Mortar mixes for masonry
Mortar binds bricks and blocks together to give strength
and stability to a wall.
Freshly mixed mortar must be soft and plastic so that it spreads easily
and makes good contact without becoming too strong. Too strong a mortar
may crack and is wasteful as it is more expensive.
1. Materials
1.1 Cement
Preferred cement types are:
- Common cement complying with SANS 50197-1.
- Masonry cement complying with SANS 50413-1; strength class 22,5X.
Cement is sold in 50 kg bags and in bulk.
Cement must be kept in dry storage. If there are hard lumps in
the cement that cannot be crumbled by hand, it is not fresh and should
not be used. The performance of products claiming conformance with SANS 50413 strength class 12,5X is not supported by independently published
data. It is illegal to sell cement not bearing the SABS mark.
1.2 Lime
Use only building lime complying with SANS 523: 2002. Do not use quick-lime
or agricultural lime. Lime is sold in 40 kg bags.
Lime should be used if the sand lacks fine material or is single sized,
as such sands tend to produce mortar with poor workability unless lime
is included in the mix.
Lime also helps the fresh mortar to retain water when it is placed against
dry cement bricks or blocks and helps to prevent cracking of the hardened
mortar.
A maximum of 40 l of lime is permitted per 50 kg of cement.
Do not use lime with masonry cement.
1.3 Sand
The sand should be clean (grass, leaves, roots, etc, are harmful) and
it should not contain too much clay. It should consist of hard particles
which range in size from dust up to about 2 mm. Pit sands generally have
these characteristics. River, dune and beach sands are often too uniform
in size (single sized) to give good results.
2. Mix proportions
The proportion of each material in the mix should suit the type of work
being done. Strength requirements and mix proportions, recommended by
C&CI, are given in Table 1.
In general terms the classes of mortar may be used as follows:
Class I
Highly stressed masonry incorporating high-strength struc-tural units
such as might be used in multi-storey load-bearing buildings; reinforced
masonry.
Class II
Normal loadbearing applications, as well as parapets, balustrades, retaining
structures, and freestanding and garden walls, and other walls exposed
to possible severe dampness.
In practice, Class II mortars are used for most applications. Although
SANS 10249 : 1993 refers to a Class III mortar, it is so seldom used that
it has been omitted from Table 1.
Other proportions may be used if these can be shown by test to be satisfactory.
Mortar must not be used after it has started to set, which usually occurs
about two hours after it has been mixed. One man – particularly if
he is a weekend builder – can probably lay a little more than 60
bricks an hour. If you are working on your own or with one assistant,
it is better to mix a number of small batches as they are required than
to mix a one-bag batch.
Do not use too thick a layer of mortar between bricks or blocks; this
is wasteful and may lead to cracking.
3. Batching the materials
A builder’s wheelbarrow is a convenient measure for large batches;
the capacity is 65 l. Steel drums of 20 l or 25 l capacity and buckets
are useful for small batches. Check the capacity of drums and buckets
when filled to the brim as this is often more than the nominal capacity.
To batch, shovel material into the measure and then strike off level with
the brim.
4. Mixing
Mixing should be done on a clean hard surface such as a smooth concrete
floor or a steel sheet. Small batches may be mixed in a wheelbarrow provided
that the volume of the batch is no more than half the capacity of the
barrow.
Sand and cement, and lime if used, should be mixed until the colour of
the mix is uniform. Then add water in small quantities, mixing after each
addition, until the mix is soft and plastic.
5. Handling
If mortar is left in the sun before being used, it should be covered with
plastic sheeting or a wet sack. Discard mortar that has stiffened so much
that it is impossible to restore workability without adding more water.
6. Quantities of materials
Quantities of cement and sand required per 50 kg bag of cement or to produce
1 m 3 of mortar are given in Table 1. Quantities required for blocklaying
depend on block size and are outside the scope of this leaflet (See C&CI
leaflet Quantities for ordering building materials.)
The addition of lime is optional. A maximum of 40 l is permitted per 50
kg bag of cement. Mix proportions do not need to be adjusted. Only yield
will increase by 5 %.
Do not use lime with masonry cement.
Note that quantities in the table are approximate and do not allow for
wastage, which could typically range from 10 - 20 %.
Cement &
Concrete Institute
PO Box 168, Halfway House, 1685
Tel (011) 315-0300 Fax (011) 315-0584 e-mail info@cnci.org.za website
http://www.cnci.org.za
Published by the Cement & Concrete Institute, Midrand, 1997, reprinted
1999, 2001, 2003, 2006.
©Cement & Concrete Institute