2.1. Mix Design Components Characterisation
The processing waste of Luserna stone was used to prepare the design mix of the mortar. Luserna stone is a Piedmont gneiss characterised by a flat-schistose texture and by a heteroblastic structure with a micro-occhiadine tendency, due to the presence of millimetric micro-porphyroblasts (0.03–3 mm) (
Figure 1). It is composed of Quartz (50%); Plagioclase (15%); Alkaline Feldspar (20%); White mica (5%); Chlorite (5%); Epidote (5%).
The two types of aggregate, derived from Luserna stone processing, were considered in this study:
waste after flaming used as a sand aggregate.
waste from sawing blocks into slabs through diamond tools, used as a filler aggregate.
A preliminary investigation was carried out for the particle size distribution analysis, performed according to UNI EN 933 Part 1 [
18]. The results obtained are shown in
Figure 2.
Luserna sand, from the flaming process, contains only mineral fractions that compose Luserna stone. Sawing sludge, from diamond blade cutting, contains a fraction from the mineral part and a metal fraction derived from the diamond tool’s wear. For this reason, magnetic separation, chemical analysis, leaching tests and XRPD (X-Ray Powder Diffraction) were performed on the Luserna sawing sludge sample. The quantification of the metal content in the sludge is necessary to define what is now a waste, a secondary raw material that can be used in other production processes (in accordance with Italian Legislative Decree 152/2006 [
19], implementation of European Directive 2008/98/EC). Chemical analysis was performed, according to Italian Legislative Decree 152/2006, by means of Fkv Ethos Easy microwave mineraliser and ICP-Mass analyser. The results obtained, with the law threshold concentration limits, are reported in
Table 1.
The results showed an excess of the concentration limits for the elements cobalt and Cr VI, but only for its reuse in the green area (Column A).
Leaching testing was performed according to UNI EN 12457-2 [
20], taking into account the threshold limits concentration defined in the appendix A of UNI 10802 [
21]. The results obtained are shown in
Table 2.
On the basis of the chemical analysis, it could be asserted that Luserna sawing sludge could be re-cycled for plaster applications in buildings.
Magnetic separation, by means of Magnetic Separator (Eriez magnetics manufacturer, L Series, Model 4, Brasil) (Kolm-type high gradient), was performed to identify the quantity of metal content in the sludge sample. One of the main requirements for plaster application is thermal insulation. A low metal content can produce a lower thermal conductivity since metals are good thermal conductors.
Table 3 shows the low percentage of magnetic (metals) fraction in the considered sludge sample.
The X-ray powder diffraction was performed with Rigaku model Geigerflex (model smartlab SE, Rigaku, Japan). The sample preparation was carried out by means of grinding, through a pestle, the sludge to obtain a fine powder (with dimensions between 25 and 10 m). This procedure is necessary to improve the homogeneity of the material.
Only the sample deriving from diamond blades sawing was analysed since the flaming sample has only components of the Luserna stone of which the composition is known. The analysis was carried out both on the sample as it is and on the sample after the magnetic separation on the magnetic fraction. This choice was due to the complexity in the interpretation of the results. The sample was composed of several minerals and several metals: for this reason, the peaks were overlapped. With magnetic separation, there was an elimination of some minerals phases, so the peaks of the metal were more evident. A comparison of spectrum for samples as it is and with the only magnetic fraction is shown in
Figure 3.
The Luserna diamond blade sawing sample showed minerals like Quartz, Feldspar and Mica and some peaks of metals that compose both the steel core of the disc and the matrix of the diamond segments. The metals identified were: Cobalt Molybdenum Oxide (CoMoO3), Aluminum Nickel (Al3Ni), Nickel Phosphide (NiP) and Nickel Chromium Oxide (NiCrO3).
In the analysis performed on magnetic fraction, we could observe a peak related to the Chlorite and other metals that were not identified in sample analysis before magnetic separation. CuS, Cobalt Iron (Co3Fe7), Nickel Molybdenum Phosphide (NiMoP2) and Silicon Carbide (SiC) were identified. All elements were contained in high-speed tool steels.
2.2. Plaster Mix Design
The preparation of plaster mix design was carried out at TEK.SP.ED s.r.l. company (Casandrino, Napoli). The plaster types differ according to the kind of binder used. In this research, the binders were hydrated lime and portland cement, with a predominance of portland cement. The mix design involves the presence of lime, cement, aggregates, water, reinforcing fibres and foam. It is known from the literature [
22] that the foam improves the thermal conductivity characteristics of the plaster but worsens the mechanical characteristics, such as compression. The foam, in this case, was used to give the final material a certain lightness, while the fibres gave greater mechanical resistance (tensile resistance).
Table 4 shows the components of the mix design and the quantities used compared to 1 m
3.
Table 5 shows the mix design with the addition of Tufo’s powder (commercial ‘‘zeolitite”, rocks with zeolite content higher than 50 wt.%, coming from a quarry site located near Comiziano, Nola-Naples (Campanian Ignimbrite)), used mainly for chemical resistance test [
23,
24] compared to specimens produced with waste derived from the Luserna stone only. Adding of Tufo powder has been carried out since the Roman Empire period [
25,
26].
The components were mixed with the S8 EVM (single-phase electric with mixer) machinery (
Figure 4) manufactured by TEK.SP.ED. s.r.l. company (Casandrino, Napoli, Italia). This machine is low power consumption, has a mixing system and a system for pumping plaster directly on site.
Once the mix design was prepared, the material was poured onto a vertical panel to simulate conventional industrial practice (
Figure 5). In this way, both the adhesion of the material to the support and its behaviour with regards to hot summer temperatures could be tested. This critical weathering condition could cause cracks and fractures, on normal plasters, that could worsen over time.
After two days, curing time (critical weathering condition), the results of plaster pouring were optimal.
Figure 6 shows the homogeneity of the product obtained and the absence of fractures and cracks.
Using the same mix design, specimens were prepared for the physical-mechanical tests (
Figure 7.). The number of specimens and their size were in accordance with the standards for each test reported in
Table 6.
2.3. Physical Properties
The following laboratory tests were performed to investigate the physical properties of the plaster: bulk density, spreading test, thermal conductivity and water absorption.
2.3.1. Bulk Density
Bulk density testing was carried out, weighing one cubic meter of material in the fresh state. For dry condition bulk density evaluation, three samples of plaster were placed in an oven at 105 °C. This was followed by two weighings, two hours apart, to verify that the mass was constant. Once the mass of the dry specimen was obtained (Ms, dry), the formula used to calculate the apparent bulk density for each specimen is as follows:
where Vs is the volume of the mold (m
3).
The average value of the three measurements was then calculated, approximating the result to 10 kg/m3.
2.3.2. Spreading Test
A spreading test was carried out using a Hagerman cone, according to ASTM D 6103 of 2017. The cone has a chamfer of 45°, an upper diameter of 70 mm and a bottom diameter of 100 mm.
2.3.3. Thermal Conductivity
Thermal conductivity testing was performed using a KD2 Pro (Meter group manufacturer, Munich, Germany) with probe RK-1 model, 6 cm long and 3.9 mm diameter. Thermal conductivity was carried out on three samples, at 15 and 68 days of curing time and in dry condition (placed in the oven for 48 h at 60 °C).
2.3.4. Water Absorption
Water absorption was carried out on 12 samples, with 6 samples not subjected to a freeze and thaw cycle, and the other 6 samples after a freeze and thaw cycle. The procedure involved placing specimens in an oven at 60 °C to obtain a constant mass. Once the dry weight (Md) of the specimens was obtained, the specimens were placed in water until saturation. Subsequently, the specimens were weighed to obtain the saturated mass (Ms). The results were expressed as the percentage of absorption obtained according to the equation:
where Ms = saturated mass (g); Md = dry mass (g).
2.4. Mechanical Properties
The following laboratory tests were performed to investigate the mechanical properties of the plaster: flexural strength, compressive strength, pull out.
2.4.1. Flexural Strength
Flexural strength was performed on three samples, according to UNI EN 1015-11 of 2007. A constant speed load of 50 N/s was applied to obtain a fracture between 30 s and 90 s. Flexural strength, after the freeze and thaw cycle, was performed according to UNI EN 12371 of 2010, which is related to natural stone, as it is not a test required by plaster standards. Twenty-five freeze and thaw cycles were performed with a four-hour duration of each cycle, consisting of two hours at −15 °C (in a saturated solution of sodium chloride) and two hours at 21 °C. The flexural test was performed seven days after the last cycle, according to UNI EN 1015-11:2007.
2.4.2. Compressive Strength
Compressive strength was performed on six samples, according to UNI EN 1015-11 of 2007. Freeze and thaw cycle was performed, according to UNI EN 12371 of 2010, related to natural stone, as it is not a test required by plaster standard.
2.4.3. Pull out
Pull out test was carried out, according to UNI EN 1015-12 of 2016, by TEK.SP.ED company on plaster pouring directly on the vertical panel after 28 days of curing.