Text 4. Mortar

1. What does lime improve when it is used in the motar?

Mortar is the material which binds the bricks and blocks together. It helps to distribute the load through a wall and seals the brick or block joints against water ingress. Mortars should have:

• good workability,

• sufficient resilience to accommodate long term thermal movement of the masonry,

• adequate bond strength,

• good resistance to water penetration.

Mortar is made from fine aggregate (usually sand) and a binding agent (nowadays usually cement). When mixed with water a chemical reaction, called hydration, occurs and the mortar sets. Early mortars were usually based on lime and sand but they were very slow to set and readily absorbed rain water (the mortar sets through a process known as carbonation where the lime gradually hardened by absorbing carbon dioxide from the atmosphere). Modern mortars use cement as the main binding agent although hydrated lime (i.e. bagged lime) is often introduced into the mix to give it a more plastic feel and to make it more 'workable'. Lime also improves the mortar's ability to cope with thermal and moisture movement. By varying the proportions of the cement the strength of the mortar can be increased or decreased as required.

2. What mix proportions ensure workability, adhesion and durability?

A given volume of sand contains approximately 25% of air space. The strength and physical properties of a mortar are determined by the type of binder that fills this air space. For example, a cement/sand mix of 1:3 will result in the air space within the sand being completely filled by the cement, producing a strong but brittle mortar. The relatively high cement content will result in shrinkage as the mortar sets. Many bricks and blocks cannot resist this shrinkage resulting in loss of bond and cracks in the mortar. By replacing some of the binder with lime (say 1:1:6, cement/lime/sand) the binder/aggregate ratio is maintained at 1:3 and the mortar has improved workability, better adhesion and longer durability.

3. What is the function of liquid plasticizers?

Liquid plasticisers can be used in place of lime to improve the nature of a mortar. Plasticisers are usually air entraining agents; in other words, air is introduced into the mix to break down the internal friction and produce more workable mortars. A mortar mix of 1 part cement to 5 parts sand, plus plasticiser, is roughly equivalent to a 1:1:6, cement/lime / sand mix. Another option is to use masonry cement. This is a pre-bagged mix of cement with added chemicals to improve the workability of the mortar. Their finished strength is lower than cement mortars so they should only be used for lightly stressed brick­work in sheltered situations. They should not be used below ground level.

Дата добавления: 2014-12-23