Carbonatación de morteros de escoria activados alcalinamente e híbridos: estudio mediante microscopía Raman Confocal y la repercusión en las prestaciones de desgaste

This work aims to contribute to reducing environmental damage caused by the manufacturing of Portland cements (PC), through in-depth exploration into the durability of two mortars manufactured from blast furnace slag: an alkaline-activated one (AAS) and a hybrid cement (HS) with less than 20% clinker. The carbonation resistance of these eco-friendly mortars is compared to that of a mortar based on Portland IV cement. From a mineralogical point of view, DTA-TG and confocal Raman microscopy (CRM) tests have been carried out, along with measurement of pH changes, compression strength and total porosity. Böhme tests have been performed to evaluate changes due to carbonation in the wear behavior of the mortars under study. Using the CRM technique, it has been possible to establish a relationship between the carbonation of the systems with the unbound carbon content, as well as identify the different polymorphic phases of CaCO3 formed. The results obtained reveal that alternative AAS and HS mortars are more difficult to carbonate than Portland cement mortars, and that the effect of this process on the porosity depends on the nature of the hydroxides previously present in the pore solution. The carbonation of the surfaces also improves the abrasive wear resistance of the mortars under study.This study was funded by the Ministerio de Ciencia, Innovación y Universidades of Spain through project RTI2018-096428-B-I00 and by the Madrid Government (Comunidad de Madrid) under the Multiannual Agreement with UC3M in the line of “Fostering Young Doctors Research” (HORATSOCM-UC3M) within the context of the V PRICIT (Regional Programme of Research and Technological Innovation).Publicad

Hybrid cements: towards their use as alternative and durable materials against wear

Hybrid cements (HC) are a possible replacement of Portland cement, as their activation process only requires a small amount of sulphates, and is carried out with water at room temperature. The present work aims to study the wear behaviour of HC manufactured from two different wastes: blast furnace slag (HS) and fly ash (HFA). Their wear behaviour was compared to Portland cements (CEM I and CEM IV). Reciprocating wear tests and rolling wear Böhme tests were carried out for pastes and mortars, respectively. Wear tracks in the pastes were analysed through opto-digital microscopy and electron microscopy (SEM). In the mortars, the mechanical resistance was measured to understand different behaviours. The results obtained reveal that HC pastes and mortars present lower wear losses than CEM IV and similar to CEM I.This work has been supported by the Madrid Government (Comunidad de Madrid) under the Multiannual Agreement with UC3M in the line of "Fostering Young Doctors Research" (HORATSO-CM-UC3M) in the context of the V PRICIT (Regional Programme of Research and Technological Innovation), and by the Ministerio de Ciencia, Innovación y Universidades of Spain through project RTI2018-096428-B-I00

Mineralogical and microstructural changes in alkali-activated and hybrid materials exposed to accelerated leaching

Alkali-activated materials (AAM) and hybrid cements (HC) have become sustainable alternatives to Portland cement (PC) due to their low carbon footprint. The main differences between AAM and HC lie in their content of clinker (none in the AAM and usually lower than 30% for HCs) and the type of activator used (strong alkaline solutions for AAM and small amounts of solid alkalis for HC). Durability problems related with microstructural changes due to decalcification and leaching of the cementitious paste have been well researched for PC pastes, but it is still not well known for AAM and HC. The present work aims to study the leaching process for cement pastes of both types of sustainable pastes. Blast furnace slag (BFS) was selected as a precursor to manufacture hybrid slag (HS) pastes and alkali-activated slag (AAS) pastes. A commercial CEM IV was selected as reference material. A 6 M NH4NO3 solution was used to accelerate leaching kinetics. After 28 days of immersion, the mineralogical and microstructural changes were evaluated. Results show that AAS pastes exhibited the highest leaching resistance of all the pastes under study, due to the absence of portlandite and the high level of polymerization of silicate chains. In HS pastes, the presence of portlandite (due to PC in the material) and gypsum (due to the activator) explains their intermediate performance, in between CEM IV and AAS.The authors have been able to carry out the present research thanks to financial support from the Ministerio de Ciencia, Innovación y Universidades of Spain (RTI2018-096428-B-I00 and PID2020-116738RJ-I00 projects) and Madrid Government (Comunidad de Madrid) under the Multiannual Agreement UC3M in the line of "Fostering Young Doctors Research" (HORATSO-CS-UC3M) in the context of the V PRICIT (Regional Programme of Research and Technological Innovation)

Durability of eco-friendly alternative materials to Portland cement

Esta tesis contiene artículos de investigación en anexo.Currently, there is a major problem regarding climate change. Solutions must be adopted to try to minimize greenhouse gas emissions (CO2, NOx, etc.), which are largely responsible for this problem. One of the main industries emiting large quantities of greenhouse gases is the cement industry, mainly due to the decarbonation process of the calcium carbonate (CaCO3) and the use of high temperatures (> 1400 ᵒC) during clinker production. Thus, the search for alternatives to ordinary Portland cement (OPC) has been a priority line of research in recent years. New cementitious materials are obtained from different aluminosilicates, either of natural origin (metakaolin) or as industrial by-products (blast furnace slag (BFS), fly ash (FA), etc.), called alkaline activated cementitious materials (AAM). More recently, the well-known hybrid materials (HM) have also been proposed. In addition, the use of different industrial by-products in the preparation of these alternative materials means a reuse of wastes, which would not end up in uncontrolled landfills. The durability of a cementitious system, regardless of whether it is OPC-based or based on different aluminosilicates, is one of the most important characteristics to evaluate, since these materials are susceptible to attack in a variety of exposures. The main aim of this Ph.D. thesis is the development of new eco-efficient construction materials, such as AAM and HM, the study of their mechanical and physical behavior, as well as their durability against different external agents, such as wear, carbonation, leaching, chloride penetration or corrosion of embedded steel reinforcements. Therefore, in this Ph.D. thesis, different precursors of aluminosilicate, such as BFS, FA and wastes from the ceramic industry, have been used. Different highly alkaline activators (for instance, sodium hydroxide (NaOH) and commercial sodium silicate (WG)) have been used for the formulation of these AAM systems. In addition, with the aim of reducing the use of the liquid alkaline activator, HM have been designed, composed of 80% of industrial by-products (BFS or FA) and 20% of OPC I, using between 3% (for BFS) and 5% (for FA) of Na2SO4 in solid state for their activation. The first partial aim was to study the mechanical behavior and wear durability of AAM and HM (both pastes and mortars). The results obtained provided the necessary information to determine the behavior of the material as a function of the type of precursor and activator used, establishing a comparison with a conventional system of ordinary type IV Portland cement (CEM IV). It was determined that the slag-based AAM and HM were those presenting the best mechanical and wear behavior. Subsequently, the resistance to carbonation and leaching of the AAS and HM materials was evaluated, comparing them with CEM IV one. Both environmentalfriendly materials showed a favorable behavior, mainly due to their low porosity and the chemical composition of their main hydration products. On the other hand, to determine the diffusion of chlorides in the mortars and the corrosion durability of the reinforcing bars, AAS, hybrid slag (HS) and hybrid fly ash (HFA) reinforced mortars samples were prepared, using thermomechanically treated (TMT) carbon steel as reinforcement. The results were compared with those of a conventional CEM IV reinforced system. The porosity is an important factor for these chloride diffusion studies. It has been found that, for AAS systems, the embedded ribbed steel presented similar corrosion intensities than steel embedded in CEM IV, despite of the cracks caused by the high shrinkage suffered by these materials when the precursor used is slag activated with sodium silicate with high SiO2/Na2O ratios (WG 1.2). Consequently, new alternative activators to replace the WG 1.2, such as WG 0.8, Na2CO3 or NaOH, were evaluated to study the shrinkage behavior and corrosion performance of these systems. MgO additions to precursor were also explored as a way to reduce shrinkage. Finally, the fatigue strength of TMT reinforced steels corroded in different systems after exposure to aggressive environments has been evaluated. The most relevant result of this Ph.D. thesis is that there are alternatives to reduce the environmental impact generated by the Portland cement industry through the development of new, more sustainable materials based on different wastes. From mechanical and durability points of view, some of the AAM and HM considered in this study present very satisfactory behavior, comparable with that of conventional Portland cement systems, suggesting, therefore, a good performance in service. Moreover, there still are materials to be further improved in certain properties that can affect their durability.Actualmente, existe una problemática importante en lo que se refiere al cambio climático. Es por ello por lo que se tienen que adoptar soluciones para intentar minimizar las emisiones de gases de efecto invernadero (CO2, NOx, etc), las cuales son las responsables, en gran medida, del problema existente. Una de las principales industrias que emiten grandes cantidades de gases de efecto invernadero es la industria cementera, debido, principalmente, al proceso de descarbonatación del carbonato de calcio (CaCO3) y al empleo de altas temperaturas (> 1400 oC) durante la producción del clínker. De este modo, la búsqueda de alternativas al cemento Portland ordinario (OPC) es una línea de investigación prioritaria en los últimos años, en donde a partir de diferentes aluminosilicatos, bien de origen natural (metacaolín) o como subproductos industriales (escorias de alto horno (BFS), cenizas volantes (FA), etc.) se obtienen nuevos materiales cementantes, denominados activados alcalinamente (AAM), o también los conocidos materiales híbridos (HM). Además, el empleo de diferentes subproductos industriales en la preparación de estos materiales alternativos supone una revalorización de los residuos, los cuales no acabarían en vertederos incontrolados. La durabilidad de un sistema cementante, independientemente de que sea en base a OPC o en base a diferentes aluminosilicatos, es una de las características más importantes a evaluar, ya que son materiales susceptibles de ser atacados en una variedad de exposiciones. El objetivo principal de la presente tesis doctoral es el desarrollo de nuevos materiales de construcción eco-eficientes, tanto AAM como HM, el estudio de su comportamiento mecánico y físico, así como la durabilidad frente a diferentes agentes externos, tales como al desgaste, la carbonatación, lixiviación, penetración de cloruros o la corrosión de armaduras de acero embebidas. Por lo expuesto anteriormente, en la presente tesis doctoral se han utilizado diferentes precursores de naturaleza silicoaluminato, tales como BFS, FA y residuos procedentes de la industria cerámica. Se han empleado diferentes activadores altamente alcalinos (hidróxido de sodio (NaOH), silicato sódico comercial (WG)) para la formulación de estos sistemas AAM. Además, con el objetivo de reducir el uso del activador alcalino, se han diseñado los HM, compuestos por un 80% de subproductos industriales (BFS o FA) y por un 20 de OPC I, empleando para su activación un 3% para las FA y 5% para BFS de Na2SO4 en estado sólido. El primer objetivo parcial planteado fue el estudio del comportamiento mecánico y la durabilidad frente al desgaste en pastas y morteros de AAM y HM. Los resultados obtenidos proporcionaron la información necesaria para poder determinar el comportamiento del material en función del tipo de precursor y del activador empleado, estableciendo una comparación frente a un sistema convencional de cemento Portland ordinario tipo IV (CEM IV). Se determinó que los AAM y HM de base escoria son los que presentaron un mejor comportamiento mecánico y frente al desgaste. Posteriormente se evaluó la resistencia a la carbonatación y lixiviación en los sistemas AAS y HM, comparándolos frente a un sistema de CEM IV, en donde ambos materiales presentaron un comportamiento favorable, debido, principalmente, a su baja porosidad y a la composición química de los principales productos de hidratación formados. Por otro lado, para determinar la difusión de cloruros en los morteros y la durabilidad frente a la corrosión de las barras de refuerzo, se prepararon morteros reforzados de AAS, híbridos de escoria (HS) y híbridos de ceniza volante (HFA), empleando como refuerzo un acero al carbono tratado termomecanicamente (TMT) y comparando los resultados con un sistema CEM IV convencional. La porosidad en estos estudios de difusión de cloruros es un factor importante, en donde aquellos sistemas AAS presentaron un comportamiento similar al CEM IV a pesar de presentar grietas debido a la alta retracción que presentan estos materiales cuando el precursor empleado es una escoria activada con silicato sódico con altas relaciones SiO2/Na2O (WG 1.2). Es por ello por lo que se evaluó el empleo de otros activadores diferentes al WG 1.2, tales como WG 0.8, Na2CO3 o NaOH, con el fin de estudiar el comportamiento y evolución de la retracción que experimentan estos sistemas, empleando, además, un porcentaje de MgO como precursor que permita reducir la retracción. Finalmente, se ha evaluado la resistencia a fatiga de los aceros TMT corroídos en diferentes sistemas tras un tiempo de exposición en ambientes agresivos. Como resultado más relevante de la presente Tesis Doctoral, se puede concluir que existen alternativas para reducir el impacto ambiental que genera la industria del cemento Portland mediante el desarrollo de nuevos materiales más sostenibles y en base a diferentes residuos. Desde un punto de vista mecánico y de durabilidad, estos AAM y HM presentan un comportamiento muy adecuado y comparable con los sistemas convencionales de cemento Portland, garantizando, por tanto, un buen comportamiento en servicio de cada uno de ellos, aunque aún son materiales objeto de estudio con el fin de mejorar más si cabe su comportamiento mecánico y durable.Ministerio de Ciencia, Innovación y Universidades of Spain (RT2018-096428-BI00). HORATSO-CS-UC3M: Madrid Government (Comunidad de Madrid) under the Multiannual Agreement UC3M in the line of “Fostering Young Doctors Research” in the context of the V PRICIT (Regional Programme of Research and technological Innovation.Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de MadridPresidente: Humberto Varum.- Secretario: Julián Jiménez Reinosa.- Vocal: Jessica Giró Palom

Sliding wear behavior of intermetallic Ti-45Al-2Nb-2Mn-(at%)-0.8vol%TiB2 processed by centrifugal casting and hot isostatic pressure: Influence of microstructure

This article belongs to the Special Issue Advanced Composite Material Design and Manufacturing Technology for Aerospace Engineering.Intermetallic alloys such as titanium aluminides (TiAl) are potential materials for aerospace applications at elevated temperatures. TiAl intermetallics have low weight and improved efficiency under aggressive environments. However, there is limited information about wear behavior of these alloys and their microstructure. The present work aims to study the influence of the microstructure in the tribological behavior of TiAl intermetallic alloy (45Al-2Mn-2Nb(at%)-0.8 vol%TiB2). Wear tests were performed on samples manufactured by centrifugal casting (CC) and hot isostatic pressure (HIP). Reciprocating sliding wear test was carried out for TiAl, it was combined with different loads and frequencies. Wear tracks were analyzed through opto-digital microscopy and electron microscopy (SEM). The results obtained reveal that CC intermetallics present the lowest volume wear lost, approximately 20% less than HIP intermetallics. This good behavior could be related to the high hardness material, associated with the main microstructure where CC intermetallic has nearly lamellar microstructure and HIP intermetallics present duplex microstructure

Wear behavior in pastes of alkali-activated materials: influence of precursor and alkali solution

Alkali-activated materials (AAM) are a possible alternative to Portland cement. Their wear performance opens a new door in the study of their durability for their use, in airport runways, highways and fabrication of construction materials. The wear behavior of AAM, under different activation conditions, is compared to Portland cement. Besides mechanical and physical characterization, reciprocating sliding wear tests were carried out. Information about the changes of structure and composition due to wear was obtained. Wear mechanism is abrasive for all pastes. Slag pastes activated with waterglass present the lowest volume wear loss, approximately 60% less than Portland cement. This good behavior could be related to the high hardness and stiffness of the material, associated with the main reaction products.This study was funded by the Ministerio de Ciencia, Innovación y Universidades of Spain through project RTI2018-096428-B-I00.Publicad

Eco-efficient hybrid cements: pozzolanic, mechanical and abrasion properties

One of the most polluting industries is the cement industry and, for this reason, alternative lines of research recommend the use of substitute materials for traditional Portland cement. This study proposes the use of industrial (slag and fly ash) and ceramic wastes for the total or partial replacement of Portland cement in the manufacturing of both alkaline-activated and hybrid cements. To carry out this study and evaluate the behavior of the proposed materials, different mortars were manufactured: Portland cement (CEM I), two alkaline-activated slag systems and six hybrid systems, with an 80&-20% waste-to-Portland-cement ratio for all the proposed wastes. An assessment of the pozzolanic activity was carried out for the different materials. The behavior of all the systems regarding mechanical resistance and durability to abrasion was studied. All the proposed materials, especially those with ceramic wastes, showed pozzolanic activity and suitable characteristics for use in the manufacturing of alternative cements. The mortar made of slag activated with waterglass presented the highest mechanical strength and lowest porosity, but the hybrid materials presented competitive results. After being subjected to the Böhme abrasion test, their effectiveness as substitutes for Portland cement is reiterated, some of them improving their durability to wear.This word was founded by the Ministerio de Ciencia, Innovación y Universidades of Spain through the project RTI2018-096428-B-I00 and by the regional project of Comunidad de Madrid, HORATSO- UC3M-C

Chloride-induced corrosion of steel reinforcement in mortars manufactured with alternative environmentally-friendly binders

There is increasing interest in exploring the durability of structures manufactured with cements that are more environmentally friendly than traditional Portland. Accordingly, protection against chloride-induced corrosion of carbon steel reinforcement in mortars manufactured with environmentally friendly hybrid cements and alkali-activated materials (AAM) needs to be analyzed in-depth. Hybrid cements are innovative alternative binders with reduced amount of clinker in their formulation (20-30%). Their precursors are aluminosilicate wastes and they are very feasible in-construction implantation because they are not activated neither with liquid solutions of strong alkalinity nor external heating. In this research, AAM mortars were manufactured using slag from industrial by-products activated with sodium silicate. Hybrid mortars were manufactured either from fly ash or from slag, and activated with Na2SO4. Portland cement (CEM IV) mortars were also included in this study as reference. Chlorides were made to penetrate into the four different mortars under study by diffusion. The porosity and chloride penetration were evaluated and related to the different materials. The electrochemical behavior of the steel reinforced mortar specimens was monitored by open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS) measurements. The corrosion rate values obtained for steel embedded in AAM were similar to conventional CEM IV mortars, and the morphology of the attack is less localized.The authors have been able to carry out the present research thanks to financial support from the Ministerio de Ciencia, Innovación y Universidades of Spain (RTI2018-096428-B-I00) and Madrid Government (Comunidad de Madrid) under the Multiannual Agreement UC3M in the line of Fostering Young Doctors Research (HORATSO-CS-UC3M) in the context of the V PRICIT (Regional Programme of Research and Technological Innovation), J.A. Donaire and D.M. Bastidas acknowledge funding from the University of Akron, Fellowship Program FRC-207160