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Various grades of mortar such as M1, M2 etc. are used in brickwork construction. Lime mortar and cement mortar are two types of mortar generally used for brickwork. The properties, strength and uses of these mortars under various circumstances should be known.
Points to Remember for Mortar in Brickwork Construction
1. The strength of brickwork does not depend on the grade of the mortar used, i.e. various mortar mixes of different grades such as M1 and M2 have different strengths, but it does not affect the strength of brickwork. For example, mortars of mix ratios 1:6 and 1:4 give the same strength of brickwork with the same type of bricks, although these have different strengths. That means, the strength of brickwork depends on the strength of bricks.
2. When the mortar mix ratio of 1:3 is used for cement to sand or (cement + lime) to sand ratio, provides a dense mortar with fewer voids.
3. Advantages of Lime Mortar – Even though the strength of lime mortar is less than cement mortar, the advantages of adding lime in mortar are as follows:
- Shrinkage in mortar is less, thus less liable to cracks due to shrinkage.
- Lime increases the workability and plasticity of the mortar mix.
- Lime has a good water retention capacity and does not evaporate quickly. Also, dry bricks are not able to suck water from the lime mortar.
- Lime increases the volume of mortar and fills the voids thus making it water resistant. So, lime mortar provides more water tightness and resistance against rain penetration.
- Bonding of lime mortar with bricks is better.
- Cement-lime mortar is more flexible and can accommodate the normal movements of brick masonry without cracking. Thus, cement lime mortar, in general, is less liable to cracking than cement mortar.
4. Lime mortar gains strength slowly and has lower ultimate strength than cement mortar. Again, lime mortar having hydraulic lime attains better and early strength. Lime mortar using fat lime don’t harden at all in wet locations.
Properties of semi-hydraulic lime are intermediate between those of hydraulic and fat lime mortars. When using fat lime, it is necessary to use some pozzolanic materials such as burnt clay in place of sand to improve the strength of mortar.
5. Cement-lime mortar of leaner mixes from ratio 1:4 to 1:8 tend to be harsh particularly if sand is coarse and not graded. Hence, plasticizers are recommended to be used to improve the workability and plasticity of the mortar.
6. Following factors affect the strength of cement mortar for the same ratio of cement and sand:
- Grading of sand
- Fineness and coarseness of sand
- Angularity and roundness of the sand particles
The plasticity property of cement mortar mix also varies with the fineness of the sand for the same ratio of cement and sand. Plasticity of cement mortar can also be increased by increasing the quantity of cement added, but this will make mortar more uneconomical.
The quantity of water to be added should be just enough to give sufficient workability to the mix which should also vary based on the above three factors. In case, the above three factors are such which leads to the use of less water, then the strength of the mortar will be more.
7. When fineness of sand increases, the workability of the cement-mortar mix increases, but also increases the surface area of the sand for which requirement increases for cement and water quantity for the same strength. If the cement quantity is not increased, then strength will be less. More water will be required to achieve the desired workability. This condition increases the water-cement ratio, therefore reducing the strength.
8. Curing is absolutely necessary to achieve the maximum strength and to get the maximum coating of the available cement around sand particles.
9. Mortar richer than a mix ratio of 1:3 is not used in brickwork masonry construction because of high shrinkage and no appreciable gain in strength of masonry, although the strength of mortar itself increases.
With the use of strong mortar, cracks will be fewer and wider, while with weak mortar, cracking will be distributed as thin hair cracks. Stresses due to differential movement of masonry due to expansion, contraction etc. are also reduced by using weak mortar, because weak mortar can easily accommodate the movements.
Hence, when strong mortar is not necessary from consideration of strength, weak mortar should be used. As lean mortar of just cement and sand is harsh, pervious, less workable and less plastic, it is preferable to use composite mortar of cement, lime and sand.
10. Lime-based mortars such as cement-lime mortar or lime mortar give higher strength of brickwork for the same mortar strength compared to cement mortar.
For example, Cement-sand ratio of 1:6 has mortar strength of 30 kg/cm2 and brickwork strength of 5.5 kg/cm2, while cement-lime and the sand-mix ratio of 1:1:6 has mortar strength of 30 kg/cm2 has higher brickwork strength of 7 kg/cm2.
11. The common defects in mortar during brickwork construction are found to be:
- Improper mixing
- Excessive water content
- Unduly thick bed
- High suction of brick and less water retentivity of mortar
- Uneven joints
- Voids in vertical joints
- Disturbance of bricks just after laying
12. Overly thick joints reduce the strength of the brickwork.
13. Addition of pozzolana increases the strength of the mortar and makes it more resistant to chemical attacks.