Some people confuse concrete with cement, but cement is just the ‘glue’ which holds the other constituents of concrete together.
There are a range of different cements but these are the three you will most commonly come across:
General Purpose Cement
This is consistent, versatile and cost effective which makes it a good choice for most building works. It can be used for Domestic concrete slabs, driveways and footpaths
Trivial Fact -You may hear standard cement referred to as Portland Cement – This is because the finished concrete has an appearance similar to stone quarried from Portland in England.
Sulphate Resisting Cement
Sulfate Resisting Cement is a blended cement designed to improve the performance of concrete where the risk of sulfate attack may be present. It also provides improved durability for concrete, and the steel reinforcement, in most aggressive environments,
Although it has an additional cost it is best for:
Geothermal areas and soils containing sulphates
Area that are frequently wet such as concrete swimming pools
Rapid Set Cement
Normally found in dry premixed concrete mixes. This is a cement with various additives that speed up the reaction to give an initial hardening within 15 minutes.
Any speeding up of the cement reaction time leads to lower final strength. This makes this product good for things like setting fence posts………… but NOT for significant structural applications.
Water Cement Ratio
One of the most important issues with cement is ensuring the ratio of water and cement is correct as this affects both the final strength and the durability. For instance:
Water cement ratio 0.5 (10L water to 20kg cement) is needed for high strength 35MPa.
Increasing the Water cement ratio to 1 (20L water to 20kg cement) will reduce the strength to 10MPa.
To ensure they don’t add too much water premix companies usually measure the water content in the sand and gravel piles and reduce the water content accordingly.
Concrete is an artificial stone with four ingredients that all contribute to the overall strength.
Large stones (Aggregate) – The aggregate provides shear strength to the concrete. In other words the concrete has to crack around rather than through the stones. A good aggregate will have a mix of stone sizes rather than just one size of stone, this help the aggregate lock together
Sand – acts to fill in the spaces between the aggregate and further helps lock it solidly.
Cement powder – Is the basis for the ‘cement paste’ (glue) which coats the surface of the first two components and holds them together.
Water – Reacts with the cement powder to form the cement paste
Just like baking a cake its important to get the proportions of the various components right for the best results.
Too Much Aggregate compared with the sand and the concrete will be ‘Boney’ and be just cement coated stones stuck together where they touch.
Too Little Aggregate, or Too Much Sand and the mix will be ‘Fatty’ There will be too little aggregate to provide shear strength and much of the aggregate will sink to the bottom.
Too Much Cement will cost you money without adding to the strength.
Too Much Water will dilute the cement paste meaning the ‘glue’ has less strength and will also cause increased shrinkage as the cement cures. (Its unusual to use too little water as the concrete will be very dry and as a result be hard to place and finish)
If you are going to lay large areas of concrete its probably be best to get premixed concrete which should have the ingredients in the correct quantities.
For small quantities you can buy bags of dry concrete mix where you just add water. Make sure you follow the instructions.
If you have got a job that is of a reasonable size but too small for premixed concrete here is a suggested mix:
Cement = 1 part.
Sand = 2.5 parts.
Stone or gravel = 3 parts.
This should be fine for paths, garden wall foundations, fence posts, washing poles, and driveways.
If you are going to order Ready Mix N(Normal)25 should be fine for a driveway.