New façade for the National Museum in Prague
Prague - Czech Republic
When renovating historic buildings, planners like to use products that resemble the historical originals. In some countries, such as Germany, there are rules for the protection of historical monuments, and these will invariably require close similarity of appearance. During the restoration of the historical façade of the National Museum in Prague, special MC products were used which fully meet the exacting demands of heritage preservation.
The reconstruction of the façades of the National Museum in Prague has been underway since 2014. After the damaged parts of the render had been removed, 35 sample areas were tested with different masonry mortars in different colours. At the end of this extensive trial phase, the client opted for Oxal SM TK 5, a special trass-bearing mortar from MC-Bauchemie for natural stone restoration. The mortar impressed both technically and optically by virtue of its natural ochre-toned sand colour.
Impressive mortar credentials
For the National Museum, MC supplied the special mortar Oxal SM TK 5 in the colour R3, referred to for short as Oxal SM TK 5 R3. It has a maximum grain size of 4 mm, but can also be mixed with additional aggregate to raise this to 8 to 10 mm.
And it meant that integration with the existing covering could also be carried out without colour and quality deviations across the entire façade surface. In addition, larger layer thicknesses of 60 to 80 mm were possible in a single application.
In contrast to the other products tested, MC's special mortar was the only one to contain an alkali-free binder with high sulphate resistance. The introduction of alkalis would mean an increase in harmful substances attacking the fabric of the building. From alkalis especially, soluble salts such as sulphates, chlorides or nitrates can form more aggressive compounds such as Glauber's salt, which have a highly destructive effect on masonry. Added to this, Oxal SM TK 5 R3 is free from organic agents such as dispersions. These will never have been constituents of historical renders or mortars and would lead to disruptive side reactions.
Rigid crack repair
During the reconstruction of the façades of the National Museum, cracks in the old masonry came to light and had to be repaired. In the structurally critical areas, these cracks were closed and sealed using spiral masonry anchors and ties and special Oxal AVG-QM mortar – a combination that has already been used successfully in many historic buildings, including in the Czech Republic.
The newly developed formula for Oxal SM TK mortar won the confidence of the clients during the test phase thanks to its ability to adapt to historical mortar and render mixtures and its natural sand colour. At the same time, the application company was able to rely on comprehensive advice and support from Lukaš Pečenka, MC’s specialist for the restoration of historical buildings in the Czech Republic.