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	<front>
		<journal-meta>
			<journal-id journal-id-type="publisher-id">rica</journal-id>
			<journal-title-group>
				<journal-title>Revista internacional de contaminación ambiental</journal-title>
				<abbrev-journal-title abbrev-type="publisher">Rev. Int. Contam.
					Ambient</abbrev-journal-title>
			</journal-title-group>
			<issn pub-type="ppub">0188-4999</issn>
			<publisher>
				<publisher-name>Universidad Nacional Autónoma de México, Centro de Ciencias de la Atmósfera</publisher-name>
			</publisher>
		</journal-meta>
		<article-meta>
			<article-id pub-id-type="doi">10.20937/RICA.53648</article-id>
			<article-id pub-id-type="publisher-id">00019</article-id>
			<article-categories>
				<subj-group subj-group-type="heading">
					<subject>Artículos</subject>
				</subj-group>
			</article-categories>
			<title-group>
				<article-title>MANUFACTURE OF LIGHTWEIGHT PREFABRICATED PANELS EMPLOYING MINE
					TAILINGS</article-title>
				<trans-title-group xml:lang="es">
					<trans-title>FABRICACIÓN DE PANELES PREFABRICADOS LIGEROS UTILIZANDO JALES
						MINEROS</trans-title>
				</trans-title-group>
			</title-group>
			<contrib-group>
				<contrib contrib-type="author">
					<name>
						<surname>Ramírez-Crescencio</surname>
						<given-names>Fermín</given-names>
					</name>
					<xref ref-type="aff" rid="aff1"><sup>1</sup></xref>
				</contrib>
				<contrib contrib-type="author">
					<name>
						<surname>Parra-Véjar</surname>
						<given-names>Jaime</given-names>
					</name>
					<xref ref-type="aff" rid="aff1"><sup>1</sup></xref>
				</contrib>
				<contrib contrib-type="author">
					<name>
						<surname>Loera-Castañeda</surname>
						<given-names>Verónica</given-names>
					</name>
					<xref ref-type="aff" rid="aff1"><sup>1</sup></xref>
				</contrib>
				<contrib contrib-type="author">
					<name>
						<surname>Aguilar-Pérez</surname>
						<given-names>Luis Antonio</given-names>
					</name>
					<xref ref-type="aff" rid="aff1"><sup>1</sup></xref>
				</contrib>
				<contrib contrib-type="author">
					<name>
						<surname>Villanueva-Fierro</surname>
						<given-names>Ignacio</given-names>
					</name>
					<xref ref-type="aff" rid="aff1b"><sup>1</sup></xref>
					<xref ref-type="corresp" rid="c1">*</xref>
				</contrib>
				</contrib-group>
				<aff id="aff1">
					<label>1</label>
					<institution content-type="original">Instituto Politécnico Nacional, CIIDIR
						Unidad Durango, calle Sigma 119, Fraccionamiento 20 de Noviembre II, 34220,
						Durango, Durango, México.</institution>
					<institution content-type="normalized">Instituto Politécnico
						Nacional</institution>
					<institution content-type="orgname">Instituto Politécnico Nacional</institution>
					<institution content-type="orgdiv1">CIIDIR</institution>
					<addr-line>
					<named-content content-type="city">Durangp</named-content>
          <named-content content-type="state">Durango</named-content>
					</addr-line>
					<country country="MX">Mexico</country>
				</aff>
				<aff id="aff1b">
					<label>1</label>
					<institution content-type="original">Instituto Politécnico Nacional, CIIDIR
						Unidad Durango, calle Sigma 119, Fraccionamiento 20 de Noviembre II, 34220,
						Durango, Durango, México.</institution>
					<institution content-type="normalized">Instituto Politécnico
						Nacional</institution>
					<institution content-type="orgname">Instituto Politécnico Nacional</institution>
					<institution content-type="orgdiv1">CIIDIR</institution>
					<addr-line>
					<named-content content-type="city">Durangp</named-content>
          <named-content content-type="state">Durango</named-content>
					</addr-line>
					<country country="MX">Mexico</country>
					<email>ifierro62@yahoo.com</email>
				</aff>
			<author-notes>
				<corresp id="c1">
					<label>*</label>Author for correspondence: <email>ifierro62@yahoo.com</email>
				</corresp>
			</author-notes>
			<!--<pub-date date-type="pub" publication-format="electronic">
				<day>13</day>
				<month>09</month>
				<year>2021</year>
			</pub-date>
			<pub-date date-type="collection" publication-format="electronic">-->
				<pub-date pub-type="epub-ppub">
				<month>11</month>
				<year>2020</year>
			</pub-date>
			<volume>36</volume>
			<issue>4</issue>
			<fpage>1019</fpage>
			<lpage>1025</lpage>
			<history>
				<date date-type="received">
					<day>01</day>
					<month>06</month>
					<year>2019</year>
				</date>
				<date date-type="accepted">
					<day>01</day>
					<month>03</month>
					<year>2020</year>
				</date>
			</history>
			<permissions>
				<license license-type="open-access"
					xlink:href="https://creativecommons.org/licenses/by-nc-nd/4.0/" xml:lang="en">
					<license-p>This is an open-access article distributed under the terms of the
						Creative Commons Attribution License</license-p>
				</license>
			</permissions>
			<abstract>
				<title>ABSTRACT</title>
				<p>The present study describes the incorporation of mine tailings from the silver
					mine La Guitarra in the manufacture of lightweight prefabricated panels (LPPs).
					Tailings have concentrations of heavy metals below the maximum permissible
					levels according to Mexican regulations. Their acidity potential (AP) of 2.22 kg
					of CaCO<sub>3</sub> per tailings ton, along with their lack of neutralization
					potential (NP), indicates a possibility for the generation of acid mine drainage
					(AMD). It was reported that concrete could be employed to prevent AMD, thus we
					proposed to stabilize mine tailings with different concrete ratios and employ
					these mixtures to fabricate concrete panels. To obtain lightweight panels, a
					foaming agent was added to the tailings/concrete mixture. Then, a fiberglass
					mesh was employed to reinforce the final structure. The resulting LPPs
					(dimensions: 254 × 139.7 × 12.7 mm) showed a flexural strength of 5.80 ± 0.06
					MPa, bulk density of 712 ± 104 kg/m<sup>3</sup>, thermal conductivity of 0.159 ±
					0.004 W/m K, mass water absorption of 59 ± 10 %, and net neutralization
					potential (NNP = NP/AP) &gt; 1.2. According to Mexican regulations, these LPPs
					are not potential generators of AMD.</p>
			</abstract>
			<trans-abstract xml:lang="es">
				<title>RESUMEN</title>
				<p>El presente estudio describe la incorporación de jales mineros de la mina de
					plata La Guitarra en la fabricación de paneles prefabricados ligeros (PPL).
					Estos jales tienen concentraciones de metales pesados inferiores al límite
					máximo permisible de acuerdo con la normatividad mexicana. Sin embargo, su
					potencial de acidez (PA) de 2.22 kg de CaCO<sub>3</sub> por tonelada de jal,
					aunado a la ausencia de potencial de neutralización (PN), indica que pueden
					generar drenaje ácido. Se ha informado que el concreto se puede utilizar para
					prevenir la generación de drenaje ácido, por lo que se propuso estabilizar los
					jales mineros con diferentes proporciones de concreto y emplear estas mezclas
					para fabricar paneles de concreto. Para obtener paneles ligeros se añadió un
					agente espumante a la mezcla de jales/concreto y se utilizó una malla de fibra
					de vidrio para reforzar la estructura final. Los PPL resultantes (dimensiones:
					254 × 139.7 × 12.7 mm) mostraron una resistencia a la flexión de 5.80 ± 0.06
					MPa, densidad aparente de 712 ± 104 kg/m<sup>3</sup>, conductividad térmica de
					0.159 ± 0.004 W/m K, absorción de agua en masa de 59 ± 10 %, y potencial neto de
					neutralización (PNN = PN/PA) &gt; 1.2. De acuerdo con la normatividad mexicana,
					este valor de PNN implica que los PPL no son potenciales generadores de drenaje
					ácido.</p>
			</trans-abstract>
			<kwd-group xml:lang="en">
				<title>Key words:</title>
				<kwd>tailings reutilization</kwd>
				<kwd>foamed concrete</kwd>
				<kwd>fiber reinforced concrete</kwd>
			</kwd-group>
			<kwd-group xml:lang="es">
				<title>Palabras clave:</title>
				<kwd>reuso de jal de mina</kwd>
				<kwd>concreto celular</kwd>
				<kwd>concreto reforzado con fibra</kwd>
			</kwd-group>
			<funding-group>
				<award-group award-type="contract">
					<funding-source>Secretaría de Energía</funding-source>
					<funding-source>Consejo Nacional de Ciencia y Tecnología</funding-source>
					<award-id>266492</award-id>
				</award-group>
			</funding-group>
			<counts>
				<fig-count count="0"/>
				<table-count count="4"/>
				<equation-count count="3"/>
				<ref-count count="35"/>
				<page-count count="07"/>
			</counts>
		</article-meta>
	</front>
	<body>
		<sec sec-type="intro">
			<title>INTRODUCTION</title>
			<p>There is a growing interest in the development of sustainable options for the mining
				activity (<xref ref-type="bibr" rid="B12">Gorman and Dzombak 2018</xref>).
				Therefore, management of mining waste has become increasingly important (<xref
					ref-type="bibr" rid="B7">Aznar-Sánchez et al. 2018</xref>). Mine tailings are
				produced after separation of the valuable products from a mineral ore. A 2016
				estimate calculates their production rate from five to 14 billion tons per year
					(<xref ref-type="bibr" rid="B28">Schoenberger 2016</xref>). Large companies
				store the mine tailings behind dammed impoundments, often termed as tailing ponds or
				tailing dams (<xref ref-type="bibr" rid="B15">Kossoff et al. 2014</xref>).
				Unfortunately, the frequency of dam failures is high resulting in both economic and
				environmental issues (<xref ref-type="bibr" rid="B25">Páez-Osuna et al. 2015</xref>,
					<xref ref-type="bibr" rid="B28">Schoenberger 2016</xref>, <xref ref-type="bibr"
					rid="B13">Jain and Das 2017</xref>).</p>
			<p>The chemical composition of tailings depends on several factors of the mineralogy of
				the ore body, the processing fluids used to extract the economic metals, the
				efficiency of the extraction process, and the degree of weathering during storage
					(<xref ref-type="bibr" rid="B15">Kossoff et al. 2014</xref>). The presence of
				sulfide minerals is common, such as pyrite (FeS<sub>2</sub>), pyrrhotite
					(Fe<sub>1-x</sub>S), galena (PbS), sphalerite (ZnS), chalcopyrite
					(CuFeS<sub>2</sub>), or arsenopyrite (FeAsS). Sulfide minerals lead to the
				formation of acid mine drainage (AMD) by exposition to atmospheric oxygen,
				oxygenated waters, and bacteria (<xref ref-type="bibr" rid="B16">Lawrence and
					Scheske 1997</xref>, <xref ref-type="bibr" rid="B26">Plumlee 1999</xref>, <xref
					ref-type="bibr" rid="B14">Kefeni et al. 2017</xref>). AMD represents an
				environmental problem due to its high acidity, toxic metals, and sulfate contents.
				When deposits of calcite (CaCO<sub>3</sub>), dolomite
					(CaMg[CO<sub>3</sub>]<sub>2</sub>), and magnesite (MgCO<sub>3</sub>) occur in
				proximity of acid-generating sulfide minerals, they can react and consume some of
				the acid generated during sulfide oxidation (<xref ref-type="bibr" rid="B26">Plumlee
					1999</xref>). Otherwise, to fully suppress AMD generation, it is necessary to
				protect sulfide minerals from air, water, and bacteria (<xref ref-type="bibr"
					rid="B14">Kefeni et al. 2017</xref>).</p>
			<p>Portland cement was previously reported as an excellent alternative for the
				encapsulation, as well as chemical fixation, of the toxic mobile elements in
				tailings (<xref ref-type="bibr" rid="B22">Nehdi and Tariq 2007</xref>, <xref
					ref-type="bibr" rid="B27">Rachman et al. 2018</xref>). Examples of its
				application include the fabrication of synthetic gravel (<xref ref-type="bibr"
					rid="B35">Zuccheratte et al. 2017</xref>), concrete pavements (<xref
					ref-type="bibr" rid="B11">Gayana and Chandar 2018</xref>), and cemented paste
				backfill (<xref ref-type="bibr" rid="B20">Lu et al. 2018</xref>). It is known that
				Portland cement can be mixed with aggregates (sand, gravel, and rock), water, and
				small amounts of chemical admixtures to make concrete (<xref ref-type="bibr"
					rid="B1">Aljerf 2015</xref>). Hence, our research interests are focused on the
				utilization of tailings as aggregates in the fabrication of concrete, particularly
				foamed concrete.</p>
			<p>Foamed concrete is obtained by adding a foaming agent to the concrete mixture,
				resulting in a product with high flowability, low cement content, low aggregate
				usage and excellent thermal insulation (<xref ref-type="bibr" rid="B2">Amran et al.
					2015</xref>). In the construction industry, prefabrication and modularization
				are preferred because they improve worksite productivity, decrease waste generation
				in site, and improve project return on investment (<xref ref-type="bibr" rid="B8"
					>Construction 2011</xref>). In consequence, we proposed the fabrication of
				lightweight prefabricated panels (LPPs) that involve the use of tailings as
				replacement of aggregates in the foamed concrete. To further improve the concrete
				post-crack performance, polypropylene fibers are added during the preparation (<xref
					ref-type="bibr" rid="B34">Yin et al. 2015</xref>) and a fiberglass mesh is
				employed as reinforcement element.</p>
		</sec>
		<sec sec-type="materials">
			<title>MATERIALS</title>
			<p>Mine tailings were provided from La Guitarra, a silver mine located at the
				Temascaltepec municipality, State of Mexico, Mexico, owned by <xref ref-type="bibr"
					rid="B9">First Majestic Silver (2015)</xref>. The chemical analysis of the
				tailings was performed by the company following procedures according to the Official
				Mexican Standard NOM-141-SEMARNAT-2003 (<xref ref-type="bibr" rid="B29">SEMARNAT
					2004</xref>). Contents of metals were determined by extraction and subsequent
				analysis through atomic absorption spectroscopy (AAS) whereas the neutralization and
				acidity potentials were determined by the modified acid base accounting (ABA)
				procedure. Both methodologies are based on the ASTM D3987 (<xref ref-type="bibr"
					rid="B4">ASTM 2012</xref>) and the modified ABA test (<xref ref-type="bibr"
					rid="B17">Lawrence and Wang 1997</xref>), respectively. Characterization details
				are summarized in <xref ref-type="table" rid="t1">tables I</xref> and <xref
					ref-type="table" rid="t2">II</xref>.</p>
			<p>
				<table-wrap id="t1">
					<label>TABLE I</label>
					<caption>
						<title>HEAVY METALS CONCENTRATION IN MINE TAILINGS DETERMINED BY ATOMIC
							ABSORPTION SPECTROSCOPY (AAS).</title>
					</caption>
					<table frame="hsides" rules="groups">
						<colgroup>
							<col/>
							<col/>
							<col/>
						</colgroup>
						<tbody>
							<tr>
								<td align="center">Element</td>
								<td align="center">Measured concentration by AAS (mg/L)</td>
								<td align="center">MPL<sup>a</sup> (mg/L)</td>
							</tr>
							<tr>
								<td align="center">As</td>
								<td align="center">&lt; 0.005</td>
								<td align="center">5.0*</td>
							</tr>
							<tr>
								<td align="center">Ba</td>
								<td align="center">0.24</td>
								<td align="center">100.0*</td>
							</tr>
							<tr>
								<td align="center">Be</td>
								<td align="center">&lt; 0.005</td>
								<td align="center">1.22<sup>†</sup></td>
							</tr>
							<tr>
								<td align="center">Cr</td>
								<td align="center">&lt; 0.005</td>
								<td align="center">5.0*</td>
							</tr>
							<tr>
								<td align="center">Ag</td>
								<td align="center">&lt; 0.005</td>
								<td align="center">5.0*</td>
							</tr>
							<tr>
								<td align="center">Pb</td>
								<td align="center">&lt; 0.005</td>
								<td align="center">5.0*</td>
							</tr>
							<tr>
								<td align="center">Se</td>
								<td align="center">&lt; 0.005</td>
								<td align="center">1.0*</td>
							</tr>
						</tbody>
					</table>
					<table-wrap-foot>
						<fn id="TFN1">
							<label><sup>a</sup></label>
							<p>Maximum permissible limit according to Mexican standards</p>
						</fn>
						<fn id="TFN2">
							<p>*NOM-052-SEMARNAT-2005 (<xref ref-type="bibr" rid="B30">SEMARNAT
									2006</xref>), <sup>†</sup>NOM-157-SEMARNAT-2009 (<xref
									ref-type="bibr" rid="B31">SEMARNAT 2011</xref>).</p>
						</fn>
					</table-wrap-foot>
				</table-wrap>
			</p>
			<p>
				<table-wrap id="t2">
					<label>TABLE II</label>
					<caption>
						<title>NEUTRALIZATION POTENTIAL (NP), ACIDITY POTENTIAL (AP), PH, AND
							HUMIDITY PERCENTAGE IN MINE TAILINGS OBTAINED FROM LA GUITARRA.</title>
					</caption>
					<table frame="hsides" rules="groups">
						<colgroup>
							<col/>
							<col/>
							<col/>
							<col/>
						</colgroup>
						<tbody>
							<tr>
								<td align="center">NP (kg of CaCO3/ton)</td>
								<td align="center">AP (kg of CaCO<sub>3</sub>/ton)</td>
								<td align="center">pH</td>
								<td align="center">Humidity (%)</td>
							</tr>
							<tr>
								<td align="center">0</td>
								<td align="center">2.22</td>
								<td align="center">3.89</td>
								<td align="center">14.3</td>
							</tr>
						</tbody>
					</table>
				</table-wrap>
			</p>
			<p>Because of their high water content, tailings were first sun-dried. Then, larger
				aggregates were separated using a sieve (mesh size of 2.5 mm).</p>
			<p>Type II Portland cement (CEMEX, Monterrey, Mexico), fiber glass mesh (60
					g/m<sup>2</sup>, FIBRAMALLA), foaming agent (Barracel, BASF), polypropylene
				fiber (Sika Fiber, SIKA), and vinyl sealant (Daraweld BA3S, GRACE) were bought at
				local hardware stores. Potable water was employed in all procedures.</p>
			<p>Molds for the LPPs fabrication were built using four pieces of galvanized steel angle
				(3/4 × 3/4 × 1/8 in). These molds were secured with hexagonal bolts and nuts to
				obtain a rectangular form of 254 × 139.7 × 12.7 mm (length × width × height). It is
				highly important that molds are greased before the application of the concrete
				mixture to facilitate their cleaning and reuse.</p>
		</sec>
		<sec sec-type="methods">
			<title>METHODS</title>
			<sec>
				<title>Determination of the neutralization potential of LPPs</title>
				<p>Specimens were tested according to NOM-141-SEMARNAT-2003 (<xref ref-type="bibr"
						rid="B29">SEMARNAT 2004</xref>), which is based on the modified ABA test
						(<xref ref-type="bibr" rid="B17">Lawrence and Wang 1997</xref>), following
					the same procedure as in the tailings characterization.</p>
				<sec>
					<title><italic>General procedure for LPPs fabrication</italic></title>
					<p>The quantities employed to obtain the different test specimens are summarized
						in <xref ref-type="table" rid="t3">table III</xref> and were based on the
						previous experience of one of the authors in the masonry work with concrete
						(I.V.F.). Typically, mine tailings, polypropylene fiber, and cement are dry
						mixed in a bucket until a homogeneous mixture is obtained. Then, water and
						vinyl sealant are added. The mixing continues until a uniform paste forms.
						The foaming agent and water were mixed in a separate container until
						homogenization prior to their addition to the paste. The resulting product
						is poured into the molds to form their final shape. Afterwards, the
						fiberglass mesh (254 cm x 139.7 cm) is placed into the mold. Finally, the
						concrete is cured every 24 hours with water for three days.</p>
					<p>
						<table-wrap id="t3">
							<label>TABLE III</label>
							<caption>
								<title>MATERIALS EMPLOYED IN THE FABRICATION OF DIFFERENT TEST
									SPECIMENS OF LPPs. NUMBERS CORRESPOND TO THE WEIGHT
									PERCENTAGE.</title>
							</caption>
							<table frame="hsides" rules="groups">
								<colgroup>
									<col/>
									<col span="4"/>
								</colgroup>
								<tbody>
									<tr>
										<td align="justify" rowspan="2">Material</td>
										<td align="center" colspan="4">Test specimen </td>
									</tr>
									<tr>
										<td align="center">1</td>
										<td align="center">2</td>
										<td align="center">3</td>
										<td align="center">4</td>
									</tr>
									<tr>
										<td align="justify">Cement</td>
										<td align="center">27.7</td>
										<td align="center">24.7</td>
										<td align="center">24.4</td>
										<td align="center">26.5</td>
									</tr>
									<tr>
										<td align="justify">Tailings</td>
										<td align="center">41.6</td>
										<td align="center">44.4</td>
										<td align="center">44.0</td>
										<td align="center">47.7</td>
									</tr>
									<tr>
										<td align="justify">Water</td>
										<td align="center">20.3</td>
										<td align="center">21.7</td>
										<td align="center">21.5</td>
										<td align="center">17.0</td>
									</tr>
									<tr>
										<td align="justify">Water/foaming agent</td>
										<td align="center">5.5</td>
										<td align="center">5.9</td>
										<td align="center">4.9</td>
										<td align="center">5.3</td>
									</tr>
									<tr>
										<td align="justify">Foaming agent</td>
										<td align="center">0.3</td>
										<td align="center">0.3</td>
										<td align="center">0.2</td>
										<td align="center">0.3</td>
									</tr>
									<tr>
										<td align="justify">Vinyl sealant</td>
										<td align="center">4.6</td>
										<td align="center">3.0</td>
										<td align="center">4.9</td>
										<td align="center">3.2</td>
									</tr>
								</tbody>
							</table>
						</table-wrap>
					</p>
				</sec>
				<sec>
					<title><italic>Characterization of LPPs</italic></title>
					<p>The apparent density, thermal conductivity, water vapor permeability,
						humidity adsorption, and water absorption were measured for the LPPs because
						Mexican Official Standard NOM-018-ENER-2011 (<xref ref-type="bibr" rid="B32"
							>SENER 2011</xref>) requires them to be reported for the
						commercialization of thermal insulating materials, with no specified
						reference values. Additionally, the flexural strength of panels was
						determined.</p>
					<p>The apparent density is defined as the ratio between the apparent volume
						(i.e., including voids) and the mass of the sample. The following equation
						was employed:</p>
					<p>
						<disp-formula id="e1">
							<mml:math id="m1" display="block">
								<mml:msub>
									<mml:mrow>
										<mml:mi>ρ</mml:mi>
									</mml:mrow>
									<mml:mrow>
										<mml:mi>a</mml:mi>
										<mml:mi>p</mml:mi>
									</mml:mrow>
								</mml:msub>
								<mml:mo>=</mml:mo>
								<mml:mfrac>
									<mml:mrow>
										<mml:mi>m</mml:mi>
									</mml:mrow>
									<mml:mrow>
										<mml:mi>V</mml:mi>
									</mml:mrow>
								</mml:mfrac>
							</mml:math>
							<label>(1)</label>
						</disp-formula>
					</p>
					<p>where <italic>ρ</italic>
						<sub>
							<italic>ap</italic>
						</sub> is the apparent density (kg/m<sup>3</sup>), <italic>m</italic> is the
						mass of the specimen (kg), and <italic>V</italic> is the volume of the
						specimen (m<sup>3</sup>).</p>
					<p>Thermal conductivity was determined by using a quasicubic arrangement, as
						proposed by <xref ref-type="bibr" rid="B10">Díaz and Tibaquirá
						(2008)</xref>. Five sides of the cube consist of expanded polystyrene
						(thermal insulator) with a panel that is placed at the top side. A lamp,
						inside the cube, was used as the heat source, and temperature differences
						were measured using an infrared thermometer. Finally, thermal conductivity
						was estimated using the Fourier’s equation of heat conduction:</p>
					<p>
						<disp-formula id="e2">
							<mml:math id="m2" display="block">
								<mml:msub>
									<mml:mrow>
										<mml:mi>Q</mml:mi>
									</mml:mrow>
									<mml:mrow>
										<mml:mi>c</mml:mi>
										<mml:mi>o</mml:mi>
										<mml:mi>n</mml:mi>
										<mml:mi>d</mml:mi>
									</mml:mrow>
								</mml:msub>
								<mml:mo>=</mml:mo>
								<mml:mo>-</mml:mo>
								<mml:mi>k</mml:mi>
								<mml:mi>A</mml:mi>
								<mml:mfrac>
									<mml:mrow>
										<mml:mi>Δ</mml:mi>
										<mml:mi>T</mml:mi>
									</mml:mrow>
									<mml:mrow>
										<mml:mi>Δ</mml:mi>
										<mml:mi>X</mml:mi>
									</mml:mrow>
								</mml:mfrac>
							</mml:math>
							<label>(2)</label>
						</disp-formula>
					</p>
					<p>where <italic>Q</italic>
						<sub>
							<italic>cond</italic>
						</sub> is the heat flow rate by conduction (W), <italic>k</italic> is the
						thermal conductivity of the panel (W/m K), <italic>A</italic> is the
						cross-sectional area normal to direction of heat flow (m<sup>2</sup>),
							Δ<italic>T</italic> is the temperature difference (K), and
							Δ<italic>X</italic> is the panel thickness (m).</p>
					<p>For water absorption experiments, test specimens were submerged horizontally
						in a container with water at 20 ± 1 ºC. After 24 h, water was removed and
						the specimen suspended to drain for 10 min. The weight difference, before
						and after submersion, corresponds to the water absorbed by the panel.</p>
					<p>Moisture adsorption was determined by placing the specimens inside an
						aluminum chamber with relative humidity of 100 % and temperature of 20 ºC
						for 24 h. The weight difference, before and after the treatment, corresponds
						to the moisture adsorbed by the specimen.</p>
					<p>Water vapor permeability was measured following a procedure similar to the
						ASTM E96/E96M method (<xref ref-type="bibr" rid="B5">ASTM 2016a</xref>). A
						test dish impermeable to water was filled with distilled water and the test
						specimen was attached to the dish by sealing. Distance from the water level
						to the test specimen was 20 mm. This assembly was placed on a horizontal
						surface and weighed periodically to determine the change in mass as a
						function of time. A slope value was determined from the linear regression of
						the straight line obtained. When the slope value is divided by the test area
						(cup mouth area), the rate of water vapor transmission (WVT) of the specimen
						is obtained. Later, permeance was obtained using the following equation:</p>
					<p>
						<disp-formula id="e3">
							<mml:math id="m3" display="block">
								<mml:mi>P</mml:mi>
								<mml:mi>e</mml:mi>
								<mml:mi>r</mml:mi>
								<mml:mi>m</mml:mi>
								<mml:mi>e</mml:mi>
								<mml:mi>a</mml:mi>
								<mml:mi>n</mml:mi>
								<mml:mi>c</mml:mi>
								<mml:mi>e</mml:mi>
								<mml:mo>=</mml:mo>
								<mml:mfrac>
									<mml:mrow>
										<mml:mi>W</mml:mi>
										<mml:mi>V</mml:mi>
										<mml:mi>T</mml:mi>
									</mml:mrow>
									<mml:mrow>
										<mml:mi>S</mml:mi>
										<mml:mfenced separators="|">
											<mml:mrow>
												<mml:msub>
												<mml:mrow>
												<mml:mi>R</mml:mi>
												</mml:mrow>
												<mml:mrow>
												<mml:mn>1</mml:mn>
												</mml:mrow>
												</mml:msub>
												<mml:mo>-</mml:mo>
												<mml:msub>
												<mml:mrow>
												<mml:mi>R</mml:mi>
												</mml:mrow>
												<mml:mrow>
												<mml:mn>2</mml:mn>
												</mml:mrow>
												</mml:msub>
											</mml:mrow>
										</mml:mfenced>
									</mml:mrow>
								</mml:mfrac>
							</mml:math>
							<label>(3)</label>
						</disp-formula>
					</p>
					<p>where <italic>S</italic> is the saturation vapor pressure at the test
						temperature (25 ºC), <italic>R</italic>
						<sup>
							<italic>1</italic>
						</sup> is the relative humidity in the dish, and <italic>R</italic>
						<sup>
							<italic>2</italic>
						</sup> is the relative humidity at the vapor sink.</p>
					<p>Finally, tension tests were carried out using an INSTRON 3360 Series Dual
						Column Tabletop Testing System. A given panel was supported near the ends
						(2.54 cm) and a central load was applied at 50 mm/min until the specimen
						fracture.</p>
				</sec>
			</sec>
		</sec>
		<sec sec-type="results|discussion">
			<title>RESULTS AND DISCUSSION</title>
			<sec>
				<title>Toxicity of LPPs</title>
				<p>From data in <xref ref-type="table" rid="t1">table I</xref>, it is clear that
					mine tailings from La Guitarra do not represent a hazardous waste in terms of
					their metals’ content according to Mexican standards (<xref ref-type="bibr"
						rid="B29">SEMARNAT 2004</xref>). However, their lack of neutralization
					potential along with their acidity potential (2.22 kg of CaCO<sub>3</sub> per
					ton of tailings) made necessary to stabilize them by making foamed concrete.
					After fabrication of the LPPs, the NP and AP of the panels were determined to
					evaluate their potential as generators of AMD. A value of NNP = 193 ± 1.5 was
					obtained, which indicates that LPPs do not represent a hazardous waste in terms
					of the Mexican standards (<xref ref-type="bibr" rid="B29">SEMARNAT 2004</xref>).
					It has been mentioned that the effectiveness of Portland cement to stabilize
					mine tailings is mainly derived by the encapsulation, as well as chemical
					fixation, of their toxic components (<xref ref-type="bibr" rid="B22">Nehdi and
						Tariq 2007</xref>, <xref ref-type="bibr" rid="B27">Rachman et al.
						2018</xref>).</p>
			</sec>
			<sec>
				<title>Characterization of LPPs</title>
				<p>The properties measured for every test specimen after characterizations are shown
					in <xref ref-type="table" rid="t4">table IV</xref>.</p>
				<p>
					<table-wrap id="t4">
						<label>TABLE IV</label>
						<caption>
							<title>PROPERTIES MEASURED FOR THE TEST SPECIMENS OBTAINED.</title>
						</caption>
						<table frame="hsides" rules="groups">
							<colgroup>
								<col/>
								<col/>
								<col/>
								<col/>
								<col/>
								<col/>
								<col/>
								<col/>
							</colgroup>
							<tbody>
								<tr>
									<td align="center">Test specimen</td>
									<td align="center">Flexural strength (MPa)</td>
									<td align="center">Apparent density (kg/m<sup>3</sup>)</td>
									<td align="center">Water absorption (% mass)</td>
									<td align="center">Humidity absorption (% mass)</td>
									<td align="center">Permeance (perm)</td>
									<td align="center">Thermal conductivity (W/m K)</td>
									<td align="center">NNP</td>
								</tr>
								<tr>
									<td align="center">1</td>
									<td align="center">5.76</td>
									<td align="center">613.4</td>
									<td align="center">71.9</td>
									<td align="center">14.2</td>
									<td align="center">31.47</td>
									<td align="center">0.162</td>
									<td align="center">195</td>
								</tr>
								<tr>
									<td align="center">2</td>
									<td align="center">5.78</td>
									<td align="center">656.7</td>
									<td align="center">59.3</td>
									<td align="center">8.2</td>
									<td align="center">26.9</td>
									<td align="center">0.163</td>
									<td align="center">192</td>
								</tr>
								<tr>
									<td align="center">3</td>
									<td align="center">5.78</td>
									<td align="center">853.1</td>
									<td align="center">46.4</td>
									<td align="center">2.1</td>
									<td align="center">43.54</td>
									<td align="center">0.159</td>
									<td align="center">192</td>
								</tr>
								<tr>
									<td align="center">4</td>
									<td align="center">5.89</td>
									<td align="center">725.0</td>
									<td align="center">59.4</td>
									<td align="center">5.4</td>
									<td align="center">22.01</td>
									<td align="center">0.153</td>
									<td align="center">194</td>
								</tr>
							</tbody>
						</table>
						<table-wrap-foot>
							<fn id="TFN3">
								<p>NNP: net neutralization potential.</p>
							</fn>
						</table-wrap-foot>
					</table-wrap>
				</p>
				<p>It is worth to mention that early experiments lead us to the use of a fiberglass
					mesh as reinforcement element in the panels. The mesh accounted for up to 95 %
					of the flexural strength in the panels. As can be seen in <xref ref-type="table"
						rid="t4">table IV</xref>, minimal variations between the test specimens were
					found; also, if the mesh is not used, cracks appear in the PPLs. Thus, we
					decided to use the mesh, then statistical changes were difficult to observe. The
					properties of foamed concrete as a function of their composition was previously
					reviewed in the literature (<xref ref-type="bibr" rid="B2">Amran et al.
						2015</xref>, <xref ref-type="bibr" rid="B21">Ma and Chen 2016</xref>) and a
					similarly extensive study was not considered necessary for LPPs.</p>
				<p>According to their apparent density values (&lt; 1800 kg/m<sup>3</sup>), LPPs
					correspond to lightweight concrete. In general, flexural strength in concrete is
					about 10 to 20 % of its compressive strength (<xref ref-type="bibr" rid="B23"
						>NRMCA 2000</xref>). This indicates that specimens 1-4 are not suitable for
					structural applications neither by density (<xref ref-type="bibr" rid="B6">ASTM
						2016b</xref>) nor strength (<xref ref-type="bibr" rid="B24">NRMCA
						2003</xref>). However, LPPs are not intended for a structural application,
					but as an alternative for non-structural applications such as housing
					(insulating walls, sidings, or roof material).</p>
				<p>Thermal conductivity values (<italic>k</italic>) are not directly comparable with
					other foamed concrete systems. This is due to the addition of mine tailings as
					aggregates. The comparison of typical thermal conductivity values for foamed
					concretes (<xref ref-type="bibr" rid="B3">Asadi et al. 2018</xref>) in the
					density range of 400-1600 kg/m<sup>3</sup> (0.15 - 0.57 W/m K) indicates that
					LPPs 1-4 are similar in terms of thermal behavior (0.15 - 0.16 W/m K). Foamed
					concrete is considered an excellent thermal insulator because of its low
						<italic>k</italic> values. Such behavior is mainly due to the enhanced
					amount of air inside the cement matrix (<xref ref-type="bibr" rid="B3">Asadi et
						al. 2018</xref>).</p>
				<p>The lack of water and moisture absorption studies in foamed concrete with
					tailings made necessary to compare our results with nearly equivalent systems.
						<xref ref-type="bibr" rid="B21">Ma and Chen (2016)</xref> have reported the
					properties of foamed concrete (target density: 550 kg/m<sup>3</sup>) made of
					ordinary Portland cement, silica fume, water, polypropylene fiber, a type of
					naphtalene-based superplasticizer and foam stabilizer agents. Water absorption,
					by volume, determined after 48 h of concrete soaking is 68.5 % and the moisture
					absorption value obtained at 100 % relative humidity is 46.4 kg/m<sup>3</sup>.
					Those values are closely related to the obtained for test specimens 1-4
					confirming the seamless integration of tailings in foamed concrete.</p>
				<p>The water vapor permeance of materials is commonly reported in the construction
					trade with the unit “perm” rather than their SI equivalent (g/Pa s
					m<sup>2</sup>). The values obtained for the LPPs (&gt; 10 perm) indicate that
					panels are classified as vapor permeable. This characteristic is convenient for
					their use in certain hygrothermal conditions (<xref ref-type="bibr" rid="B19"
						>Lstiburek 2002</xref>).</p>
			</sec>
		</sec>
		<sec sec-type="conclusions">
			<title>CONCLUSIONS</title>
			<p>Mine tailings obtained from a silver mine were mixed with Portland cement, water, and
				other additives to obtain foamed concrete, which is useful in the fabrication of
				LPPs. Their properties were similar to those reported for common foamed concrete,
				indicating that tailings are suitable as a replacement of aggregates in foamed
				concrete mixtures. Moreover, characterization indicates that LPPs have a net
				neutralization potential of 192, proving the stabilization of tailings and that
				there is no risk of AMD generation.</p>
			<p>
				<xref ref-type="bibr" rid="B18">Lottermoser (2011)</xref> describes waste-management
				practices with a hierarchy in the following order: (<italic>i</italic>) prevention,
					(<italic>ii</italic>) reuse, (<italic>iii</italic>) recycling,
					(<italic>iv</italic>) energy recovery, and (<italic>v</italic>) treatment and
				disposal. As tailing´s generation is inevitable, their use as aggregates replacement
				in foamed concrete is a promising alternative for their handling. Other uses of
				foamed concrete include their use as backfill material for heat preservation pipes,
				foundation for the highway roads, fire insulation, trench reinstatement, etc. (<xref
					ref-type="bibr" rid="B33">Tan et al. 2014</xref>, <xref ref-type="bibr" rid="B2"
					>Amran et al. 2015</xref>). This leaves us with plenty of opportunities
				remaining unexplored in the replacement of aggregates with mine tailings.</p>
		</sec>
	</body>
	<back>
		<ack>
			<title>ACKNOWLEDGMENTS</title>
			<p>We would like to recognize the support of Secretaría de Energía-Consejo Nacional de
				Ciencia y Tecnología (Fondo SENER-CONACyT), project 266492, Engineering and science
				applied to the semi-desert energy sector in the north of the country, to perform
				this study.</p>
		</ack>
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