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P2: La Muralla (The wall)

Second stop

La Muralla (The wall)

The walls have a defensive purpose. Their design and materials must be resistant; this is their function. Generally, materials that are available or that will work well are used. This is the case of the medieval walls of Girona.
They are mainly made of limestone rocks (the rocks most commonly found around Girona, along with sandstone) and fragments of bricks, joined by cement or lime, depending on when that portion of the wall was built. From this point, all of these materials can be observed.

The limestone rock is mostly composed of calcium carbonate (CaCO3) and, in the case of the so-called Girona stone, it often contains small marine organisms such as nummulites, which indicates that these rocks formed in a marine environment.

The carbonates react with dilute acids to give rise to carbon dioxide and the corresponding calcium salt. This process helps the chemical weathering of the rocks.

CaCO3 + 2 HCl —> H2O + CaCl2 (aq) + CO2
In certain places, bricks or fragments that fill small holes can be seen. These materials are made from clay, which has been used since ancestral times, and was the raw material used to make ceramics, vases, pots, as well as many construction materials.

From this point, you can see several Modernist tiles, enamel tiles that are mostly green or yellow.

The different materials that make up the wall are joined by a matrix or cement. This material has evolved over time, although its use is very old. For example, the Romans used a mixture of lime and tile dust or volcanic ash, which, when mixed with water, resulted in a hydraulic cement (which hardens over time). Often this mixture also contained sand or small rocks to increase its volume and resistance.

In the eighteenth and nineteenth centuries there was a change due to two discoveries that allowed for the manufacture of a strong, hard hydraulic cement that did not dissolve in water. These discoveries led to the creation of Portland cement, a basic material used to make concrete (mixed with stones and sand), mortar (mixed with fine sand), and stucco (mixed with chalk).

Did you know?

Marble is a rock that has the same chemical composition as limestone, that is, mostly calcium carbonate. Its appearance is different because it has been formed under different conditions, especially under higher pressure. Marble has been widely used as kitchen counter tops due to its ease of being cut into sheets and also because of its beauty, especially when polished. Over time, other materials have become popular, such as granite and sandstone.

The economic factor has an effect: years ago it was easier to obtain and work with marble, but it has some drawbacks. It reacts with acids and it is easily stained because it is quite porous. In the kitchen, some acids that can stain the marble are used frequently. Lemon juice contains ascorbic acid (vitamin C), and vinegar, acetic acid: two enemies of calcium carbonate.