take up to a month before it reaches its full strength. These timescales vary depending on the temperature.

How Long Does Cement Take to Set in Cold Weather?

It takes 96 hours for cement to set at temperatures between 0°C and 5°C. This is 48 hours longer than it would usually take when pouring concrete in warm weather. While it does take longer to harden, it will not reduce the overall strength.

How Long Does Rapid Cement Take to Set?

Blue Circle extra rapid cement takes 30 minutes to set. It will have hardened enough for foot traffic in 4-6 hours and can be driven over after 8-12 hours. It is used to reduce inconvenience when conducting repairs in busy areas. Even though the curing is accelerated, it still achieves the same strength as regular cement.

What Temperature is Too Cold for Cement to Work?

Temperatures less than 0°C are too cold for cement to work effectively. At temperatures below freezing, the water in the concrete mix will freeze and reduce its strength, leaving it unfit for use. 

When there is frost on the ground or it was cold the night before, wait for the ground to warm up before creating concrete.

How Long Does Cement Last?

Manufacturer instructions on cement bag showing recommended shelf life

Cement lasts for up to three months once it has been produced. You can find the use-by date on the plastic packaging. It has a best before date due to a chemical compound included in the product called chromium, which may be dangerously high after this.

Increased levels of chromium can cause an allergic reaction if it comes into contact with skin. Cement is relatively cheap and using an old bag is not worth the risk.

How to Tell if Cement Has Gone Bad?

You can tell cement has gone bad if there are lumps in the mixture. It should be a fine powder, so if you can see clumps it should not be used. The lumps suggest that moisture has penetrated the bag and will have compromised its strength.

How Much Sand and Cement Do You Need to Lay Bricks?

You will need approximately 1kg of sand for every brick that is to be laid. This means you will need 1,000kg of sand and 250kg of cement for 1,000 bricks. This is a good rule of thumb that makes it easy to estimate how much sand you need for your wall.

How Much Does Cement Cost?

Cement costs £5.31 per 25kg bag, which works out at £0.21 per kg. These costs are based on retail prices from builders merchants and include VAT. Customers can get a more competitive price if they sign up for a trade account or buy in bulk.

Why Does Concrete Crack?

Concrete cracks when the water it contains begins to evaporate. This is known as shrinkage and often happens when too much water is added to the cement and sand mixture. To prevent it from happening in future, reduce the amount of water in your mix.

How Many Bags of Cement on a Pallet in the UK?

There will be between 40 – 56 bags of 25kg cement on a pallet in the UK. This number varies depending on the supplier and is not a one-size-fits-all answer. For a definitive answer, check with the manufacturer before purchasing.

How to Dispose of Old Bags of Cement?

When disposing of old cement you should first harden it by adding water. This will create a block of concrete and prevent airborne powder, which can cause eye and skin irritation. The block can then be taken to your local household waste centre and disposed of as building rubble.

Cement Mix Ratio For General Purpose Concrete

Cement mix ratios for concrete and cement on the back of a bag of Blue Circle general purpose cement

General-purpose concrete will use 1 part cement, 2 parts sharp sand, and 3 parts aggregate. When using ballast, this ratio will be 1 part cement and 4 parts ballast. Concrete for foundations needs to be stronger and using a 1:9 ratio would be more suitable.

Cement to Sand Ratio for Plastering, Pointing, and Rendering

A 6:1 sand to cement ratio is suitable for plastering walls and a 4:1 ratio should be used for ceilings and pointing work. When rendering, the first coat, known as a scratch coat, should use a 3:1 ratio and the second coat, known as a floating coat, should use a 4:1 ratio.

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