In renovation renders with Poraver® as a lightweight aggregate, salt crystallization takes place within the air pores of the expanded glass granules.
Unlike classic renovation renders that work with air-entraining agents, considerable improvements can be achieved by using renovation render formulations with Poraver: Poraver gives the renders a low density with a stable air-entrained matrix, shortens the mixing time and significantly improves workability.
Efflorescence in construction
Renovation renders are used for the renovation of damp and humid walls. The moisture causes efflorescence and salt crystallization, which, in turn, can result in structural damage. Both existing and new buildings are affected. Efflorescence can be as a result of structural deficiencies or unsealed basement walls, for example. The renovation of these buildings first needs proper sealing against the damp and removal of the salt in the walls without generating further damage. To achieve this, one possibility is the use of renovation renders.
The problem of efflorescence can have many causes, as well as manifestations. Efflorescence can occur on renders, pavements, cement blocks, bricks etc. Most commonly, efflorescence is formed by carbonation of lime (Ca(OH)2) or salts (sodium chloride, sulfate or nitrate). These materials can be either part of the material’s formulation, the hardening reaction or can be absorbed from the environment in combination with moisture. In addition to the visual problems on a render surface, such as the efflorescence or hygroscopic spots already mentioned, the continuous crystallization of salt will lead to a delamination of paint and the gradual destruction of the render caused by the increased stress of the building material structure during frost.
Renovation of efflorescence
In the case of efflorescence caused by the carbonation of lime Ca(OH)2 the issue can be solved by adding pozzolanic co-binders or by using aluminate cements to catch, or even avoid the formation of portlandite.
Efflorescence based on salt disposal in combination with a leakage needs a different approach. Simply sealing with a waterproofing layer of the wet area and renewal of the render will not solve the problem, as water will just rise higher due to the capillary effect and the same damage will occur at the next level. The renovation salt-contaminated brick work or walls has to be done in two steps:
- Stopping of the water supply
- Drying out of the wall and allowing a crystallization of the salt without damaging the new render layer.
Requirements and formulation of renovation renders
Mortar systems that support this out drying and crystallization are described in “WTA Merkblatt 2-9-04/D” in addition to EN 998 “Specification for mortar for masonry. Rendering and plastering mortar”. The special design of the render formulation will allow the water to evaporate easily and the salt will crystalize in the pore system of the renovation render without damaging the binder matrix. To achieve their special properties, renovation renders have to fulfil the following requirements:
- Air content: > 25 vol.-%
- Dry density: < 1.400 kg/m3
- Capillary water absorption (24h, disc): > 0.3 kg/m2
- Water intake: < 5 mm
- Water vapor coefficient (m-value): < 12
- Porosity: > 40 vol.-%
- Salt resistance: passed
A stable pore volume and high salt storage capacity with Poraver®
One challenge of the render’s formulation is to achieve a high volume and stable pore system and a balanced hydrophobicity. The pores have to be stable during mixing and application, but also need to be relatively large in size to stop any capillary action and to have enough space for the salt to crystallize. Typically, pores in renders are ‘generated’ by using air entraining agents but the disadvantage of air entraining agents is their dependency on mixing time and speed, as well as their interaction with hydrophobic agents, especially at high air content levels. One way to stabilize the air content in renovation render is to use Poraver® expanded glass to reduce density and to add a specific pore geometry.
In our research we wanted to find out if Poraver® granules are able to store salt crystals. In this case they would not only stabilize the render due to their low density, but also function as a source of reliable and stable pores, supporting the pores formed by air entraining agents. Prisms of the materials were prepared and infiltrated with salt according to ‘WTA Merkblatt 2-9-04/D’ for renovation mortar systems. From those prisms small samples were cut approx. 0.5 cm below the surface with efflorescence. The samples were freed from dust by means of compressed air and sputtered with gold.
SEM (scanning electron microscope) pictures of different areas were prepared and the chemical composition at different points were determined by EDX (energy dispersive X-ray spectroscopy) measurements. Those pictures show particles inside the pores of Poraver®. EDX diagrams show, in addition to the peaks formed by Poraver® (Al, Si, Mg, O, Na), a strong peak of chlorine and an increased peak for sodium. At another area, sulfur from Na2SO4 could be detected. So crystals of both salts (NaCl and Na2SO4) that were used for the infiltration could be detected inside the Poraver® granules. Comparative tests on air voids in the renders without Poraver® in the matrix showed similar results.
Salt cannot only crystallize in air pores of a renovation render, but also in the pores of a lightweight aggregate as long as the pore diameter is big enough and the pore system is open to the surface. Poraver® does not only stabilize the air pores inside the render by reducing the density, but also acts as active pore volume that allows salt to crystallize without damaging the render. The main advantage of the stable Poraver® pore volume is, that it is not dependent mixing times and application. In this way Poraver® is a hassle-free formulation and provides a stable and proven product quality not reliant on ‘perfect’ site conditions.