Durability of lightweight geopolymers for passive fire protection: steel corrosion behavior in chloride-rich environment

Different technologies are currently developed as promising passive fire protective coatings, due to the fact that fire protection of steel structures is an important requirement for structural components for several civil and industrial applications. Among the others, geopolymers have attracted lot of attention as promising materials suitable for high temperature applications. An optimized mix-design makes their amorphous structure more stable, when exposed to direct fire or heating from high temperatures, compared to ordinary Portland cement-based materials (OPC). However, the durability of a fire protective coating strongly depends on its adhesion on steel and its ability to prevent and/or mitigate steel corrosion phenomena. For these reasons, the understanding of the corrosion behavior of steel coated with geopolymer-based fireproofing coatings is necessary for ensuring the service life of the structure. This study aims at characterizing the corrosion behavior of carbon steel coated by different geopolymeric mortars applied as passive fire protection systems. In particular, fly ash-based geopolymeric mortars were applied as coatings on carbon steel plates. They were lightened by the combination of lightweight aggregates, e.g. expanded perlite, and chemical foaming agents, such as hydrogen peroxide (H2O2), in order to ensure good properties at high temperatures. In addition, geopolymeric paste and mortar containing quartz aggregate were also prepared as reference samples. The corrosion process was evaluated using an electrochemical approach. The samples have been tested by accelerated ageing methods, such as exposure to salt spray chamber to simulate a chloride-rich environment, such as marine aerosol. The monitoring process has been done applying non-destructive techniques and it is still ongoing. In particular, open circuit potential (OCP) and linear polarization resistance (LPR) have been recorded during the exposure. In parallel, polarization curves have also been carried out at different stages of the ageing exposure to better characterize the corrosion condition of the steel substrates. In addition, adhesion between the different geopolymeric coatings and the carbon steel plates has been evaluated before and after the artificial ageing in the salt spray chamber. Finally, density and porosity measurements were also carried out to better characterize the physical properties of the geopolymers. In this contribute, preliminary results are reported about short-term exposure. They show that in absence of any aggressive species, fly ash-based geopolymeric mortars provide a highly alkaline environment in the early curing time, enabling the passivation of carbon steel. Finally, steel corrosion behavior has been analyzed as a function of the pore structure of the geopolymeric matrix

Fly ash-based one-part alkali activated mortars cured at room temperature: Effect of precursor pre-treatments

One-part or “just add water” alkali activated materials (AAMs) have attracted a lot of attention thanks to the use of solid alkaline activators that makes these materials more suitable to commercialization compared to conventional AAMs (two-part). This is mainly because large quantities of caustic solutions should be handled for producing conventional AAMs. So, one-part AAMs have a great potential for in-situ applications. However, heat curing (<100 ◦C) has been demonstrated to be the best condition to obtain optimized performances of one-part AAMs. This study investigates how to obtain high strength one-part alkali mortars cured at room temperature, considering a newly developed mix design, precursor pre-treatments and curing time. The mechanical performances (i.e., elasticity modulus, compressive and flexural strength) of the developed materials were reported, as well as physical properties, in terms of water absorption, open porosity and pore size distribution and microstructure (by means of FEG-SEM observations coupled with elemental analysis by EDS and FT-IR measurements). Class F fly ash have been activated by potassium hydroxide (KOH) and anhydrous sodium metasilicate. It was found that high strength one-part AAMs can be achieved by activating coal fly ash with a mix of KOH and anhydrous sodium metasilicate at room temperature. In particular, room temperature-cured one-part mortars obtained by pre-treated fly ash exhibited mechanical performance similar to those obtained by heat-cured mortars (at 70 ◦C, tested after 7 days), reaching a compressive strength (Rc) greater than 60 MPa at 28 days of curing when mechanochemical activation of fly ash was applied

Self-compacting concrete with recycled concrete aggregate: Study of the long-term properties

This paper investigates the shrinkage and creep of self-compacting concrete prepared with coarse and fine recycled concrete aggregates (up to 40% of total amount of aggregates). Physical properties and porosity measurements are studied and related to the mechanical properties. Results highlight that self-compacting characteristics are maintained when recycled aggregates are utilized and their good quality promotes high mechanical properties. Creep behavior and pores size distributions are more influenced by the content and assortment of recycled aggregates, although their effect is more limited compared to what occurs in traditional concrete with recycled aggregates

Molecular model of geopolymers with increasing level of disorder in the atomic structure

Concrete is the most used building material on Earth, but the production of its main binding component, cement, is responsible for 8% of worldwide CO2 emissions. A greener alternative cementitious material is provided by geopolymers, which can be synthetized from calcined clays and industrial by-products. A key issue, that limits the applicability of geopolymers in the construction sector, is an insufficient understanding of the relationship between their chemistry and development of long-term properties. Reducing these uncertainties requires an integrated approach combining modelling and experimentation. The binding phase of geopolymers often consists of sodium-alumino-silicate-hydrates (N-A-S-H), obtained through the reaction of a sodium silicate solution with an alumino-silicate source. Theoretical models describe this structure at the molecular scale as “pseudo-crystalline” [1] but, the existing models, based on solely amorphous or crystalline structures, are not always in agreement with this definition and with experimental results. For this reason, a defective crystalline structure is proposed here as a baseline geopolymer cell, featuring both amorphous and crystalline attributes (Figure 1). This new structure is created by creating vacancies in a sodalite crystalline cage, which is then stress-relaxed and reorganised to achieve full polymerisation of Al and Si tetrahedra while respecting the Loewenstein\u27s principle. Results are compared with experimental data and with other simulation results for amorphous and crystalline molecular models, showing that the newly proposed structures better capture important structural features with impact on mechanical properties, reconciling experiments with the “pseudo-crystalline” model. Specifically, the comparison with the experiments addresses the effect of Si:Al molar ratio and water content on a range of structural and mechanical properties such as skeletal density, ring structure, bong-angle distribution, X-ray diffraction (Figure 1) and X-ray pair distribution function. The simulation results confirm the necessity of a defective structure able to detect both order and disorder in geopolymers experiments. The proposed defective molecular model provides a starting point for the multiscale understanding of geopolymer cements, as well as for investigating the molecular interactions between geopolymer cements and various adsorbates, e.g. for applications in environmental engineering and nuclear engineering. Please click Additional Files below to see the full abstract

Editorial: Women in Science: Materials

Editorial on the Research Topic Women in Science: Material

Investigation of lightweight geopolymer mortars as fireproofing coatings

Fireproofing coatings are passive fire protection (PFP) systems adopted to increase the fire safety of structural components in several civil and industrial applications. They are generally spray-applied systems that behave like thermal barrier for heat transfer to the substrate. Their use is aimed at slowing down the temperature rise of the substrate and maintaining the temperature of the component below its critical temperature (e.g. steel loses about one-half of the strength at 500 °C), thus providing time to control or extinguish the fire. When good resistance to high temperature is required, geopolymers are considered highly competitive materials thanks to the intrinsic thermal resistance of their structure. For this reason, this study investigates the possibility of using fly ash-based geopolymers, activated at room temperature, as fireproofing coatings for steel components. Lightweight geopolymer mortars (LWGs) were synthesized at room temperature using low-calcium coal fly ash as precursor and 8 M NaOH and sodium silicate solutions as activators. The weight ratio between the sum of the alkaline solutions and the fly ash was maintained constant, whereas the amounts of 8M NaOH and sodium silicate solutions in the mix were varied, thus obtaining two different geopolymer matrices with different compositional SiO2/Al2O3 and Na2O/SiO2 ratios. The mix design was completed using expanded perlite (EP) as aggregate and hydrogen peroxide solution as foaming agent to increase thermal insulation properties and to decrease products density, both essential features for fireproofing coatings. Physical, mechanical and thermal properties of the lightweight geopolymers were investigated as a function of the compositional parameters and of the amount of lightweight aggregate and foaming agent. Furthermore, considering that the performances of a fireproofing coating are temperature dependent, the variation of thermal conductivity and specific heat as a function of temperature were studied to provide data on the heat transfer to the substrate during heating. Thermogravimetric analysis confirmed the remarkable weight stability at high temperature of all the investigated geopolymers, which showed a total mass loss always lower than 8% at 900°C. Results showed that the use of expanded perlite as lightweight aggregate, combined with the foaming agent, allowed obtaining lightweight geopolymer mortars characterized by bulk density of 0.77 g/cm3 and thermal conductivity of 0.23 W/mK at T = 20°C. These features are comparable to the ones of commercially available cementitious-based fireproofing coatings. Results obtained from the experimental characterization were used to simulate the performance of the most promising LWG as fireproofing coating during a fire accident. A finite volume software set-up was used to simulate the temperature rise of steel components covered by different thickness (15, 20, and 25 mm) of the selected LWG, under cellulosic and hydrocarbon fire curve conditions. The performance of a commercial Portland cement-based fireproofing mortar (LWC) was also simulated for comparison. The simulations confirmed that the selected lightweight geopolymer mortar was effective in delaying the increase of the steel temperature, providing a protection for the steel substrate for at least 30 minutes in the case of cellulosic fire conditions. In addition, the thinnest layer (15 mm) of LWG coating considered in this study exhibited the same behavior of a 20 mm layer of cementitious-based product

EFFECTIVE ELASTIC CONSTANTS OF PLAGIOCLASE FELDSPAR AGGREGATES IN DEPENDENCE OF THE ANORTHITE CONTENT - A CONCISE REVIEW

Recent work on the elastic constants of plagioclase feldspars is reviewed. Based on the 21 elastic constants (stiffnesses) reported in the literature for triclinic plagioclase monocrystals of different composition, the effective elastic constants of dense, single-phase, polycrystalline plagioclase aggregates with isotropic microstructure are calculated via standard VoigtReuss-Hill averaging. Master curves show the good agreement of the constants obtained via the two approaches (experiment and simulation) and recall the fact that values reported in the older literature have been underestimated. Fit relations are given for the calculation of Young modulus, shear modulus, bulk modulus, Poisson ratio and density in dependence of the composition. These fit relations may serve as handy tools for obtaining relevant input information for the calculation of effective elastic constants of multiphase ceramics produced with Ca and Na containing raw materials

Strengthening mechanisms in cement-stabilised rammed earth

There is currently little scientific understanding of stabilised rammed earth (RE) and the re- lationship between water-cement ratio and compressive strength. For traditional (unstabilised) RE materials, it is standard practice to compact the soil mix at its optimum water content to achieve maximum dry density and hence maximum strength. However, this may not also apply to cement-stabilised rammed earth (CSRE). A recent investigation (Beckett and Ciancio 2014) showed that CSRE samples stabilised with 5% cement and compacted at a water content lower than optimum performed better than samples compacted at optimum or higher. This seems to be in agreement with the well-known effect in concrete materials, according to which the lower the water-cement ratio, the stronger the cementitious products hence the higher the compressive strength. This paper investigates the effect of water cement ratio in CSRE samples. Results of an experimental programme are presented and used to discuss the appropriateness of the water-cement ratio for RE materials

A Comprehensive Microstructural and Compositional Characterization of Allogenic and Xenogenic Bone: Application to Bone Grafts and Nanostructured Biomimetic Coatings

Bone grafts and bone-based materials are widely used in orthopedic surgery. However, the selection of the bone type to be used is more focused on the biological properties of bone sources than physico-chemical ones. Moreover, although biogenic sources are increasingly used for deposition of biomimetic nanostructured coatings, the influence of specific precursors used on coating’s morphology and composition has not yet been explored. Therefore, in order to fill this gap, we provided a detailed characterization of the properties of the mineral phase of the most used bone sources for allografts, xenografts and coating deposition protocols, not currently available. To this aim, several bone apatite precursors are compared in terms of composition and morphology. Significant differences are assessed for the magnesium content between female and male human donors, and in terms of Ca/P ratio, magnesium content and carbonate substitution between human bone and different animal bone sources. Prospectively, based on these data, bone from different sources can be used to obtain bone grafts having slightly different properties, depending on the clinical need. Likewise, the suitability of coating-based biomimetic films for specific clinical musculoskeletal application may depend on the type of apatite precursor used, being differently able to tune surface morphology and nanostructuration, as shown in the proof of concepts of thin film manufacturing here presented

B-IMPACT project: eco-friendly and non-hazardous coatings for the protection of outdoor bronzes

Application of protective coatings is the most widely used conservation treatment for outdoor bronzes. Eco-friendly and non-hazardous coatings are currently needed for conservation of outdoor bronze monuments. To fulfil this need, the M-ERA.NET European research project B-IMPACT (Bronze-IMproved non-hazardous PAtina CoaTings) aimed at assessing the protectiveness of innovative coatings for historical and modern bronze monuments exposed outdoors.In this project, two bronze substrates (historical Cu-Sn-Zn-Pb and modern Cu-Si-Mn alloys) were artificially patinated, by acid rain solution using dropping test and by “liver of sulphur” procedure (K2S aqueous solution)to obtain black patina, respectively. Subsequently, the application of several newly developedprotective coatings was carried out and their performance was investigated by preliminary electrochemical tests. In the following stepsof the work, the assessment of the best-performing coatings was carried out and their performance was compared to Incralac, one of the most widely used protective coatings in conservation practice. A multi-analytical approach was adopted,considering artificial ageing (carried out in representative conditions, including exposure to rain runoff, stagnant rain and UV radiation)and metal release, as well as visual aspect (so as to include aesthetical impact among the coating selection parameters) and morphological and structural evolution of the coated surfaces due to simulated outdoor exposure. Lastly, also the health impact of selected coatings was assessed by occupational hazard tests. The removability and re-applicability of the best-performing coatings were also assessed.The best alternatives to the conventional Incralac exhibited were: (i) fluoroacrylate blended with methacryloxy-propyl-trimethoxy-silane (FA-MS) applied on patinated Cu-Sn-Zn-Pb bronze and (ii) 3-mercapto-propyl-trimethoxysilane (PropS-SH) applied on patinated Cu-Si-Mn bronz