*2.1. Binder Characterization*

In order to study the mineralogical, chemical and colorimetric properties of the binders used in the present research, X-ray fluorescence (XRF), X-ray diffraction (XRD) and standard colorimetric observation methods were used, in line with the CIELab 1976 system.

A compact, high-performance wavelength dispersive X-ray fluorescence spectrometer, the Zetium model (Malvern Panalytical Company, Worcestershire, UK) by the brand PANalytical, was used to perform the X-ray fluorescence test (XRF) at the Scientific Instrumentation Centre of the University of Granada (CIC).

A Bruker D8 DISCOVER diffractometer (Dectris, Baden-Daettwil, Switzerland) with a DECTRIS PILATUS3R 100K-A detector (Dectris, Baden-Daettwil, Switzerland), from the Scientific Instrumentation Centre of the University of Granada (CIC), was used for the XRD test. The Xpowder [64] program (v. 8, Daniel, Granada, Spain) was used to determine the composition. Figure 1 and Table 3 show the results obtained from the XRD and XRF tests.

**Table 3.** Chemical composition by XRF analysis (wt %) of each raw material. Data normalized to 100% (LOI-free), loss on ignition.


The preparation of the samples involved grinding the raw materials in an agate pestle and paste and subsequently sieving them (mesh sieve ASTM N◦ 45, diameter <0.354mm).

A Konica Minolta CM-2500c Spectrophotometer (I.T.A. Aquateknica, S.A. Valencia, Spain) from the University of Granada was used in order to be able to carry out the colorimetric characterization and calculate the CIELab-1976 chromaticity coordinates [65] of the binders, pure pigments and their mixes after 28 and 120 days. The measurements obtained for diffuse spectral reflectance were in the visible range of 360–740 nm, at 5 nm

intervals, with a D65 illuminant at 10◦. The specular reflection component was excluded from all of the measurements, following CIE recommendations [66].

### *2.2. Sample Design and Preparation*

The research was carried out using four different pastes: the first composed entirely of pure plaster; the second made of plaster and air lime; the third made of plaster and hydraulic lime; and the fourth made of plaster and sodium silicate. In all four cases, the pastes were mixed with the pigments described above.

For the compression and flexural tests, several samples with dimensions of 160 × 40 × 40 mm were prepared in a plastic mold. For the permeability test, the samples dimensions were 40 × 40 × 20 mm, being prepared in a plastic mold.

A dosage was specified in terms of the volumes of the mixture components, the final compositions being those shown in Table 4. In order to prepare the samples, the different components (plaster, lime/plaster + pigment) were dry-mixed, before finally adding potable water in order to facilitate the mixing. For the samples containing plaster and water glass, the plaster binder and pigment were dry-mixed before adding the sodium silicate (liquid). This binder (sodium metasilicate) was purchased commercially in proportions of 25% active material, sodium silicate (Na2SiO3), and 75% water. The compaction times in the demountable molds and subsequent demolding were 24 h for all samples.

**Table 4.** Specification and dosage of each gypsum-based paste mix (by volume %).


All of the samples were placed in a Weiss Technik climatic chamber, ClimeEvent model (Xian LIB Environmental Simulation Industry, Shaanxi Province, China), with the following characteristics: temperature range −42 ◦C to 180 ◦C and relative humidity of 10–98%.

The environmental setting and hardening conditions for the PPS, PALS and PHLS mixtures were 22 ◦C and RH 70% inside of a climatic chamber for 120 days. The samples of the PWGS paste had particular conditions of 60 ◦C for 24 h in an oven, causing the concretion of the binder, and were subsequently kept in the same environmental conditions (in a climatic chamber) as the rest of the samples.

The average time used for sample setting was 28 days, except in the case of the PWGS mixes, which set after 24 h. All samples were considered to have fully hardened after 120 days.

Figure 2 shows both the pigments and the binders used in the research.

**Figure 2.** Samples of pigments and binders used in the research.
