Characteristics of Good Mortar
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.
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.