Roundtower Trass is a natural pozzolanic material derived from volcanic deposits in the Rhine Valley. It has been used for centuries as an addition to lime mortars and renders, particularly where improved resistance to water and weathering is required while maintaining good vapour permeability.
When added to lime, Trass reacts with the lime binder through a pozzolanic reaction. This reduces the amount of free lime in the finished mortar and can improve its hydraulic properties, durability and resistance to water absorption without creating an excessively strong or impermeable material.
Trass is particularly useful in traditional building and conservation work where lime mortars need to provide increased resistance to driving rain, reduced capillary water absorption or improved durability while remaining compatible with vapour-open masonry.
Key Benefits
- Natural pozzolan – derived from volcanic deposits and traditionally used with lime.
- Reduced water absorption – lowers capillary water absorption in lime mortars and renders.
- Good vapour permeability – helps maintain the moisture-managing properties of lime-based construction.
- Reduced free lime – modifies the characteristics of the lime binder and reduces lime leaching.
- Reduced risk of efflorescence – can help reduce lime migration and surface staining.
- Improved durability – provides greater resistance to weathering and is particularly useful in exposed locations.
- Salt-resistant properties – suitable for applications where resistance to salt-laden environments is required.
- Compatible with lime binders – can be used with suitable hydrated lime, lime putty and Natural Hydraulic Lime mortars.
Uses
Roundtower Trass can be added to lime mortars, renders and plasters where reduced water absorption and improved durability are required.
Typical applications include:
- Traditional lime mortars
- External lime renders
- Internal lime plasters
- Exposed masonry
- Areas subject to wind-driven rain
- Conservation and restoration work
- Masonry in salt-laden environments
The appropriate addition rate will depend on the lime binder, aggregate and intended application. As a general guide, Trass is commonly added at 10–20% by volume of the binder. Mix design should always be assessed according to the requirements of the individual project.
Technical Information
| Property | Specification |
|---|---|
| Product Type | Natural pozzolanic material |
| Origin | Volcanic deposits, Rhine Valley |
| Colour | Natural brown/creamy beige |
| Bulk Density | Approx. 1000 kg/m³ |
| Pozzolanic Activity | Medium |
| Typical Addition Rate | 10–20% by volume of binder |
| Application | Lime mortars, renders and plasters |
Traditional Use
Trass has a long history of use in lime construction throughout northern Europe. Its use is particularly valuable where a lime-based material needs greater resistance to moisture and weathering without losing the vapour permeability that makes lime suitable for traditional masonry.
As with all lime mortar mixes, the binder, aggregate, Trass content and application should be selected to suit the existing masonry and the specific requirements of the project.
