The chief properties of hardened mortar are strength, development of
good bond with building units, resistance to weathering and those of
green mortar mixes are mobility, placability and water retention. In
addition, the mortar should be cheap and durable and should not affect
the durability of building units in contact. The joints made with mortar
should not develop cracks.
1. Strength
The strength of masonry depends upon both the mortar and the building
unit (brick, stone or block). A very strong mortar with weak building
units will be of little use. It is also important to consider whether
full strength is required within a short time. In cold weather, when the
strength of lime or cement mixes develops slowly, this is likely to
affect the choice of mix.
Strong cement mortars are most likely to lead to shrinkage cracks,
and should, therefore be avoided except where high strength is an
essential requirement. On the other hand the use of much weaker mortar
say, 1:10 cement mortar is not satisfactory since reduction in cement
content leads to less workability, less cohesion and will produce porous
joints of low frost resistance. Strength of hardened mortar depends on
the activity of binding materials, the water-cement ratio, consumption
of binding material and the quality of sand. It has been found that:
- The density and strength of mortars made of the same class of aggregate decrease as the proportion of fine aggregate is increased.
- It requires about twice as much cement to produce a mortar of given strength when fine sand is used as it does with coarse sand.
- When the percentage of mixing water is increased beyond that required to form a placeable mix, the density and strength of mortar reduces. The proportionate effect is greatest at the early ages.
- Even small percentage of mica if present considerably lowers the tensile strength and adversely affects the compressive strength.
- There is a loss of compressive strength by the replacement of less than 25 per cent of cement by hydrated lime.
- Cement lime mortars are helpful in autogenous healing of cracks.
2. Resistance To Penetration Of Rain
The mortar for plastering should protect the masonry joints and units
by forming an impermeable sheet. A satisfactory bond between the
building units, mortar and plaster should be ensured.
3. Mobility & Placability
The term mobility is used to indicate the consistency of mortar. The
placeability is the ease with which the mortar mix can be applied with a
minimum cost in a thin and uniform layer on the surface. Depending on
its composition a mortar may have a consistency ranging from stiff to
fluid. Mortars for masonry, finishes and other works are made
sufficiently mobile. The mobility of mortar mix determines its
placability. Mortars prepared from Portland cement alone are frequently
deficient in cement paste, stiff and non-placeable and often
plasticizers are added.
4. Water Retention
It is characterized by the ability of mortar not to stratify during
transportation and to retain adequate humidity in a thin layer spread
over a porous bed. A mortar mix of low water retention will show the
defects after hardening. Mortar may lose so much water that the amount
left may be insufficient for its hardening and required strength.
Mineral and organic plasticizing agents may be added to enhance water
retention.