PHLS + Pigment Samples

The results for this group of samples are the highest of all those tested, reaching maximum values of 9.91 MPa ± 0.02 for the samples containing added pigment (PHLS + PGE (Green Earth pigment) and 9.79 MPa ± 0.03 for the samples without added pigment; percentage differences are less than 1% between them. In relation to the other groups, a significant increase was also confirmed, these being 45% compared to the pure plaster samples (PPS), 42.7% compared to the plaster and air lime (PALS) samples and 11.6% compared to the pastes containing geopolymers (PWGS).

Overall, it was observed that the compressive strength results for the samples containing pigments experienced improvements in resistance capacity, the least resistant being the PPS pastes, followed by the PALS and the PWGS, and the most resistant being the PHLS pastes.

The literature consulted [70,71] confirms that, in general, an increase in compressive strength for this type of paste is the consequence of a variation in the microstructure of the hardened matrix.

Furthermore, [72,73] established that an increase may also be the result of chemical reactions between the components (plaster + hydraulic lime) in the presence of water. The presence of plaster as a replacement for the lime results in faster hydration, and consequently, accelerated setting.

Meanwhile, it was observed that the addition of water glass to the mixes studied resulted in the formation of amorphous silica, which acted as a siliceous aggregate in the resulting paste, thereby increasing the mechanical strength.

Finally, in the samples containing hydraulic lime, an increase in the hydration rate was confirmed due to the presence of calcium silicates, whose role is that of an active additive (catalyst) in these pastes.

The analysis of the samples containing pigments for the four pastes concluded that in all cases, the maximum value was associated with the samples containing the PGE (Green Earth pigment). Previous studies [73–75] confirmed that MgO in both the pigment itself and in the binding material acts as a fraction of aggregate material, increasing the compressive strength of the mixes containing added pigments.

#### 3.3.2. Flexural Strength

Where flexural strength is concerned, the results show a very similar tendency to those observed in the compressive strength testing (Tables 6 and 7). In this case, it is observed that strength increased when part of the plaster was replaced by other binders, such as air lime, sodium silicate or hydraulic lime.

During the period of study, the maximum values were obtained for the plaster + hydraulic lime samples (PHLS samples). Those maximum values were up to 3.94 MPa ± 0.03 (average value of 3.92 MPa). Next were the plaster + water glass samples (PWGS samples), which reached an average value of 3.50 MPa, representing a percentage decrease of 10.7%. In third place were the plaster + air lime samples (PALS samples), which obtained average flexural strength of 2.93 MPa. Finally, the minimum values were achieved by the pure plaster samples (PPS samples), their average value being 2.25 MPa.

In percentage terms, the differences between the groups are significant. The plaster + hydraulic lime samples were 10.7% stronger than the plaster + water glass samples, 25.2% stronger than the plaster + air lime samples and 42.6% stronger than the pure plaster samples.

After comparing the results of the samples containing added pigments, no real difference was observed among them. On comparing the samples without added pigments with the colored samples in each group, it was observed that in the plaster + hydraulic lime samples (PHLS samples) the flexural strength results were very similar to those for the sample without added pigment, there being a difference of <1% in all cases. The samples containing plaster + water glass (GWS sample) obtained similar differences, slightly greater than for the previous group, although still less than <1%. The samples containing plaster + air lime (PALS samples) displayed differences of 2%. Lastly, for the pure plaster samples (PPS samples) the differences were also minor, slightly above 2% (2.17%).
