Life Cycle Assessment of Alkali Activated Cement compared to Ordinary Portland Cement

Approximately 8% of the global emissions of CO2 are originated by the cement industry, which consumes on average between 4 to 6 GJ per ton of cement. Ordinary Portland Cement (OPC) is the most used cement for construction purposes. Every year, around 4 billion tonnes (Gt) of OPC are manufactured. For each kg of OPC produced, 0.81 kg of CO2 is generated. Therefore, seeking cements with more environmentally friendly manufacturing process, economically viable, and socially relevant is necessary. One of the most promising materials are the Alkali-Activated Cements (AAC), where its components are an aluminosilicate precursor and alkaline activators. The precursor used in this study is Weathered Bottom Ash (WBA), a waste obtained from the Municipal Solid Waste Incineration (MSWI). On the other hand, the alkaline activators are sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). The name of the AAC developed is Sustainable-AAC (Sust-AAC). This project is focused on searching for new materials that could reduce the use of OPC as a building material. To be able to assess the associated environmental impacts, a comparison between AAC and OPC (CEM I) through a Life Cycle Assessment (LCA) following the standards ISO 14040:2006 and ISO 14044:2006 is performed. The LCA methodology allows identification and quantification of relevant inputs and outputs of the system, thus, evaluating the potential environmental impacts associated. The system boundary of this project is cradle-to-gate and the functional unit of the assessment is 1 ton of commercial cement. The OPC inventory is carried out through the values obtained in GaBi Software and the Sust-AAC inventory is made from the previous studies performed in the DIOPMA research group, on a laboratory scale. The results show that the OPC has higher impact on global warming, energy consumption, water consumption, and mineral extraction categories compared to Sust-AAC. In OPC manufacturing, the kiln stage is the most energy intensive stage (by the chemical reaction and by the fossil fuel requirement) and therefore, has the most significant environmental impact in terms of CO2 emissions and energy consumption. In contrast, the highest environmental impacts on the Sust-AAC are due to the production of Na2SiO3. The main reason for the energysaving is because Sust-AAC production does not need a kiln with high temperatures. In addition, the use of waste as raw material promotes a circular economy and, at the same time, reduces the extraction of natural resources. Then, the environmental performance in the Sust-AAC is promising compared to OPC. Sust-AAC is suitable to be used as lightweight material and as insulation material for thermal insulating applications. This application can contribute to realising operational energy savings and performance benefits

Evaluation of mechanically activated kaolin as alkali-activated material precursor

The cement industry is one of the most polluting worldwide, therefore the finding of new binding materials is mandatory. In this regard, alkali-activated materials (AAMs) rise as one of the most promising alternatives to ordinary Portland cement. AAMs are produced through the reaction of aluminosilicate solid precursors with alkaline activator solutions, forming a gel that hardens resulting in a binder material. Many precursors have been assessed, and recently, the trend is to use waste or by-products as precursors since aside from reducing the carbon footprint in the cement production, second life is given to residues that otherwise would probably be disposed of. However, the use of waste and by-products presents some drawbacks, e.g., the variability of the composition of the waste depending on the place and the season, as well as the availability of waste. The huge demand of raw materials for the cement industry could not be reached with residues. Hence, the optimal precursor depends on the local availability of suitable raw materials [1]. Clays and clay minerals are worldwide available and can easily meet the raw materials’ demand of the cement industry [2]. Nonetheless, the reactivity of clay precursors is low, and a previous activation of the material must be performed. The most common activation process is carried out through a thermal process, where the clay structure undergoes a dehydroxylation stage to remove the hydroxyls and amorphize the structure. Depending on the clay type and properties, the thermal dehydroxylation requires temperatures between 500 and 900 ºC. The energy consumption during this process is an important downside in front of waste-based precursors. Therefore, the activation of the clay precursor should be achieved with low energy-requiring processes. In this sense, mechanical activation (MA) arises as a feasible activation method, which could reduce the required energy compared to thermal dehydroxylation. Nevertheless, the reactivity of mechanically activated clays is barely studied. This work aims to assess the reactivity of mechanically activated kaolin for its use as precursor for AAMs production.Peer reviewe

Potential reactivity assessment of mechanically activated kaolin as alternative cement precursor

This work aims to assess the potential reactivity of a mechanically activated kaolin for its use as an alternative cement precursor. The mechanical activation was successfully achieved by grinding at different rotation speeds (250, 300, and 350 rpm) and times (60, 90, and 120 min), obtaining a highly amorphous and reactive material. The amorphization was monitored through XRD with amorphous content estimation and FTIR deconvolution. The potential reactivity was evaluated through chemical attacks, obtaining SiO2 and Al2O3 availabilities up to 95 wt% and 93 wt% of total SiO2 and Al2O3 content. XRD and FTIR also allowed the identification of the nonreactive phases on the insoluble residue after chemical attacks. NMR analysis revealed that part of the aluminium was in IV coordination, limiting the reactivity in the alkali activation process. This work demonstrated the effectiveness of mechanical activation as a greener treatment than thermal dehydroxylation to increase the raw kaolin's reactivity.Peer reviewe

Depth-sensing indentation applied to polymers: A comparison between standard methods of analysis in relation to the nature of the materials

Mechanical data (hardness and elastic modulus) from instrumented indentation testing are often extracted assuming linear elasticity in the initial portion of the unloading. The method is nowadays widely accepted as a convenient tool to interpret depth-sensing data, however it is a matter of controversy when applied to polymer materials due to their time-dependent behavior. More recently, Loubet and co-workers applied continuous stiffness measurements (CSM), consisting of superimposing a small oscillation to the quasi-static component of loading, to the study of the mechanical properties of polymers and proposed a new model to account for the apparent increase in the contact area detected at the first stages of contact. The present work offers a comparative study between the Loubet's model using CSM and the procedure yielding a single reading from the onset of unloading. A wide range of thermoplastic polymer materials including glassy and semicrystalline polymers have been investigated. The most important equations employed for each method are summarized and the advantages and disadvantages of employing one procedure or the other are discussed. The differences found between the results obtained from both approaches are discussed in relation to the nature of the polymer material. A comparison between mechanical data extracted from indentation measurements and from classical dynamic mechanical analysis is offered. © 2013 Elsevier Ltd. All rights reserved.One of us (AF) thanks the MICINN (Ministerio de Ciencia e Innovación), Spain, for financial support under the grant FIS2010-18069.Peer Reviewe