Читать книгу Monument Future - Siegfried Siegesmund - Страница 88

In this paper we present preliminary results in order to understand the properties of gypsum renders as a function of their composition, binder and aggregates, but also of the samples preparation, especially the amount of mixing water, It is difficult to draw general conclusions, as the sampling process is not easy and obtained samples cannot be considered as representative of the whole population of renders in the Vexin Français area.

Another difficulty of the study is the age of the samples. This study has been done during a Master degree research project of 6 months. Samples from buildings are at least 100 years old but laboratory samples have less than 3 months. This difference is especially important for samples with lime. In new 70samples we find some portlandite, not found in old samples, not even in Saint Clair sur Epte sample, fabricated entirely with lime binder.

Nevertheless, some conclusions can be inferred from this work:

a) Historical studied renders are very different from one to the other, even if most of them are composed of more than 80 % of gypsum. Aggregates granulometry and composition, amount of charcoal, number of layers, colour, etc. vary from one sample to the other.

b) The amount of calcite or quartz aggregates is more important in renders of farms or enclosure walls than in façades. Decorative elements are composed almost exclusively of gypsum. The composition also depends on the location of a building in respect to the areas of plaster of Paris production.

c) Mixing water and the amount of lime control the properties of renders. In this way, the plasterer’s skills determine the quality of the products.

d) Porosity cannot be directly related to the water absorption/desorption of plasters. Sorption/ desorption test seems more adequate to characterise hydric behaviour of plasters than porosity measurements.

e) Permeability of plaster is directly related to the capacity of samples to desorb the absorbed water upon drying.

f) It is difficult to compare new and old plasters due to the alteration of their properties with time.