· Concrete Mix and Mortar/Screed Mix are similar in some respects.
· They are both made from the same basic materials: cement, water
· and aggregate (sand, stone or gravel).
· But, when it comes to their uses as building materials, concrete and screed mix are very different.
· : Quality concrete is used for construction and structural tasks.
· : While mortar/screed is used/applied as a top layer to a concrete/brick base.
Physical differences between concrete and screed:
Concrete and screed are both made from a mixture of cement, water and aggregate. The main difference between the two materials is the type of aggregate that is used.
When mixing concrete, you add coarse aggregate like gravel to the mix. These small stones are normally 20mm or less in size. This coarse aggregate gives the concrete its strength, durability and workability, and makes it suitable for structural work.
For mixing screed mix, you use fine, sharp sand particles, with a maximum grain size of about 2mm, cement, water and sometimes lime.
Sharp sand is more coarse than building/soft sand and is more perfect for mixing with other sands to prevent cracking during the drying process. It’s often used in situation where a slightly thicker layer of mortar is required such as chimney flaunching, bedding roof tiles and many garden projects will require sharp sand.
Plasterers sand is not as coarse as sharp sand, but not as fine as soft sand. Its washed to remove salts and clay residue that could cause efflorescence salting.
Lime can be used as an alternative to cement in some mixes or used in conjunction with it. Lime allows materials such as bricks to breathe, it’s commonly used on older period buildings with soft, permeable bricks that would likely crack if cement was used.
Plasticiser is a liquid that is added to the mix to make the materials easier to work with and also slows down the curing process slightly. It makes a typical mortar mix sticky and easier to point over.
Water proofer is different to plasticiser, it does what it says and is often used when rendering.
Frost proofer, accelerators and colour additives can be included in the mix as required.
What Happens if you Make the Mortar too Weak?
Not only will the mortar fail to bind sufficiently, it may also crumble after a short amount of
time or wash away after minimal weathering.
What Happens if you Make the Mortar too Strong?
Too strong a mix, e.g. made with too much cement and your mortar many dry to quickly, thus
it will dry to quickly, thus it will shrink and crack. Cracking can be avoided by reducing the
amount of cement in the mixture and ensuring the mortar is wet enough.
The best mix will depend on the type of brick and the location of the wall. For most domestic builds such as a house building, use one of these mixes:
· 4 parts soft sand with 1part cement, add water and plasticiser.
· For soft older bricks already bedded onto a lime based mortar, a lime and sand
mix should be used, the ratio will depend on the type of lime and how much water content it has.
· For retaining walls or anywhere likely to be in regular contact with water, use 3
parts soft sand and one part cement, 1 part lime is optional depending on the type of brick or block.
For chimney repointing in a modern home, the best mix will depend on how exposed the chimney is and its location. For chimneys in wind swept rainy parts of the country, use a mix of 3 parts soft sand and 1part cement. For softer or more permeable bricks, 4 parts sand, half part cement and 1 part lime can be used. Plasticiser is optional.
Chimney flaunching will receive a lot of rain so the mix should be strong, this prevents it washing out. Because flaunching is laid several centimetres thick, add sharp sand to the mix to
avoid cracking during the drying process… 2 or 3 parts soft sand, 1 part sharp sand, 1 part cement and half-part lime is optional.
For older period homes, a lime-based should be used instead of cement.
Paving Mortar Mix:
· For bedding under the slabs use 5 parts sharp sand, 1 part soft sand and 1 part cement.
· For pointing use 4 parts soft sand and 1 part cement.
· For high traffic areas, a strong mix of 3 parts soft sand and 1 part cement can be used.
Render Mortar Mix (Modern Buildings)
The first coat of render should ideally be slightly stronger than the second coat. While some tradespeople make the two mixes the same strength, the second coat should never be stronger than the first coat.
A pure soft sand mix shouldn’t be used when rendering. It’s preferable to use either sharp sand, plasterer’s sand or a mixture of the two. Soft sand can be added to the mix but it shouldn’t
comprise of more than 25% of the entire mix.
The first coat of render can be 2 parts plasterer’s sand and 2 parts sharp sand, 1 part cement and half part lime. Waterproofer can be applied to the first coat if the property is modern.
The second coat must be slightly weaker than the first, so 3 parts plasterers sand, 2 parts sharp sand, 1 part cement and half lime. Plasticiser can be used in the second coat.
Locations that are sheltered, a slightly weaker mix can be used. For the first coat 4 parts sharp
sand can be used and for the second coat 5 parts sharp sand can be used.