Mixing lime correctly involves combining it with aggregate (usually sand) and water in specific proportions to create a usable mortar or plaster. The process ensures the lime is properly hydrated and achieves the desired consistency and workability.
Steps for Mixing Lime Mortar/Plaster:
Here's a step-by-step guide on how to mix lime, focusing on traditional lime mortars. Note that this is a general guideline; always refer to the specific instructions for the type of lime you are using (e.g., hydrated lime, non-hydraulic lime, or hydraulic lime) and the intended application.
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Prepare Your Mixing Area: Choose a clean, level surface. This could be a mixing trough, a wheelbarrow, or a cement mixer. Ensure you have access to clean water.
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Gather Your Materials: You'll need:
- Lime (appropriate type for your project)
- Aggregate (sand – use a well-graded sharp sand for best results)
- Clean water
- Mixing tools (shovel, trowel, or mixer)
- Gloves and eye protection
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Dry Mixing (Important for Even Distribution):
- Manual Mixing (Wheelbarrow/Trough): Add one part sand, followed by one part lime, followed by two parts sand. This layering helps with the initial mixing.
- Mechanical Mixing (Cement Mixer): Add one part sand, then one part lime, then two parts sand.
- Thoroughly mix the dry ingredients together for at least five minutes until you achieve a consistent color and texture. This ensures the lime and sand are evenly distributed before adding water.
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Adding Water Gradually:
- Slowly add water to the dry mix while continuously mixing.
- The key is to add water gradually to avoid over-saturating the mix.
- Mix until you achieve a workable consistency. It should be plastic and sticky, similar to peanut butter.
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Assessing the Consistency:
- The correct consistency is crucial. The mortar/plaster should hold its shape when a small amount is placed on a trowel. It shouldn't be too runny or too stiff.
- If it's too dry, add a small amount of water. If it's too wet, add more sand and lime in the same proportions (e.g., for every two shovels of overly wet mix, add roughly half a shovel of lime and a full shovel of sand). Mix thoroughly after each addition.
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Slaking (For Non-Hydraulic Lime, optional but beneficial): Traditionally, non-hydraulic lime putty is ideally slaked (allowed to sit and mature) for several weeks or months before use. This improves its workability and performance. While not always practical, allowing even a few days for the mixed lime to sit and hydrate can make a noticeable difference.
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Using the Mix: Use the lime mortar/plaster as soon as possible after mixing. If you need to store it, keep it in an airtight container to prevent it from drying out.
Key Considerations:
- Lime Type: Different types of lime require different mixing techniques and proportions. Always follow the manufacturer's instructions. Hydraulic lime will set even underwater, while non-hydraulic lime sets by carbonation with the air.
- Sand Quality: Use clean, sharp sand that is free of contaminants. The sand's particle size distribution influences the mortar's workability and strength.
- Water Quality: Use clean, potable water.
- Safety: Lime is caustic. Wear gloves and eye protection to prevent skin and eye irritation.
- Avoid Overwatering: It is much easier to add water than to remove it from an overly wet mix. Add water in small increments and mix thoroughly after each addition.
