Page 30 - Haddonstone

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6.5 Mortars

It is vital that the correct mortar designation is • Mortars containing lime give a stronger
specified when using cast stone products. The bond than those containing air-entrainment
designer should note however that this is often and are particularly useful because the
of a different designation to that used for the improved bond increased flexural strength
surrounding brickwork. Failure to allow for this, and better resistance to rain penetration.
particularly where the mortar is stronger, can Air-entrained mixes tend to be more
result in cracks appearing in long units as a resistant to frost damage, particularly
result of concentrating the effects of differential when the mortar is at an early age. With
movement. The cracks are usually of little mortars containing lime however, there
structural significance but are unsightly. The is also the added advantage of a certain
reason is that too much restraint is offered by self healing action, known as autogenous
strong mortars and that this can cause distress healing. This has the effect of sealing
to the cast stone by preventing the shrinkage small cracks (often caused by movement)
process taking place. The cracks will occur which occur in the mortar. Free lime in
both in the cast stone and in the mortar below the form of calcium hydroxide, which is
onto which the units are bedded. This can formed during the hydration of the cement,
cause debonding of the unit from the mortar is washed from the cement matrix when
and hence instability. If strong mortar is used the joint becomes wet. As the free lime
for the jointing/pointing process then damage washes to the surface of the joint, it reacts
may occur to the arrises as the strong mortar with the carbon dioxide to form calcium
shrinks away, perhaps pulling some of the arris carbonate which is non-water soluble. This
with it. Strong mortars shrink considerably and self seals the fissure and in time makes
also have a higher bond strength. the joint more water proof.

• Sand and cement mortars are not To improve the water repellent quality of
recommended because they lack the the joint in exposed conditions, proprietary
required workability to allow the masons water-proofers are often added to the
to lay the units at an economic rate. mortar. However, the most commonly used
water proofing additives such as Styrene
• Mortars should be able to resist frost and Butadiene, which is a co-polymer latex
develop durability fairly quickly. However, emulsion (SBA or SBR) cannot typically be
as has been stated earlier, the strongest specified because they are only suitable with
mortars are not always the best. Where a either a designation (i) or (ii) mortar, which is
mortar of a stronger strength is required too strong for cast stone components.
for durability reasons, reference should
be made to Table NA.2 of NA to BS EN For further information please refer to NA to
1996-1-1: 2005. BS EN 1996-1-1: 2005.

The recommended grade of mortars are:

Types of Mortar Binder Constituents Designation

Cement:Lime:Sand A Portland cement and lime with or without (iii) 1:1:5 to 6
an air entraining additive (iv) 1:2:8 to 9

Masonry Cement:Sand Masonry cement containing Portland cement (iii) 1:3.5 to 4
and lime in the approximate ratio 1:1 and an (iv) 1:4.5
air entraining additive
(iii) 1:4 to 5
Masonry cement containing a Portland cement (iv) 1:5.5 to 6.5
and inorganic materials other than lime and an
air entraining additive

Cement:Sand A Portland cement and an air entraining additive (iii) 1:5 to 6

(plasticised) (iv) 1:7 to 8

All proportions by volume