What Sand is Best for Lime Mortar?

Choosing the Right Aggregate

When mixing traditional lime mortar, it is easy to focus entirely on the binder. While selecting the correct strength of Natural Hydraulic Lime (NHL) or lime putty is crucial, the lime is only half of the equation.

The sand acts as the skeleton of your mortar. If you get the aggregate wrong, even the highest quality lime binder will fail. So, what sand should you be using for heritage pointing and building?

Why Soft Building Sand Fails

One of the most common mistakes made by general builders when transitioning to heritage work is using standard building sand (often called soft sand).

Building sand is typically used with modern cement. The grains of soft sand are very fine, smooth, and rounded. Furthermore, soft sand often contains a high proportion of clay. When mixed with lime, these smooth, clay-heavy particles require more water to achieve a workable mix. As that excess water evaporates, the mortar shrinks significantly, leading to severe cracking and a weak, powdery joint that will ultimately fail and fall out of the masonry.

The Solution: Sharp, Well-Graded Sand

For a successful lime mortar mix you must use a sharp, well-graded, washed sand.

• Sharp (Angular): Unlike the smooth spheres of soft sand, sharp sand has rough, angular edges. These jagged edges interlock with one another, providing immense structural strength and preventing the mortar from slumping out of the joint.
• Well-Graded: “Graded” means the sand contains a varied mixture of particle sizes, from tiny grains up to 2mm or 3mm grits. The smaller particles fill the voids between the larger ones, creating a dense but breathable matrix.
• Washed: The sand must be washed to remove any silt or clay impurities that would interfere with the lime’s ability to bond.

A well-graded sharp sand ensures that the mortar remains highly breathable, allowing water vapour to easily escape the building fabric without causing the bricks to flake or crumble.

Matching the Sand to the Historic Fabric

When repointing a historic building, the sand you choose dictates the final colour and texture of the mortar joint. Traditional mortars were mixed using local sands, giving period properties their distinct regional characteristics.

By experimenting with different coloured sharp sands, or blending a coarse grit with a finer sharp sand, you can seamlessly match the existing historic pointing.

The Ready Mix Alternative

Sourcing consistently graded, high-quality sharp sand from local builder’s merchants can be incredibly frustrating. Moisture content and grading can vary wildly from bulk bag to bulk bag, making it difficult to maintain a consistent colour and strength across a large project.

To eliminate this variability, our Dry Ready Mix Lime Mortars are pre-blended using kiln-dried, expertly graded aggregates. Whether you are using our PureLime Mortar or our EcoMortar range, the aggregate has been precision-matched to the lime binder, ensuring a perfect, reliable mix simply by adding water.

If you are unsure which mortar specification is correct for your masonry, consult our Technical Data Sheets or get in touch with our conservation team for expert guidance.