Getting Started - What is limecrete made from?

Limecrete has three main components, Limecrete binder, aggregate and screed fibres, the mix ratio is 3 to 1 (3:1) by volume, with 3 parts aggregate to 1 part lime and approximately 1 bag (0.9kg) of screed fibres per tonne of aggregate.

Limecrete Binder: The active ingredient that binds the limecrete together is Natural Hydraulic Lime, we use an NHL 5 for Limecrete as this creates a strong slab that cures relatively quickly and allows other work to continue. While the NHL 5 is not as vapour permeable as some of the softer limes (NHL 3.5 / NHL 2) the higher compressive strength and quicker set time makes it more suitable for the limecrete, the reduced vapour permeability does not affect the overall performance of the slab and will not create any risk for the adjoining masonry especially as it will be isolated by the expanded cork edge insulation.

Aggregate: As standard we supply a recycled aggregate for the limecrete that is processed from reclaimed building materials to reduce the use of quarried aggregates, this has a maximum grain size of 4-5mm and has suitable grading curve for use in a screed. However, the aggregate used for the limecrete can vary from a sharp sand (4mm down to dust) to a conventional concreting ballast (20mm down to dust) depending on the preference of the installer.

Lightweight Aggregate: As an alternative to a standard limecrete we supply a lightweight aggregate (pumice) which provides more insulation than a standard screed. This can be used when underfloor heating is to be installed in a separate screed, or for when electric underfloor heating (UFH) will be installed on top of the slab below the floor finish, as the additional insulation helps to direct the heat up into the room above. We would not recommend the pumice aggregate as the material when UFH pipes are being installed in the slab as the higher thermal resistance will significantly increase response times for the floor and decrease the efficiency of the heating system.

Screed Fibres: Screed fibres are mixed into the limecrete to provide additional flexural strength and to help control shrinkage as the limecrete dries, we would recommend that the screed fibres are always included in the limecrete mix.

Water: The quantity of water required to create a workable mix will vary based on the moisture content of the aggregate, this can vary significantly even within the same bag especially if the bags are stored outside. Water will settle towards the bottom resulting in a higher moisture content towards the bottom of the bag.

A guideline for water content is to use around half the volume of lime, this should be added cautiously to ensure the mix does not become too wet and more or less may be required. Please note that the limecrete will become more fluid as it mixes so adding too much water can result in a very loose mix after the required 15 minutes of mixing.