Everything about white cement

  WTA czy WTF?

The WTA, the general German recommendations and parameters that are to be characterized by plasters, as well as renovations, have become the determinant of the renovation and preservation of monuments for the last three decades. The same recommendations or guidelines have been developed in Germany and can be accepted with certainty that they relate to the old plaster and mortar that were used in central and western Germany, as well as in areas west of the Rhine valley (today in northern France, The Netherlands or Belgium).

One of the most important parameters characterizing these plasters is the resistance to salts: NaSO4, NaCl and NaNO3. It is even advisable to prepare the right water containing all three dissolved substances in the correct amount and test their effect on the given plaster sample, of course standardized and so on.

In order to understand where these salts could have taken in the plasters, one should move south to ancient Italy, and more precisely to the Romanum Empire. It was here that something called Roman concrete was invented. Its composition was about the following: ca. 45% CaO (lime roasted, finely ground),ca. 35% SiO2, ca. 10% Al2O3 (studies ranged from a few to slightly over 10%) and other compounds (iron oxide predominated in principle Fe2O3). This is the first clear, in the limestone plaster era - the lime roasted was not uncommon, but the second and next? Well - it was and is roughly the chemical composition of the ashes and volcanic dust from the area of Pouzzola in Italy, and the name of the pucolate to this day means a binding compound in the presence of water.

By mixing these ingredients in the appropriate quantities after adding water there was a rapid reaction, not only producing enormous amounts of heat, but also significantly increasing the volume of the mixture. Of course, former specialists mastered the way the reaction was much more calm and predictable. There are suspicions that sea water containing dissolved salt was added or simply in the absence of ordinary salt water.

The Rhine Valley is the place where volcanic tuffs are used today as a supplement to restorative plasters to limit the appearance of efflorescence. In the old days, they provided the hydraulic strength of the limestone mixture and its resistance to adverse weather conditions.

Former masters of plastering the tuffs, that is, dust and volcanic ash, added to slaked lime. These tufts consist of a compound called ettringit having the following chemical formula: Ca6Al2(SO4)3(OH)12•26H2O. It is quite readily soluble in water on the one hand and on the other hand under the influence of temperature (if the water evaporates) relatively easily dissolves on CaSO4 and other sulphates. One of the parameters determining its quality is the sulfate content of the cement. Under the influence of moisture begin to behave similarly to yeast, increasing volume and if the mortar is weaker (and the mortar never to the strongest did not belong) can destroy it. Lime cake used in the old days naturally filled the entire mixture a lot, which meant that there were places where these sulphates (originating in Germany from ettringite) had the possibility of expanding and not destroying the plaster.

The next two compounds:which means NaNO3 and NaCl, there are probably related to where saltwater is derived to calming the reaction itself. NaNO3 indicates rock salt, NaCl - salty water, although it could equally be water with salt obtained from another source (for example, brine). Sources of rock salt are so far in central Germany. These compounds are not only quite aggressive, they are also seen as rash on plasters that are unsightly and undesirable.

Can something be thrown away in the old technologies? Under no circumstances - they were effective both in Italy and in Germany. With one difference - in sunny Italy they were used without the addition of sand and almost certainly dust and volcanic ash differed in composition (the German ones contained ettringite). In addition, Italy is always described as "sunny" and northern Germany is famous for its moisture and fog. Problems with the use of salt and its release on plasters were much smaller in Italy than in Germany. That is why in Germany such recommendations for plaster were developed and not others, taking into account one more interesting thing. Dust and volcanic ash contain silicon compounds SiO2, Which as a microsilica are currently used as a water retentor (ie water storage tank for cement) and in a certain sense sealing concrete (if the microsilica is suitably finely ground).

Taking into account everything you can assume that plaster in former Germany had three apparently contradictory features: on the one hand they were aerated - it provided them with lime pie, which was the basis for the then plasters, and on the other had limited water absorption - The activity of very fine grains containing SiO2 was manifested, and for that they were hydraulic (compounds SiO2 and Al2O3).

There were no places where dust and volcanic ash could be found in the former lands of Poland. Massive limestone was obtained - there was no problem with it, as evidenced by the better or worse-preserved lime, for example, in the Cracow-Czestochowa Jura. The stone was removed from the local crowbar, but in no case was it tuff or volcanic ash. Interesting news on how distances were carried out gives a description of how and for what it was built in XIX century Cracow - basically all the quarries and places within a radius of 30-40 km, no more from the then city. It can therefore be assumed that local conditions have forced the way of obtaining hydraulic mortar. They made the use of such and not other methods. Having lime in abundance, both burnt and slaked, it was the basis for modification, not what is now called the Rhine trails. There is no reason to believe that other countries behave differently. The rules were everywhere the same..

Studying books and recommendations (lectures, textbooks), it is easy to see that the additions of ashes (derived from the burning of the right kind of tree, much less often and much later - coal) are exceptions. Much more often the methods are: "Take a piece of lime, a third part of crushed lime, a third of raw bricks, some of the oil, mix thoroughly ..." and so on. All have one thing: they use what is at hand, what is available and not expensive. The routes in the Rhine valley were both far and expensive, as the cost of transport at such a distance was very high.

The twentieth century is a cement which, by its very nature, is hydraulic, which has made the problem cease to exist. For lime it initially meant the role of plasticizer and added in such quantity that according to today's standards such concrete would be at most a strong mortar.

For us, here in Southern Poland, limestone mortars (also those modified with other ingredients) do not contain NaCl or NaNO3, or NaSO4, although sulphate-based compounds contain cement, although the early 20th century had much less of it than today.

This in turn means that in fact the plasters used in Poland, even in the present western Poland, but once belonging to Germany (Lower Silesia, Pomerania) really differ greatly in the parameters recommended by WTA.

In fact, we ourselves, taking into account the local parameters and methods used in the past, should develop these parameters. Of course we should also consider:

  • changing the quality of the brick and its production techniques
  • she specificity of local additives added to the mortar (eg ashes or slag from smoking in blast furnaces - Silesia, since its intensive industrialization)
  • possibility of applying wood ash (eg Podkarpacie region)
  • historical changes in plaster composition (appearance of cement)
  • various propaganda actions in the literature on construction news (eg cement brick), along with the plasters and mortars recommended by them
  • changes in cement quality

Literature on the subject is, the results of research by solid conservation companies and the same conservators should be. This will allow us to develop our own recommendations on what is in Poland, and most importantly it will allow us to adapt the restoration materials to real needs.

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