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Dealing with cement is something that I avoided for as long as I possibly could. If something interests me I can’t get it done quick enough, but otherwise I avoid doing it ever, if possible. What is it that I don’t like? Well having thought long and hard about it, it’s all down to the fact that it is dirty and wet! It doesn’t do much for keeping your hands as soft as your face, either. Anyway, I finally got down to mixing some cement mortar when I noticed the huge gaps between all of the ridge tiles on the roof. I don’t like heights, either, so it was a great combination.
What is the difference between cement, concrete and mortar? Cement itself is generally the minority ingredient, in terms of quantity, that is used as a binder to make concrete and mortar. Mortar is the compound that is used for bricklaying. Concrete is the compound used to form blocks, foundations, slabs and walls. They are both formed by the mixing of cement with aggregates, either sand and/or stones (coarse aggregates).
Cement is powdered lime and clay. It is catalysed by water and must be kept completely dry in storage. In effect this tends to mean that it has a short shelf life. If it has already formed lumps, don’t use it. Portland cement is the most common type in use. It’s worth bearing in mind that cement is the more expensive element of the compound, so it’s perhaps just as well that you use more aggregate than cement.
The aggregate that you use and how you combine it with the cement and water in a mixture is the key to producing mortar and concrete. Sand is used in mortar and concrete. It consists of relatively fine particles, the majority of which will pass through a 1.25mm sized sieve. You will hear references to ‘sharp’ and ‘soft’ sand, which aren’t particularly descriptive terms. The former is suitable for concreting and contains coarser particles. The latter should be used for mortar, having finer particles. Colour varies from shades of orange to yellow. It is only important if the final appearance of the dried product matters. You probably don’t have a sieve, but handle the sand; it should remain loose, not form a ball, and not stain your hands too much.
Coarse aggregates are stones that are added to make concrete. These vary in size from 5mm up to 40mm, the larger sizes for a coarser mix.
The strength and flexibility of concrete varies widely, and you will find contradictory information about what is the most suitable mix proportions for different tasks. The cement, proportions and quality of aggregate, water content and method of mixing, batching and so on all affect its properties. It is often said that you should be accurate when mixing, but is mixing by the bucket or shovel full likely to achieve this? I don’t think so, but somehow it seems to have worked out as an effective building material for a long time in many situations.
The other important element to consider are plasticisers and other additives. As the the name suggests, you can add these to the concrete or mortar mix to make it more flexible. This helps to reduce the appearance of cracks. Lime is the old fashioned way of doing it, but it is an unpleasant material to use, and there is a liquid alternative, usually PVA. Other additives include coloured pigments, frost and water proofers. The former helps if you need to work in cold weather, water having a tendency to freeze at 0oC.
Mixing Mortar and Concrete by hand
Measure out the required quantities by volume. You can do this using a bucket or by the shovel full. Pour these out onto any flat surface, an old piece of shuttering board is ideal. Mix all the ingredients together dry by turning them over with a shovel. Once it is all mixed up, create a hollow, rather like the top of a volcano. Follow the instructions on the packaging for any additives establishing wether they need to be added to the water before you start mixing it in. Pour water into the middle of the hollow, rather like you are peeing in it! Don’t over do it, as too much water makes it sloppy and reduces the strength of the mix. Bring in the the dry material from the edges first and turn it over with the shovel. Keep turning everything over and add more water a little at a time. Stop adding water once it’s reached a consistency where it is stiff enough to hold the ridges made by your shovel or trowel. If you’ve already passed that stage, you can rescue it by adding more of the dry mix. Don’t mix up more than you can readily use, because it can dry out significantly in less than an hour, depending on the conditions.
Typical mixes that you can use are:
•Mortar for laying bricks – 1 part cement to 6 parts soft sand, plus plasticiser, if you like
•Concrete general purpose – 1 sharp sand, 2 cement, 3 coarse aggregate
•Concrete for foundations – 1 sharp sand, 3 cement, 6 course aggregate
Obviously, mixing by hand isn’t the best solution for large amounts. The next step is to use a mixer, or even have some ready mixed concrete delivered by the lorry load. There you go.