The main properties of hardened mortar are strength, development of good bond with building units, resistance to weathering and mixing mobility of green mortars, pliability and water retention. In addition, the mortar should be cheap and durable and should not affect the durability of the building units in contact. The joints made of mortar should not have cracks.
The strength of the masonry depends on both the mortar and the construction unit (brick, stone or block). A very strong mortar will be of little use with weak construction units. It is also important to consider whether full power is required within a short period of time. In cold weather, when the strength of lime or cement admixtures develops slowly, this is likely to influence the choice of mix.
Strong cement mortars are most likely to cause shrinkage cracks, and should therefore be avoided, except where high strength is an essential requirement. On the other hand, the use of a very weak mortar, say, 1:10 cement mortar is not satisfactory because the reduction in cement content leads to lower workability, less cohesion and will produce porous joints of less frost resistance. The strength of the hardened mortar depends on the activity of the binding material, the water-cement ratio, the consumption of the binding material and the quality of the sand. it is found that:
- The density and strength of mortars composed of the same class of aggregates decrease with an increase in the proportion of fine aggregates.
- When finer sand is used, it requires almost twice as much cement to produce a mortar of the given strength.
- When the percentage of water required to form a mixable mixture increases, the density and strength of the mortar decrease. The proportional effect is greatest at a young age.
- Even small percentages of mica present significantly reduce tensile strength and adversely affect compressive strength.
- Replacing less than 25 percent of cement with hydrated lime results in loss of compressive strength.
- Cement lime mortars are helpful in autogenous healing of cracks.
2. resistance to rain penetration
Mortar for plastering should protect masonry joints and units by forming an impervious sheet. A satisfactory bond must be ensured between the construction units, mortar and plaster.
3. mobility and ease
The term mobility is used to indicate the consistency of mortar. Placeability is the ease with which a mortar mix can be applied to a surface in a thin and uniform layer with minimal cost. Mortar can have a consistency ranging from hard to liquid, depending on its composition. Mortars are designed to be mobile enough for masonry, finish and other works. The mobility of the mortar mixture determines its consistency. Mortars prepared from Portland cement alone often lack cement paste, are rigid and non-placeable and often have added plasticizers.
4. water retention
It is characterized by the ability of the mortar to not stratify during transportation and to retain sufficient moisture in a thin layer spread over the porous bed. Mortar mixture of low water retention will show defects after hardening. Mortar can lose so much water that the balance may be insufficient for its hardening and required strength. Mineral and organic plasticizing agents may be added to enhance water retention.