What is trass?
Trass is finely ground volcanic tuff used as an addition to lime or cement mortar. It is a natural pozzolan: on its own trass does not set, but combined with lime and water it forms compounds that harden even under water. This turns an ordinary lime mortar into a hydraulic, water-resistant mortar. The main source in north-western Europe is the Eifel region in Germany, especially around the Brohl and Nette valleys, which is why it is also known as Rhenish trass.
How it works
Trass contains reactive volcanic glass rich in silica and alumina. This reacts with the calcium hydroxide from lime (or released when cement hydrates) to form calcium silicate hydrates, the same type of compounds that give concrete its strength. The results:
- the mortar becomes denser and less permeable to water;
- free lime is bound, so there is less lime efflorescence;
- the mortar stays relatively flexible and less brittle than a pure cement mortar.
Uses
- Hydraulic works: historically used for locks, quay walls, bridges and foundations in or near water. Rhenish trass was shipped down the Rhine to the Netherlands for this kind of work.
- Base courses: the lowest courses of a wall laid in trass mortar (Dutch trasraam), so that ground moisture rises less far into the wall.
- Natural stone: trass mortars and trass cement are used for bedding and pointing natural stone because they are less prone to staining and efflorescence.
- Restoration: for historic masonry where a hard cement mortar would cause damage.
Practical tips
- For natural stone, use the mortar recommended by the stone supplier; trass mortar is often suitable but not for every stone type.
- Trass makes mortar set more slowly than pure cement mortar. Protect fresh work from frost and drying out.
- On historic buildings, use a mortar matched to the original mix; if in doubt, have a mortar analysis done.
Related terms
- Pozzolan
- Hydraulic
- Lime mortar
- Natural hydraulic lime
- Damp-proof course
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