Preparation of lime mortar and cement mortar have different procedures, which are discussed in subsequent paragraphs.
7.8.1 Preparation of Lime Mortar
Ordinary lime mortar and plaster are made by grinding lime with sand. There are three methods of preparing lime mortar.
In the first method, a dry mixture of lime and sand are placed in a trough or on a platform in the specified proportion. The required quantity of water is sprinkled over the mixture in stages and mixed each time after the water is sprinkled. Then the mixture is ground to a plastic mixture by pounding with heavy wooden hammers.
The second method is adopted when a large quantity of mortar is needed. A circular annular trough is made and the mixture is placed into it. The mixture is ground by rotating a wheel through the trough with the help of bullocks. This type of mortar production is called a bullock-driven mortar mill. The mixture is ground for about 200 revolutions to get a suitable mortar.
The third method is the modern method, which is used for preparing large quantities of lime mortar. Here, a machine-driven pug or mortar mill is used. For brickwork, the grinding is done for three minutes and longer for plaster for plasterwork, pointing and terrace works. While the construction work is in progress, the mortar is raked up continuously.
Ordinary lime mortar can be stored for a maximum period of 3 days after mixing. Hydraulic lime should be used within 2–4 hours after grinding. The lime mortar should be kept moist by covering it with a wet sack cloth and frequently moistened.
7.8.2 Preparation of Cement Mortar
Cement mortar is mixed by hand for small construction purposes, while large quantities of cement mortar may be mixed by a machine.
1. Hand Mixing
Sand is measured in boxes, and the specified quantity of sand is spread first. Cement is measured by the weight of bags or by considering the volume of 0.035 m3. Cement is spread over the sand. Then they are dry mixed by turning over and over and backward and forward using a shovel or a spade till the mixture attains a uniform colour.
The quantity of mix that has to be used within 30 minutes is separated and mixed again and formed into a heap. After making a small depression at the top, water is added to the centre of the depression and mixed well. Adequate water must be added, as excess water may draw the cement out. The water required is roughly about 70% of the weight of cement for 1:3 mortar. The whole mass is mixed thoroughly for 10–15 minutes by hand mixing.
2. Machine Mixing
Machine mixing of cement mortar is done in a concrete mixer. The sand and then the cement are put into the mixer. Initially, it is dry mixed and then water is gradually added. Wet mixing is then continued for more than a minute till the mixture is brought to a plastic consistency.
Cement mortar has to be used before it begins to set. The entire cement mortar has to be used within a maximum time of 2 hours. While using the cement mortar, water may be added to the mortar when the mortar gets stiffened. This process may be continued to restore the necessary consistency. Such retempering may be allowed only up to a period of 2 hours, beyond which the mortar should not be used.
7.8.3 Preparation of Lime–Cement Mortar
In this case, it should be ensured that the lime is slaked well. Then the lime is mixed with sand and ground well to form an intimate mixture. Water is then added and mixed well to form a regular lime mortar. The lime mortar is transferred to a mechanical mixer to which the required cement is added and mixed with additional water. Mixing is continued for 3–5 minutes to get the required consistency. This mortar can be used similarly as that of cement mortar within 30 minutes.
Leave a Reply