Concrete and Mortar Guide

Being able to mix concrete and mortar is an essential skill for any ardent DIY-er. Concrete is used in many places around the home, for laying a firm sub-floor or a base for paving, whilst Mortar is the most important bit in any wall. Both Mortar and Concrete is made with a mix of sand and cement, with Concrete also containing coarse aggregate (small stones) for extra strength.

Cement

The standard type of cement, the type you will most likely use for general DIY tasks, is OPC or Ordinary Portland Cement. OPC is air setting, meaning that the moisture in the air will cause it to harden if not sealed in bags. It is grey and usually available in 50kg bags. Although there are several other types of cement available, the other two main types you are most likely to come across are White Portland Cement and Masonry Cement. White cement is used in areas where the finished appearance is important, whilst masonry cement has additives to increase its usability in mortar and render mixes.

Aggregates

Aggregates are used to give body and strength to a concrete or mortar mix. Coarse aggregate is small stones, usually less that 20mm in diameter, and fine aggregate is basically sand. Sharp Sand (which has larger grains) is used for concreting or laying paving slabs and Soft Sand is used for bricklaying, etc. It is also possible to buy mixed aggregate containing both coarse and fine aggregates.

Ready-Mix

Mixing your own concrete or mortar with separate bags of cement and aggregates is often the cheaper option, but if you only have a small job to do, you can buy bags of ready-mix or pre-mixed concrete and mortar. Pre-mixed cement and mortar simply needs water added to it in the correct quantities. For bigger jobs, you can also buy wet pre-mixed concrete and get it delivered. However, you need to make sure you have enough helping hands to get the wet concrete in place before it sets.

Additives

There are several things that can be added to a mortar or concrete mix. It is possible to get pigments to change the colour of both mixes, but these need to be used with care. Most pigments will affect the usability of the mix, and you might need to think about adding lime or plasticizer to balance this.

Concrete and Mortar Mixes

There are several different mixes you can employ. The proportions of each individual element you add are dependent on the job at hand. A strong mix is one with a higher percentage of cement, whilst a weak mix uses less.

Concrete

General Purpose Concrete - 1:2:3 mix

Ideal for most uses except foundations and exposed paving. It is composed of one part cement, two parts sand and three parts coarse aggregate. If using combined aggregate, this mix would be 1:4, one part cement to four parts combined aggregate.

Foundation Concrete - 1:2 ½:3 ½ mix

Ideal for wall foundations or bases and laying paving slabs, etc. One part cement, two and a half sand and three and a half coarse aggregate. If using combined aggregate, this mix would be 1:5.

Paving Concrete - 1:1 ½:2 ½ mix

Used for exposed paving such as driveways and garage floors. One part cement, one and a half parts sand and two and a half parts coarse aggregate. Combined aggregate would need a mix of 1:3 ½.

Mortar

Standard Mortar 1:5 mix

Used for internal or sheltered bricklaying. Mix one part cement to 5 parts soft sand. Add a small amount of lime or one part plasticizer to increase the workability.

Strong Mortar 1:4 mix

Used for exposed brickwork. Mix one part cement to 4 parts soft sand. Again, add a small amount of lime or plasticizer to increase the workability.

Mixing Concrete and Mortar

It is always best to mix concrete or mortar on a piece of board (spot board). This helps to keep the mix free from stones or dirt on the floor, and also avoids leaving a concrete stain. Start by mixing the separate components thoroughly whilst dry. This applies to pre-mix as well. Once the mix looks uniform in colour, make a small well in the middle of the mix and slowly add water. Work the mix in from the edges until the first bit of water is absorbed. Now make a new well and repeat the process. Continue doing this until you have a uniform mix. If you draw your spade across the mix and it leaves a smooth finish with no water seeping out, the mix is just about right.