Mortar

Mortar is a plastic mixture of water and binding materials used to join concrete blocks, bricks or other masonry units.

It is desirable for mortar to hold moisture, be plastic enough to stick to the trowel and the blocks or bricks and finally to develop adequate strength without cracking.

Mortar need not be stronger than the units it joins. In fact cracks are more likely to appear in the blocks or bricks if the mortar is excessively strong.

There are several types of mortars each suitable for particular applications and of varying costs. Most of these mortars include sand as an ingredient. In all cases the sand should be clean, free of organic material, be well graded (a variety of sizes) and not exceed 3mm of silt in the sedimentation test. In most cases, particle size should not exceed 3mm as the mortar will be "harsh" and difficult to work with.

Lime mortar is typically mixed 1 part lime to 3 of sand. Two types of lime are available. Hydraulic lime hardens quickly and should be used within an hour. It is suitable for both above and below ground applications. Non-hydraulic lime requires air to harden and can only be used above ground. If smoothed off while standing, a pile of this type of lime mortar can be stored for several days.

Cement mortar is stronger and more waterproof than line mortar, but is difficult to work with because it is not 'fat' or plastic and falls away from the blocks or bricks during placement. In addition, cement mortar is more costly than other types. Consequently it is used in only a few applications such as a damp-proof course or in some limited areas where heavy loads are expected. A 1:3 mix using fine sand is usually required to get adequate plasticity.

Compo mortar is made with cement, lime and sand. In some localities a 50:50 cement-lime mix is sold as mortar cement. The addition of the lime reduces the cost and improves the workability. A 1:2:9, cement-lime-sand mix is suitable for general purposes, while a 1:1:6 is better for exposed surfaces and a 1:3:12 can be used for interior walls or stone walls where the extra plasticity is helpful.

Mortar can also be made using pozzolana, bitumen, cutback or soil. A 1:2:9 lime-pozzolana-sand mortar about equals a 1:6 cement-sand mortar. Adobe and stabilizedsoil blocks are often laid in a mortar of the same composition as the blocks.

Tables 3.16 and 3.17 provide information on the materials required for a cubic metre of various mortars and the amount of mortar per square meter for several building units.

Starting with cement mortar, strength decreases with each type, although ability to accommodate movement increases.