Mixing cement for garden projects, written by professional landscapers.
This quick guide shows you how to mix cement for garden projects such as paving, fencing, decking and rendering. It includes ratios, consistency, calculating quantities and other useful tips.
Ingredients
- Cement
- Builders sand
- Mortar plasticiser
- Water
All of these materials will usually be available from a local builders merchant who will be able to deliver them if need be.
Note: The words concrete and cement are sometimes used interchangeably however cement is an ingredient mixed with sand and other things to make concrete.
Mixer or wheel barrow?
It goes without saying that a cement fixer is the ideal tool for the job however they are expensive and take up a lot of space. Not ideal for a one off job or if you don’t have room for another bulky item in the shed.
If you are mixing in a barrow we advise using a good quality plastic barrow as they tend to outlast the metal ones by far. Cheap barrows tend to rust and break quickly so getting a good one is often cheaper in the long run.
Cement mixing paddles
Unless you are looking or a long frustrating workout we would recommend using a mixing paddle to get a smooth even cement mixture.
Cement mixing paddles are a useful garden tool because they can be used for mixing soil, resin, paint and other commonly used garden substances.
Here are some of the best ones suitable for domestic use.
Ratio
Concrete has many uses which means that there are many different types needed. All different types of concrete have different ingredients and ratios of builders sand and cement to make them suitable for their purpose and keep costs to a minimum.
For garden projects we recommend a ratio of 1 part cement to 6 parts builders sand. This means for every 10 kg of cement you should use 60 kg of sand.
Typically cement comes in 25kg bags as standard. We found the easiest way to work out the ratio to be half a bag to roughly 10 shovels of sand.
the ratio does not have to be perfect every time but if you get it in the broad area of 6 to 1 you will have no problem.
Consistency
Concrete for needs to have the correct consistency especially for paving. Too wet and it won’t hold firm during the setting process. To dry and it won’t adhere to the paving which will mean loose slabs or supports.
Unfortunately there is no no scientific method available to the layman to make this perfect. It depends on the existing water content of any sand and and and how much of the dry materials you have put into the mix.
Still today the most accurate way of judging this is by eye and touch. The the cement mixture should be sticky but not sloppy, it should be one consistent colour with no dry or sandy patches.
It’s advisable to add water slowly whilst mixing. Add a little water then wait for it to mix, then add a little more. Do this until you have the right consistency. With practice you will get much faster at doing this.
Mortar plasticiser???
This stuff is a little trick we learnt from experienced bricklayers. You can pick it up from most hardware stores. Following the guidance and the safety measures on the container, pour 5ml into each mix. It’s best if this goes in with the water to ensure it is mixed through.
Mortar plasticiser will increase the plasticity of the concrete while you are working with it. This makes it much easier to shape and move around. We never mix concrete without this, but if you don’t want to spend the extra money it will still set hard without.
Here are two of the types we commonly use and rate highly.
Quantity
Based on the 6 to 1 ratio you will need to order the correct quantity of each material. Working this out can be a little tricky, i will walk through an example to show you how this is done. In the example I will be laying 10 square meters of paving.
Calculating the quantity of sand and cement needed for paving (example)
Step 1: Calculate the volume of mixed concrete you will need. This is the square meters (10) multiplied by the depth in meters (50mm = 0.05meters)
10*0.05 = 0.5 cubic meters
Step 2: Change the cubic meters of cement into its equivalent weight. Generally, materials are sold by weight, most materials will have a different weight compared to their volume. For builders sand and cement 1 cubic meter = roughly 1.6 metric tonnes (unless the sand is wet).
0.5*1.6 = 0.8 tons (800kg)
Step 3: How much of each material to order. To account for miscalculations and the base not being 100% level (its ok to be a couple of cm out either way in some places) its good practice to order 10% more than we need.
800*1.1 = 880 (lets round this to 900kg)
Then to work out the quantities of each ingredient we divide this by 7
900/7 = 128.5
Multiply this amount by 1 for the cement and by 6 for the sand
128.5kg of cement
128.5*6 = 771kg building sand
Step 4: Work out the best quantities to order. We have worked out how much we need but we can’t order to the exact kg. Cement comes in 25kg bags as standard and building sand will be much cheaper to order in a ton bag than in smaller bags.
My order
6 x 25kg bags of cement (get the plastic wrapped bags, the paper ones often split or get wet)
1 x 1000kg (850kg if they have it) builders sand
Mixing the cement
Before you begin to mix it’s a good idea to plan your mixing station. Concrete has a habit of ending up all over the place so make sure you mix somewhere this doesn’t matter. Alternatively you can use boards, sheets and bin bags to cover things you want to protect.
When using a normal sized cement mixer or barrow use half a bag (12.5kg) of cement to 75kg of sand (around 10 shovels). To reduce the mess and for best mixing follow these steps.
- Load up the sand
- Pour in half a bag of cement
- Add 5ml of mortar plasticiser to the water (use a watering can or bucket)
- Add around 1 litre of water
- Wait until partly mixed (around 20 seconds)
- Keep adding water then waiting until its consistency is correct
Pro tip: It’s very easy to add too much water here. Make sure to wait until the water is mixed though before adding more.
Make sure to use the mixed concrete within half an hour to avoid it setting and becoming brittle.
If you are mixing the cement in winter make sure to read our guide on laying paving in winter which goes over how to protect your cement from frost and rain damage.
