Among the traditional gypsum plasters, there are several types of coatings :
• Gypsum paste (pure gypsum paste)
• Gypsum mortar (gypsum and sand)
• Gypsum and slaked lime (with or without sand)
Traditionally in Portugal, gypsum-based coatings are applied on a substrate after it has been coated with a render, whose function is to smooth and prepare the surface for the execution of the finish. However, if the substrate’s surface is very irregular, such as rough stone masonry (a frequent situation only in ancient buildings), it is necessary to previously smooth the surface by applying filling mortar in the most depressed areas .
The characteristics of the substrate determine the number and constitution of the coats to be used. The quality of execution of the substrate determines the number of coats needed to obtain a plane surface, thus influencing the coating’s thickness. The texture, porosity and suction of the substrate influence its adhesion capacity to the overlying layer, determining the content of the mix to use. However, if great water contents are used in the coating’s execution to decrease suction propensity, efflorescence may occur later on the coating’s surface. The strength of each coating layer must decrease from the substrate outwards, to prevent potential cracking and loss of adherence [10, 22].
Base coat Smoothing plaster
———— Gypsum plaster
Fig. 3.11 Coats of a traditional gypsum plaster (adapted from )
• Spatterdash, 3-5 mm (cement and sand)
• Base coat or undercoat
— Single layer, 10-15 mm
— Double layer (filling mortar and “mortar”), 20 mm
• Finishing coat (6-8 mm)
— Smoothing plaster, 3-5 mm (lime and sand paste + gypsum or simply gypsum and sand)
— Gypsum plaster, 3 mm (gypsum and slaked air lime paste or simply gypsum with setting retarder)
The main function of the spatterdash is the preparation of the substrate for the application of the next coat, ensuring a good adherence thanks to its rough surface. Spatterdash is a discontinuous coat, with maximum thickness between 3 and 5 mm, and is made of a mortar with a high cement content (e. g. a volumetric proportion of 1:2 (cement-sand)) and with enough water to guarantee cement hydration and that the substrate’s absorption is even. The spatterdash can be applied by slapping it against the wall with a broom dipped in mortar or mechanically by spraying. When this coat is being applied, the substrate must be clean and dampened .
The base coat, also called undercoat, has its main function of smoothing the surface, ensuring verticality, planeness and unwarping of the walls and being the final barrier against humidity from the exterior or the substrate. Furthermore, it must confer a good adherence to the subsequent coats. The final thickness of the base coat must be within the 10-15 mm range, for single-layer applications, and may reach 20 mm if two layers are applied (filling mortar and “mortar”), which is necessary when the substrate is very irregular. The mortar to be applied must be mixed binder, with volumetric proportions between 1:0.5:4 to 4.5 and 1:1:5 to 6 (cement-lime — sand), always complying with the decreasing trend of binder content from the inside outwards [6, 10, 23].
The application must start after the spatterdash’s mortar has developed most of its initial drying shrinkage (a minimum of 3 days) and is made manually by slapping the mortar against the substrate and uniformly pressing it with a trowel. The roughness of the base coat’s surface must be compatible with the finishing coat to be applied. If the base coat is made with two layers, the latest remark applies only to the second layer, which is under normal circumstances applied after the first layer has developed most of its initial drying shrinkage and is then dampened [10, 23].
When a metal or fibre glass reinforcement is applied, the base coat must be made with two layers, where the first one is applied with pressure in order to penetrate through and involve the mesh and the second one is made after the first one completely dries and hardens .
The finishing coat is applied in two layers, the first one called smoothing plaster and the second one gypsum plaster, with a total thickness between 6 and 8 mm.
The composition and volumetric proportion of the mix is different: in the first one, it is 1:2:1 or 1:1:4 (gypsum-slaked lime paste-sand) and in the second one 1:0 to 0.25:0 to 1 (gypsum-slaked lime-sand) .
The smoothing plaster prepares the surface for the application of the final finishing coat (gypsum plaster). It must be applied with a trowel, starting from the border (lower in the case of walls), then spreading the mortar onto the wall’s upper area with the trowel, until the opposite border is reached.
The plaster is obtained by mixing the lime paste with the sand, adding the gypsum only immediately before application, due to its short setting time. This mix allows a useful application period between 10 and 30 min .
In the case of walls, the points and lines technique can be used. To that purpose, gypsum points (markers) are made on the wall, starting from the upper level of the wall and coming down. Their horizontal spacing should be around 2 m and the vertical spacing between 1.5 m and 2.5 mm. Next, the points in the same vertical line are united by mortar strips, forming the lines, and then the area in between each two consecutive lines is filled with mortar, later smoothed with a ruler [6, 23].
The gypsum plaster’s main purpose is to give the surface its target final aspect. To that purpose, this coat must generally have a smooth finish, even though it may incorporate frame and ornaments, in which case it is considered a decorative coating. Similarly to the previous coat, the gypsum plaster is applied with a trowel. However, being the final coating layer, its finish is more demanding, with a plasterer’s spoon or a small trowel. When a plain gypsum paste is used, mixing is done in a large bowl where the amount of water necessary (50-70 % of the gypsum’s volume, around 18 L per 25 kg of gypsum) is previously poured. Then the powder gypsum is sprayed on it in small layers until they become saturated, and that is when the paste is quickly mixed and application starts at once. Sometimes slaked lime is added to the gypsum or gypsum is mixed with lime water with the objective of decreasing its expansibility, increasing its hardness after drying and/or for economic reasons (lime is less expensive than gypsum) [6, 10, 24].
Traditional gypsum plasters can have several types of final finish (Figs. 3.12, 3.13, and 3.14): polishing; painting; colouring the plaster itself; decorative elements (frames, moulded ornaments, painted ornaments, fresh painting and faked painting); and stucco (imitation of marble or other natural stones) [6, 10].
Fig. 3.12 Frames around a moulded ornament in a plastered ceiling