How to Mix Hydraulic Lime

‍Choosing the Right Equipment

For smaller projects, a conventional cement mixer can suffice for mixing hydraulic lime plasters. However, if you’re tackling larger endeavours, such as extensive masonry work, it's advisable to use a roller-pan or paddle mixer. Drum mixers can be prone to balling, which can compromise the quality of your mix. The equipment you choose can significantly impact the consistency and effectiveness of your hydraulic lime mortar. 

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Accurate Measurement

Accurate measurement of materials is paramount when mixing hydraulic lime. Using a gauging box or bucket to measure materials by volume, as measured by shovel can lead to inconsistent quantities. This precision ensures that your mortar mic remains consistent throughout the project.

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Mixing Procedure

Follow these steps to mix hydraulic lime effectively:

• To an empty mixer add one part sand.
• Follow with one part lime.
• Then, add two parts of sand.
• Mix the dry ingredients for at least five minutes. 
• Gradually add water until you achieve the desired consistency, being cautious not to overwater the mix.
• Once the desired consistency is reached, continue mixing for another twenty minutes. The mortar will initially appear dry, but as mixing time increases, it will become ‘fattier.’ Avoid using plasticisers, as they can compromise the mix. 

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Water Content Matters

The amount of water you add directly affects the strength and durability of your mortar. Excessive water weakens the final result, while too little water prevents crucial chemical processes from occurring. Therefore, add water sparingly until you reach a usable consistency. Keep in mind that the masonry background can influence mortar strength. Dry backgrounds may absorb moisture from the fresh mortar, so dampen the background before applying mortar to maintain control over the drying process.

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Storage Considerations

Proper storage of your materials is essential to ensure their effectiveness:

• Natural hydraulic limes are typically supplied in water-resistant paper bags. Keep these bags dry, as moisture can irreversibly damage them. Once opened, fold over the top of the bag and store it in a dry area. Lime can remain usable in this state for a few days, but after that, it should be discarded.
• Aggregates should also be covered when not in use. Exposure to the elements can wash out fines and cause the material to separate over time. Protect your aggregates to maintain their quality. 

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By following these steps and tips, you can effectively mix hydraulic lime for your construction projects. Remember that precision, the right equipment, and proper storage are key to achieving strong, durable mortar that will stand the test of time. Whether you’re working on historical restorations or new construction, mastering the art of mixing hydraulic lime is a valuable skill for any builder or conservationist.

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