**1. Introduction**

Plaster is a commonly-used material due both to its abundance and the fact that it is easy to extract, transform and distribute [1]. In addition, there are other features, such as its easy preparation, durability and versatility [2], along with its quick setting and hardening when exposed to air. It is a material that is often used in the construction sector due to its low cost, its excellent thermal insulation and soundproofing properties, its high flame resistance and its low energy consumption during the production process [3,4]. It is also easy to recycle using suitable preparation processes based on the theory of hydration and dehydration [5].

In building construction, plaster is used for decorative trimmings and coatings of walls and ceilings; for stucco work and rendering; in interiors and exteriors; as a binder in ceramic and stone materials; and even for building products such as bricks and blocks, laminated plasterboards, plasterboard sandwich panels, etc. [6,7].

Despite its aforementioned advantages, its extreme fragility and limited mechanical strength and resistance to water mean that this material is unsuitable when its use requires it to withstand certain specific stresses, to bear shock loads or to be located in external environments [8].

**Citation:** Sáez-Pérez, M.P.; Durán-Suárez, J.A.; Verdú-Vázquez, A.; Gil-López, T. Study and Characterization of Special Gypsum-Based Pastes for Their Use as a Replacement Material in Architectural Restoration and Construction. *Materials* **2022**, *15*, 5877. https://doi.org/10.3390/ ma15175877 Academic Editor: Alessandro P. Fantilli

Received: 15 July 2022 Accepted: 22 August 2022 Published: 25 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

For some time, research has been being carried out into how to improve the mechanical properties of pastes so as to broaden the scope of their use. This fact has led to new mixes being studied in which natural and artificial fibers are added [9–16]. Usually, in the case of plasters or gypsum-based pastes, the way that this is achieved is by reinforcing them using fiberglass [17–19].

The search for materials which have lower environmental impacts and greater efficiency has led to artificial fiber-based additives being replaced by others containing natural fibers [3]. However, the high cost of natural fiber in comparison with that of plaster, along with the limited interaction of these additives with the plaster matrix, mean that these compounds are not as competitive as artificial ones.

Within the construction sector, the use of gypsum-based pastes is necessary for the majority of monuments, giving this material significant relevance in conservation and restoration projects affecting the world's cultural heritage. Its origin dates back to the sixth millennium BC in Greece, and it was widely used and developed by the Romans [20]. Consequently, knowledge of this type of compound, and its application, is fundamental for mitigating the state of degradation and loss of built heritage [21–24]. Despite its importance, there has been little interest in studying it or defining new strategies for its use in conservation for a number of years.

Fortunately, in recent decades, the features of mortars and pastes have become a priority in material characterization studies, particularly those which have compositional or microstructural issues [25–34], excluding aesthetic matters [21,35–38]. In this regard, incorporating polymers in traditional building materials, such as mortars, adds great value when compared to conventional building materials. The addition of polymers makes it possible to obtain good levels of mechanical strength, good adhesion properties, abrasion and weathering resistance, waterproofing and excellent insulating properties [39].

New research projects seek to increase the durability of plaster pastes exposed to atmospheric influences. Recent studies have analyzed the improvements brought about by introducing modifying additives to the plaster binder, such as: polymeric compositions, fine minerals and nano-disperse components. Zhukov et al. have analyzed the addition of hardening resins to plaster through polycondensation and the application of nanoaggregates [40]. The addition of a polymer to the plaster mix produces a framework of dehydrated crystalline aggregates during the hydration of the plaster, whilst the resin, when it hardens, forms a continuous polymeric matrix. This causes the polymeric plaster to increase its strength over time due to the continuous polymerization of the resin.

Other research projects seek to analyze the influence of micro-aggregates (microspheres, hydroxyethyl methyl cellulose polymer and/or aerogel) on the thermal conductivity coefficient and thermal diffusivity. Using these additives makes it possible to reduce thermal conductivity by up to 23%, compared to the unmodified plaster samples [41]. The authors conclude that the polymer provoked a change in the structure of the plaster compound, giving it a lower density and greater porosity.

On the other hand, the porosity increasing can also have some disadvantages, since a key element to take into account in the conservation of gypsum-based pastes is the permeability of the compound [42]. As such, it is necessary to study the porosity, sphericity and pore size distribution, assessing the changes depending on the mortar composition [43]. Recent studies have made progress by using mercury intrusion porosimetry (MIP) and micro-computerized tomography (μCT). Thanks to these techniques, it is possible to visualize pores, air voids, aggregates and binder distributions within a sample [25].

However, when it comes to heritage, the maintenance and preservation of buildings require broader interventions also addressing issues of aesthetic nature. In this respect, the colorimetry rules and studies can be key in the field of conservation and restoration [44–46]. Knowledge of the chromatic possibilities of pigments and their techniques is fundamental when the objective is to carry out an intervention for recovery of built heritage [47,48].

In this kind of intervention, where it is not possible to modify the aesthetic characteristics of the element, colorimetry has become a highly useful tool. In an intervention of this type, a correct choice of materials must be made after evaluating the effectiveness of the procedures to be used and the chromatic modifications that can happen. In this way, it is necessary to develop a colorimetric study at least, before and during the preparation of the restorative pastes.

There are recent studies which have already applied the study of colorimetry in the field of monumental heritage, analyzing the performances of treated pigments [49–56]. For this reason, the use of pigments in different applications within the field of materials engineering and interventions in architectural heritage is necessary in order to achieve matching visual and aesthetic characteristics [57,58].

Generally, the colorimetry studies have focused on the characteristics of the pigments, mainly on production processes and their formulation, the saving of resources, the product finish and the most suitable application methods for protecting the environment. However, it is important to underline the conditions that the materials will be expose for establishing their performance and effectiveness.

The present article focuses upon the application of different pigments in plasterbased pastes for use in construction, be it to new builds or to the restoration of cultural heritage sites. It analyzes the intended use of the pigments studied, the suitability of their physical and chemical properties and the characteristics of the materials. Their colorimetric implications were analyzed in two different studies after 28 days and 120 days.

#### **2. Materials and Methods**

The research carried out involved designing four gypsum-based pastes containing added pigments. Different binders and pigments were used to produce them, making it possible to add significant color to the pastes, and increase their final mechanical strengths without unduly reducing their insulation or water vapor permeability values.

Both the limes and plaster used were provided by CTS Spain. The plaster used contained a minimum of 90% of hemihydrate, giving it the highest quality, as can be seen in the XRD (Figure 1). According to the distributor, a microfiltration process is applied to the air lime after it has been slaked in tanks used specifically for that purpose, and it is then aged for a period of no less than six months. According to [59], air lime is a type CL90 calcium lime (CL), the CaO + MgO content of which is ≥90%. The MgO content is ≤5%, the CO2 content is ≤4%, the SO3 content is ≤2% and the usable lime (Ca(OH)2) content is ≥80%. Where hydraulic lime (NHL5) is concerned, this was genuinely natural pure lime obtained from the calcination of loamy limestone, without additives, at production temperatures of 1200 ◦C, which according to [59], has a usable lime (Ca(OH)2) content of ≥15%, a SO3 content of ≤2% and compressive strengths ranging from 5 to 15 MPa. The sodium metasilicate (water glass) was acquired from Alquera Ciencia SL (Spain), with a SiO2 content of 26.40 ± 1.50%, Na2O content of 8.00 ± 0.60% and water content of 65.60 ± 2.00%. The true density is 38 ± 1.00 Be, and the pH is 12.50 ± 1.00. The mixture has a viscosity of 80 MPa·s.

**Figure 1.** X-ray diffractograms for the plaster, hydrated air lime, hydraulic lime and water glass [60]. Abbreviations for names of rock-forming minerals.

In relation to the different pigments used (eight), these were supplied by Kremer Pigmente. The color range selected enables the use of a broad spectrum of red, blue, green, ochre and yellow colors, specifically for use in the preparation of pastes, putties and mortars for applications in architectural heritage work. The compositional parameters, by manufacturer, color code [61] and name, are set out in Tables 1 and 2.


**Table 1.** List of pigments along with an indication of the identification used in the study carried out, trade name, color index, composition and acronym.

**Table 2.** List of raw materials used for binders along with an indication of the identification used in the study carried out, trade name, color index, composition and acronym.

