What is leaching?
Leaching is the slow dissolving and washing out of substances that are bound in solid form within a building material. Rain, groundwater or aggressive liquids carry these substances out of the material and into the soil, groundwater or surface water. In Dutch it is called ‘uitlogen’.
Examples in construction
- Metals: zinc, copper and lead from gutters, downpipes, flashings and lamp posts. Every shower dissolves a little metal into the run-off.
- Treated timber: the preservative salts in pressure-treated wood (mainly copper compounds today, plus chromium and arsenic in older timber) can partly wash out when wet.
- Sealants: plasticisers and oily components migrate out of joint sealants and leave stains on natural stone and facades.
- Concrete: in livestock buildings, acids and salts from manure and silage effluent attack the cement paste. Lime dissolves, the surface roughens and in time the reinforcement can become exposed.
What affects it?
The more water a material absorbs or the more water flows over it, the faster it leaches. Acidity matters too: acidic water dissolves metals and lime faster. Dense, well-cured concrete therefore releases less than a porous material.
Leaching tests
Leaching tests show how much a material can release at most. The usual ones are a column test for granular materials such as recycled aggregate or soil, and a diffusion or tank test for shaped products such as blocks and slabs. In the Netherlands the results are checked against the emission limits for building materials in the Soil Quality Decree (Besluit bodemkwaliteit). Which tests apply elsewhere depends on local regulations and the intended use. Contaminated material can be bound with cement and additives into an immobilised material that hardly leaches.
Application
Leaching matters when choosing roof and facade materials, reusing rubble and recycled aggregate, harvesting or infiltrating rainwater and building agricultural floors.
Tips and points of attention
- Rainwater harvesting: water from zinc, copper or lead surfaces contains dissolved metals. Avoid using it for a pond (copper is toxic to aquatic life) or the vegetable patch.
- Cut down on lead: for new flashings, a lead replacement flashing is often possible.
- Run-off staining: expect marks below copper and zinc details on light render and natural stone.
- Livestock floors: specify concrete with a suitable exposure class for chemical attack, or protect it with a coating.
Related terms
- Corrosion
- Concrete corrosion
- Pressure-treated wood
- Sheet lead
- Lead replacement flashing
- Immobilised material
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