All About Concrete, Mortar and Aggregate Material
Concrete and Mortar
Aggregates and cement are mixed together to make mortar or concrete. Mortar is used as an adhesive that can hold bricks and blocks together, or hold paving slabs in place. It can also be used as an exterior finish. Concrete, which contains coarse aggregates, is used for foundations and hard landscaping. Different kinds of mortar and concrete, suitable for various uses, can be achieved by varying the proportions of the ingredients. Aggregates and sands can be bought in bags, in bulk or in ready-mixed bags.
How much do you need?
Seek advice from your supplier on how much of each constituent you need for your project. To give you a rough idea, to lay 100 bricks with general-purpose mortar you would need: 55 pounds of cement, 220 pounds of sand and 22 pounds of lime. In practice, this is one bag of cement, four bags of sand and half a bag of lime. The table opposite gives the proportions needed for a variety of mixes.
The adhesive in a mortar mix, cement binds together the components and dries to a stable, hard finish. It must be stored in a dry atmosphere and is unusable if it gets damp.
Portland: Gray in color; the most commonly used cement.
White Portland: A lighter version of Portland cement.
Fast-set: Sets very quickly, so is ideal for small DIY tasks.
Sulfate-resistant (Type II Portland cement): For cement in contact with a clay-rich soil, or soil that is high in sulfates.
Expansive: Hydraulic cement that expands during the hardening process.
These are quicker to use, but are more expensive.
Mortar mix: A general-purpose mortar.
Slab mix: For laying paving.
Concrete mix: A general-purpose concrete.
Using a Mortar Tub
Pour the appropriate portion of a premixed bag of ready-to-mix concrete into a tub (Image 1).
Add the minimum amount of water recommended. The less water used, the stronger the dried, finished product will be (Image 2).
Use a garden hoe to mix the concrete. This way, you will reduce the strain on our back (Image 3).
Mix the concrete, adding water if needed, until the mixture is the consistency of soft peanut butter (Image 4).
Particles smaller than 3/8 inch in diameter are considered a fine aggregate; anything larger is a coarse aggregate. Both materials are used to make a mortar or concrete mix. Fine aggregate varies in color depending on where it comes from. Washed aggregate contains fewer of the impurities that can weaken the adhesion of a parging mix to the surface and stain the finish.
Crushed stone: Large stones and gravel — the coarsest aggregate. Used as a base for concrete and other hard-landscape surfacing.
Ballast: An "all-in-one" combination of fine aggregate and larger stones or gravel. Ideal for use in general concrete work where exact proportions of individual aggregates are not required.
Coarse aggregate: Graded stone, also known as gravel. It may be used in concrete, or as a drainage aid, or as a finished surface for a drive or path.
Sharp sand: Coarse sand with fairly large particles. Often used in concrete mixes, but may also be used to produce a very hard, durable mortar.
Builder's sand: Builder's sand is fine-textured and is used in mixes for laying blocks or bricks. An even finer grade of sharp sand is used for parging.
Kiln-dried silver sand: Very fine, dry sand, light in color. Mainly used dry to grout exterior paved areas.
These can be used in all mixes, according to your requirements.
Lime: Cement already contains lime, but adding more makes mortar easier to work with and less likely to crack when set. Traditional mortar mixes (without cement) are based on lime. Lime retains water well, and is less likely to shrink as it dries out. Nonhydraulic lime is sold as powder or as a putty containing water. Use powder for cement-based mortar, and putty for a traditional building mortar. Hydraulic lime sets more quickly, is harder and is less widely used.
Plasticizer: Makes mortar more workable, and is used as a modern equivalent of lime. It normally comes in liquid form.
Cement pigment: Powdered pigment that colors cement.
Waterproofer: May be mixed with mortar, especially when parging is to be applied in an area prone to damp. Some parging waterproofers slow down the parging's drying, keeping it workable for a longer period.
Accelerator: Speeds up curing time, and can be used to protect the mixture while it dries if frost may be a danger.
Copyright 2009 Dorling Kindersley Limited
Text copyright 2009 Julian Cassell and Peter Parham