A conventional cement mixer can be used to mix hydraulic lime plasters. However, for larger projects a roller-pan or paddle-mixer is preferable. Lime mortars mixed in drum mixers are prone to balling. The following mixing procedure can go some way to reducing this problem.
It is vital to ensure that when measuring materials this is done by volume. A gauging box or bucket will be necessary for this task. Measuring by shovel is not acceptable as quantities will be inconsistent.
1. Start with an empty mixer.
2. Add one part sand
3. Followed by one part lime
4. Followed by two parts sand.
5. Mix dry for at least five minutes.
6. After five minutes slowly add water until the desired consistency is reached, it is very important not to drown the mix by adding too much water.
7. Once the desired consistency is reached mix for a further twenty minutes. (The above example is based on a mix ratio of 1:3). The mix, to begin with, will appear dry but as mixing time increases the mortar will become much ‘fattier’. If too much water is added the risk of shrinkage will increase and the final strength reduced. Do not use any plasticisers.
The addition of water should be considered carefully, as it will directly affect the ultimate strength and durability of a mortar. The more water introduced into the mortar mix, the weaker will be the final result. However, too little water will prevent the chemical processes taking place and weaken the material. Generally, water should be added sparingly, until a useable consistency is achieved.
The masonry background may also affect mortar strength. Dry backgrounds can quickly ‘suck’ moisture from newly applied mortar. This should be controlled by dampening down the background prior to mortar application.
Storage of Limes & Aggregates
NATURAL HYDRAULIC LIMES are supplied in water resistant paper bags. If the bags are allowed to get wet they may be irreversibly damaged. Also, once opened the exposure to air will start to weaken the hydraulic set. As a result any part opened bags left at the end of the day should be carefully folded over at the top and put into a dry store. In this state the lime will remain useable for a further 2 or 3 days. Thereafter it should be discarded.
AGGREGATES should also be covered. If left exposed fines can be washed out and the material as a whole can gradually separate.