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Sand Selection
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Selecting a Sand
Sand and larger sized aggregates make up the larger proportion of
most mortars. Colour, texture and overall strength are all strongly
affected by the choice of aggregate.
The aggregates most commonly used with hydraulic lime are sand and
grit, although for the purpose of matching historic mortars various
impurities may have to be added. A good sand should be a washed
sharp sand with angular grains to ensure good bonding qualities.
Soft building sands should be avoided as their rounded grain shape
can result in excessive shrinkage.
Sands used should be well graded with a range of grain sizes, which
for most plaster, render and mortar work will range from 5mm down to
75 micron. Larger sized aggregates may be used in some mortar or
pointing work. As a rule of thumb for pointing, the maximum size of
aggregate should be no bigger than one third of the joint width.
Sands, which contain a clay or silt content of more than 4% should
be avoided, as these will inhibit the contact between lime binder
and aggregate.Sands which have a high fines content
also be avoided as the larger surface area of these will require
more water to be used in the mixing. This higher water content will
induce shrinkage and can affect flexural and compressive strengths.
Monogranular sands should be avoided as they will possess poor
workability qualities and will inhibit good vapour exchange i.e. the
ability to breath.
Typical sand Grading Examples
General mortar work
% retained on sieve:
5.00mm 5%
2.36mm 8%
1.18mm 16%
600 micron 25%
300micron 31%
150micron 12%
150micron 3%
Sand Void Percentage
Once you have identified a good local sand, it is worthwhile to
establish its void percentage. Knowing the voids in the sand tells
how much binder is required in order to fill the voids. Insufficient
binder to fill the voids will weaken the material and make it
vulnerable to frost damage.
The procedure is simple:
- Take a container of known volume (a two litre jug or jar is
ideal) and fill it level to the top with the selected sand.
- Remove the sand and dry it completely in an oven on a
tray.
- Replace it in the container to a level top.
- Take a measured jug of water and gradually add the water
until bubbles stop rising and the water has saturated the sand.
- The void ratio can then be calculated from the amount of water
used.
If for example the amount of water used was approx 30% of the
total volume then a mix ratio of 1 part lime to 3 parts sand (by
volume) would fill the voids.
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Sand Selection |
Mortar Mixing | Pointing
| External Rendering |
| Internal Plastering |
Harling/Roughcasting | Limewashing
| Limecrete |
Aftercare |
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