Talk:Salts in building materials: Difference between revisions
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=== ''Cement mortar'' === | === ''Cement mortar'' === | ||
Cement binders may contain up to 1% of soluble alkalis. Beside other cement phases that contribute to increased strengths, these alkalis could be | Cement binders may contain up to 1% of soluble alkalis. Beside other cement phases that contribute to increased strengths, these alkalis could be responsible for the formation of salts. This is why cement is considered a risk in conservation. Even cements labeled low in alkalis (referring to [[DIN]], German Standards), may still contain up to 0.5% of soluble alkalis (see table). | ||
''The composition of standard cements according to DIN 1164 (in mass , %) (<bib id="Knoblauch.etal:1992"/>) | ''The composition of standard cements according to DIN 1164 (in mass , %) (<bib id="Knoblauch.etal:1992"/>) | ||
Revision as of 11:20, 18 August 2012
Authors:Hans-Jürgen Schwarz
back to Origin of Salts
Abstract[edit]
Building materials are often the main source of salt forming ions on objects. They occur in differing quantities, depending on their origin and the storage conditions of the object, but also depending on e.g. raw materials and firing atmosphere (bricks).
Natural stone[edit]
To date, little study has been carried out on the water-soluble constituents of natural stone. Their contents, however, are probably worth mentioning only in building stones, which are strongly affected by weathering or have been formed by erosion and weathering. This particularly applies to sedimentary rock such as sandstone.
Levels of salt forming ions in sandstone varieties from the Bern region, Switzerland (after [Blaeuer:1987]Title: Verwitterung der Berner Sandsteine
Author: Bläuer, Christine
) data in [mg/kg]
| Variety\ Ion | Cl- | NO3- | SO42- | Na+ | K+ | Ca2+ | Mg2+ |
| 1. | 13 | 5 | 85 | 40 | 68 | 536 | 84 |
| 2. | 12 | 2 | 46 | 33 | 94 | 492 | 137 |
| 3. | 7 | 1 | 9 | 26 | 70 | 410 | 86 |
| 4. | 9 | 1 | 8 | 30 | 73 | 504 | 85 |
| 5. | 8 | 11 | 16 | 40 | 384 | 130 |
Brick[edit]
Bricks contain differing levels of salt forming ions, depending on the raw material.
Some historic firing techniques play an additional role, but today’s firing technique, which uses natural gas, is almost certainly not going to contaminate the material.
Altogether, the contamination levels are clearly higher in brick, compared to natural stone.
An indication for contamination may be a white veil appearing directly after finishing the brick facing of a façade. At times the veil disappears again, soon after it appeared.
Often a salt contamination takes place during the construction. On the one hand, this takes place when the moisture from fresh mortar soaks into the brick, on the other hand the veil may be caused by pointing techniques. The veil is later removed through the application of acid.
Mortars and plasters[edit]
Cement mortar[edit]
Cement binders may contain up to 1% of soluble alkalis. Beside other cement phases that contribute to increased strengths, these alkalis could be responsible for the formation of salts. This is why cement is considered a risk in conservation. Even cements labeled low in alkalis (referring to DIN, German Standards), may still contain up to 0.5% of soluble alkalis (see table).
The composition of standard cements according to DIN 1164 (in mass , %) ([Knoblauch.etal:1992]Title: Bauchemie
Author: Knoblauch, Harald; Schneider, Ulrich)
| Cement | Portland cement (OPC)- Clinker | Blast furnace cement (Hüttensand bzw. Ölschieferabbrand) + CaSO4 | Trass – pozzolan cements containing lava + CaSO4 | Max. SO3-content at a specific surface of | |
| Mass % | Mass % | Mass % | 200-4000 [cm²/g] | > 4000 [cm²/g] | |
| OPC | 100 incl. CaSO4 | - | - | 3,5 | 4,0 |
| Iron Portland cement | 65 | 35 | - | 3,5 | 4,0 |
| HOZ | 15-64 | 36-70 70-80 |
- - |
4,0 4,5 |
|
| TrZ | 60-80 | - | 40-20 | 3,5 | 4,0 |
| PÖZ | 65-90 | 35-10 | - | 3,5 | 4,0 |
| PÖZ | - Portland-oil shale-cement |
| OPC | - Ordinary Portland cement |
| EPZ | - Portland cement, iron |
| TrZ | - Trass cement |
| HOZ | - Blast furnace cement |
Limits for the composition of cements with low effective alkali content (low-alkali cement) ([Knoblauch.etal:1992]Title: Bauchemie
Author: Knoblauch, Harald; Schneider, Ulrich)
| Total alkali content in % (Na2O-equivalent) |
Slag sand in M % | |
| OPC | ≤ 0,60 | - |
| HOZ | ≤ 1,10 | ≥ 50 |
| HOZ | (≤ 2,00) | ≥ 65 |
Thus, 1 gram of Na2O may lead to 4.6 gram natron or 5.2 gram mirabilite. When considering the quantities of cement that have been used on structurally sensitive buildings, it becomes more conceivable that the appearance of salt forming ions is highly likely. For instance, in Königslutter Cathedral over 20.000 kg of cement were introduced into the building. Given the amount of water introduced with the cement mortar or slurry, consequential damages from salt efflorescence are easily explained.
Lime mortar[edit]
Lime mortars may contain varying proportions of magnesium, depending on regional differences. These dolomite lime mortars react with sulfate to form magnesium sulfates. This reaction is shown in the following, simplified equation:
CaMg(CO3)2+ SO42- → CaCO3 + MgSO4 aq + CO32-
Lime putty is substantially free of salt forming ions (except possibly Mg).
Magnesia binders[edit]
Magnesia binders (Sorel cement) consist of MgO and MgCl2 (but also magnesium sulfate) at a mass ratio of 2.0-3.5 : 1, excluding water. The water-soluble salt acts as a stimulator. In conjunction with the resulting magnesium hydroxide, the set products will be of the approximate composition:
MgCl2 5Mg(OH)2 • 8H2O or
MgCl2 • 3Mg(OH)2 • 8H2O
and MgCl2 • 2MgCO3•Mg(OH)2.
The final product may additionally contain MgO, Mg(OH)2 and MgCl2 aq . These magnesium salts are to a degree very hygroscopic, or may lead to further damaging salts, especially sulfates.
Gypsum plasters and mortars[edit]
Gypsum plasters and mortars are usually based on calcium sulfate dihydrate (CaSO4•2H2O). Due to their low resistance to weathering (very slightly water-soluble) they are typically used in the interior of buildings. Gypsum has been used in construction for as long as lime mortars. It was and still is used mainly as plaster, for plasterboards, flooring, for stuccowork and as an ingredient for fillers. However, in some regions (e.g. in Germany: Thuringia, Lüneburg) gypsum mortar has also been used for the construction of exterior walls. This mortar is more resistant to weathering, because it is mainly based on anhydrite (CaSO4), but when exposed to high moisture levels or rain, it transforms into calcium sulfate dihydrate. Due to cement mortars being used for repairs, some buildings have been completely destroyed. The leaching of gypsum and/or cement components brings upon the formation of ettringite and thaumasite (and other salts), which can burst walls as they expand.
Literature[edit]
There were no citations found in the article.
Weblinks[edit]
Magnesia binders, short history and uses in the construction industry: http://www.premierchemicals.com/corner/articles/cements.htm
Modellvorhaben: Optimierung und Erprobung dauerhafter Gipsmörtel für die Instandsetzung umweltgeschädigter national wertvoller Kulturgüter u. a. am Beispiel des Klosters Walkenried: http://www.dbu.de/projekt_18320/_db_1036.html
SLeithaeuser 12:40, 18 August 2012 (CEST)brought in the gypsum, is that o.k?