# Characterization of Natural Gypsum Materials and Their Composites for Building Applications
## 1 Introduction
### Acoustical and Mechanical Characteristics: State of Art  
Noise insulation in building is also an important issue to be taken into account, in order to achieve optimal comfort conditions. The acoustic properties of plasters and concrete have been widely investigated in the literature [11; 12; 13; 14]. [PERSON] et al. [11] studied the effect on the acoustic properties of innovative insulating coatings made of natural calk and granular silica aerogel, with a final coat of 2 mm. They showed a better acoustic absorption coefficient when compared to the same plaster without granules. However, the \(\alpha\)-value strongly depends on the final coat; normal incidence values are not very high at all (about 0.13, increased to 0.18 when removing the final coat). The acoustic absorption performance of clay-based plasters and natural earth of the north-western Sicily were investigated in [12]; they are mainly used for interior wall coatings in green buildings, as an alternative to the more common gypsum or lime plaster. Natural earth plasters are characterized by open porosity, higher than that of commercial materials, with a consequent increase in sound absorption, while premixed clay-based plasters do not show a marked difference in the absorption behavior. Moreover, for medium-high frequencies, the effectiveness of the noise mitigation seems to depend on the size of the sandy aggregate. However, the absorption performance of green-plasters is comparable to the one of traditional lime-based or gypsum-based binders and fired clay bricks.  
In a recent study in 2018, [PERSON] and [PERSON] [13] investigated the influence of mixtures produced with river sand aggregate and hydraulic lime binder on the absorption properties, considering the effects of volumetric ratio, aggregate particle sizes, and different types of additives. Results showed that by increasing the binder volume ratio, the sound absorption tends to decrease, but a positive effect on compressive and flexural strengths is highlighted. The size of the aggregate particles is also very important: the use of fine sand (particles in the 0-2 mm range) reduces the \(\alpha\)-values. Furthermore, it was found that polypropylene fiber, flax fiber, and crumb rubber additives in the mixture allow us to increase the sound absorption coefficient at low frequencies (up to 750 Hz, 900 Hz, and 800 Hz, respectively), with a reduction in compressive and flexural strengths (about 40-45%).  
The acoustic properties of hemp concretes were investigated in [14] in terms of the absorption coefficient and sound insulation. The hemp-clay and hemp-lime mixtures are characterized by a similar acoustic behavior, in which binder fluidity and clay type have no effect. Considering densities below 375 kg/m\({}^{3}\), a peak of absorption equal to 0.6-1 was measured in the 500-1200 Hz; when the density increases above 500 kg/m\({}^{3}\), the sound absorption curve flattens, while the transmission loss increases. However, the width of the absorption peak and the air resistivity for a given density are lower for hemp-lime than for hemp-clay.