How do I use a cement mixer properly?

A mixer transforms the amount of work you can get through, and most people are never shown how to use one. The two things that go wrong are gauging — losing track of the ratio between batches so the mortar changes colour and strength across a wall — and cleaning, which is easy for thirty seconds and a demolition job an hour later.

Safety

What you'll need

• A mixer, sited level and stable
• A gauging bucket used for every ingredient
• A wheelbarrow that fits under the drum
• Waterproof gloves, boots and eye protection
• A shovel of gravel or hardcore for cleaning the drum
• A hose or a full water butt within reach

Step by step

1. Site it level, stable and on boards.A mixer walks about as it runs. Set it on firm level ground or scaffold boards, at a height where the barrow goes underneath, with room to work all round it and the cable run where nobody will trip on it or drag it through wet mortar.
2. Check the electrics before anything else.Plug into an RCD-protected socket or an RCD adaptor, inspect the cable for damage along its whole length, and keep the plug and connections up off the ground and out of any standing water.
3. Gauge with one bucket, every time.Consistency between batches matters more than absolute precision. Use the same bucket for cement, sand and ballast, count out loud, and write the recipe on the mixer with a marker if more than one person is loading it.
4. Start the drum before you load it.Never load a stationary drum and then start it — the load lands in one lump and stalls or strains the motor. Get it turning, then feed material in.
5. Put most of the water in first.Roughly three quarters of the water you expect to need goes in before any solids. It stops cement caking on the drum walls and it means the mix comes together much faster.
6. Load sand or ballast, then cement.Aggregate first so the cement is not thrown against a dry drum. Then the cement, then the remaining water in small doses, watching the mix each time. It is easy to add water and impossible to take it out.
7. Mix for two to three minutes after the last ingredient.Under-mixing leaves streaks and uneven strength. Over-mixing — beyond about five minutes — starts to break down the mix, drives off water and, in mortar, over-aerates it. Watch for uniform colour and a smooth rolling action, not a sliding lump.
8. Judge consistency in the drum, then in the barrow.Mortar should roll over rather than slide, hold a ridge and stay on a trowel held at an angle. Concrete should hold its shape when heaped and slump only slightly. Tip a little out and look at it properly before committing the batch.
9. Discharge fully into the barrow.Tip slowly with the drum still turning, and scrape the barrow out completely between loads. Material left in the drum from the last batch throws the next one out and starts building the crust that ruins mixers.
10. Clean the drum before it sets, not after.As soon as the last batch is out, put in a few litres of water and a shovelful of gravel or broken hardcore and let it turn for a couple of minutes. The gravel scours the drum. Tip it out, rinse, and do it again if anything is still clinging.
11. Never chip a running drum.If build-up needs knocking off, switch off at the socket, wait for the drum to stop completely, and chip it out with the machine isolated. Hands and tools never enter a turning drum.
12. Dispose of the washings properly.Concrete washings are strongly alkaline and it is an offence to let them enter a drain, a gully or a watercourse. Tip them into a lined pit or a tub in a corner of the garden, let the solids settle and the water evaporate, and take the set lump to the tip as rubble.

Tips

• A tipping mixer with a barrow under it is twice as productive as one you have to shovel out of. If you are hiring, hire the size that lets a barrow underneath.
• Mix the first batch slightly dry and adjust. Sand carries a lot of variable water and yesterday's rain changes how much you need today.
• If you are mixing all day, mark the drum height for a full batch. It stops the batches creeping bigger as the day goes on.
• Cover the mixer between batches in hot sun. The drum gets hot and starts flash-setting mortar on the walls.

If it doesn't work