Mortar is a durable, gap-filling adhesive that:
- acts as a cushion to spread vertical loads evenly down into the brickwork
- allows adjustment of the position of each brick to line, level and plumb, as it is laid
- sticks the bricks together for stability
- forms a seal between them to resist rain penetration.
"Mortar should never be stronger than the bricks" is a well-known maxim in the bricklaying world. In this context, "stronger" does not mean its load-bearing capacity, but the mortar's hardness and permeability. Hard, dense mortar should not be used to surround porous, open-textured bricks. In such brickwork rainwater wets the bricks, but the impervious mortar stops the water diffusing and prevents the brickwork drying out freely. Ideally both the brickwork and mortar should have a similar porosity.
Sand for use in mortar should have a well-graded range of particle sizes. The proportion of solid material to "void" in such sand is 3:1; this ratio dictates the proportion of materials in a mortar mix since binder is used to fill these voids.
For more than a century, Portland cement has been used as a binder for mortar. A mix of one part cement to three parts sand produces a very strong, hard, water-resistant jointing material. However, it is too strong and too hard for most applications and, because it is brittle, the masonry it binds is likely to crack if it is subjected to movement caused by structural settlement or changes in the masonry's temperature or moisture content.
To create a weaker mortar, Portland cement is used in combination with other standard materials. Different mixes have evolved that reduce its strength, hardness and brittleness and to provide mortars with a balance of properties to suit a variety of applications.
Reducing cement content and substituting filler material to preserve the ratio of binder to sand reduces the dense, hard characteristics of 1:3 cement-and-sand mortar. One such filler is hydrated lime; it has no binding property, but it is beneficial as filler because it is good at retaining water, which promotes the mortar's bonding and improves its workability. This mix is known as cement-lime-sand mortar, or "compo". This mix has a long-term advantage in that it self-heals minor cracks, which makes the brickwork tolerant of minor movement.
An alternative to using hydrated lime is to create minute bubbles of air in the mortar to compensate for using less cement in the mix. To do this, an air-entraining additive known as a plasticiser is added and the mortar is mixed for a prescribed time. This type of mix is called air-entrained, or plasticised, cement-sand mortar. Reducing the cement content does reduce a mortar's durability. However, air entrainment improves resistance to frost damage and sulphate attack.
General purpose mortars, developed in the 1990s at BRE, then the Building Research Establishment, combine the advantages of lime and air-entrainment. They are made by adding air-entrainment to Portland cement-sand mortar mixes or by using masonry cements, which contain a plasticiser with a high lime content.
Table 1 lists four types of mortar based on Portland cement as a binder and the proportions, by volume, of the constituents in various mixes.
Brick Bulletin June 2004
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Understanding brick mortars