Salts in building materials

Author:Hans-Jürgen Schwarz
English Translation by Sandra Leithäuser
back to Origin of Salts

Abstract

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

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

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

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 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 M%) ([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
EPZ	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 clear 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

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+ SO₄^2- → CaCO₃ + MgSO₄ aq + CO₃^2-

Lime putty is substantially free of salt forming ions (except possibly Mg).

Magnesia binders

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

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

[Blaeuer:1987]	Bläuer, Christine (1987): Verwitterung der Berner Sandsteine., Universität Bern, Url
[Knoblauch.etal:1992]	Knoblauch, Harald; Schneider, Ulrich (1992): Bauchemie, Werner Verlag

Weblinks

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

crystallization of a dissolved salt