Study of porosity of nano-crystalline materials and of hydration of photocatalytic mortars containing titania with nuclear magnetic resonance and atomic force microscopy

Aim of this thesis is the study of the physicochemical properties of photocatalytic self cleaning construction materials with Titania (TiO2). The work is focused on the production and study of (a) mortars and pastes made of white cement and titania, (b) masonry mortars produced with white cement and titania and (c) restoration mortars produced with lime and titania. Initially, paste and mortar samples of white cement mixed with titania (in powder form) were produced and studied according to standard EN 196. Consequently, paste hydration and evolution of pore size distribution were determined with NMR, SEM and Mercury Porosimetry, while a tribological study on paste surface was also realized. According to the study, titania exhibits a similar behaviour to that of a filler as it improves cohesion and homogeneity of pastes and mortars. However, due to the lack of hydraulic properties, cement hydration does not benefit from titania, neither do strength development and porosity evolution. It was proven that titania’s hydrophilic properties play a major role on the matter, as TiO2 particles adsorb water molecules needed for hydration. Because of this, cement grains hydrate with a slower rate when titania is present. In addition, the tribological study revealed the evolution of surfacial roughness, as defined through the hydration process. Moreover, self cleaning masonry mortars were produced with white cement, lime, sand, perlite and titania. Limited application of titania lead to the creation of mortars with satisfactory properties as defined by standard EN 1015. Titania’s effect was observed in this case as well, yet in a lesser degree. Finally, self cleaning restoration mortars were produced with lime and titania. Combination of the two materials lead to the creation of improved self cleaning construction materials, as it was proven that carbonation benefits greatly from the photocatalytic process and in specific from the production of CO2 due to decomposition of organic pollutants. Hence, the materials produced exhibit self cleaning properties as well as improved physicochemical characteristics and can contribute greatly in the effective protection both of built heritage and of our environment.Στόχος της παρούσας διδακτορικής διατριβής είναι η μελέτη των φυσικοχημικών ιδιοτήτων των φωτοκαταλυτικά αυτοκαθαριζόμενων δομικών υλικών με τιτανία (TiO2). Η διατριβή επικεντρώθηκε στην παραγωγή και μελέτη (α) παστών και κονιών μίγματος λευκού τσιμέντου και τιτανίας, (β) επιχρισμάτων με βάση το λευκό τσιμέντο και την τιτανία και (γ) κονιαμάτων αποκατάστασης με υδράσβεστο και τιτανία. Σε πρώτη φάση παρήχθησαν δοκίμια παστών και κονιών αναμιγνύοντας λευκό τσιμέντο με τιτανία σε μορφή σκόνης, και μελετήθηκαν οι ιδιότητες αυτών με βάση το πρότυπο ΕΛΟΤ ΕΝ 196 παράλληλα με την ενυδάτωση (NMR, SEM) και τη πορώδη δομή (Ποροσιμετρία Υδραργύρου, NMR), ενώ πραγματοποιήθηκε και μελέτη τριβολογίας (AFM). Από τις αναλύσεις που διεξήχθησαν προέκυψε ότι η τιτανία συμπεριφέρεται ως πληρωτικό υλικό καθότι αυξάνει τη συνεκτικότητα και την ομοιογένεια των δειγμάτων παστών και κονιών. Ωστόσο, λόγω της έλλειψης υδραυλικών ιδιοτήτων δεν συνεισφέρει στην διεργασία της ενυδάτωσης και κατ΄επέκταση στην ανάπτυξη αντοχών και στη μείωση του πορώδους. Όπως αποδείχθηκε, σε αυτό συμβάλει σημαντικά η υδροφιλική συμπεριφορά της τιτανίας, λόγω της οποίας τα σωματίδια TiO2 συγκρατούν το απαιτούμενο νερό της ενυδάτωσης. Αυτό έχει ως αποτέλεσμα οι κόκκοι του τσιμέντου να ενυδατώνονται με πιο αργό ρυθμό στα δείγματα με τιτανία. Η μελέτη της τριβολογίας ανέδειξε την χρονική μεταβολή της επιφανειακής υφής, η οποία προκαλείται από την εξέλιξη της ενυδάτωσης. Σε δεύτερη φάση, παρήχθησαν και μελετήθηκαν αυτοκαθαριζόμενα επιχρίσματα τοιχοποιίας με λευκό τσιμέντο, υδράσβεστο, άμμο, τιτανία και περλίτη. Η περιορισμένη προσθήκη τιτανίας οδήγησε στην παραγωγή υλικών με αποδεκτές ιδιότητες, εντός των ορίων που θέτει το πρότυπο ΕΛΟΤ ΕΝ 1015. Αν και η ιδιαίτερη συμπεριφορά της τιτανίας παρατηρήθηκε και στην προκειμένη περίπτωση, δεν αλλοίωσε σημαντικά τα χαρακτηριστικά των επιχρισμάτων. Τέλος, παρήχθησαν και μελετήθηκαν αυτοκαθαριζόμενα κονιάματα αποκατάστασης με υδράσβεστο και τιτανία. Ο συνδυασμός των δύο υλικών οδήγησε στην παραγωγή βελτιωμένων αυτοκαθαριζόμενων δομικών υλικών, καθώς αποδείχθηκε ότι η φωτοκαταλυτική αντίδραση συνεισφέρει στην διεργασία της ενανθράκωσης μέσω της παραγωγής CO2. Ως εκ τούτου, παρήχθησαν υλικά με αυτοκαθαριζόμενες ιδιότητες αλλά και βελτιωμένα φυσικοχημικά χαρακτηριστικά που δύναται να συμβάλουν όχι μόνο στην αποτελεσματική προστασία των κτιρίων της ιστορικής κληρονομίας αλλά και στην προστασία του περιβάλλοντος

Selective hydrogen gas sensor using CuFe 2 O 4 nanoparticle based thin film

Hydrogen gas sensors based on CuFe2O4 nanoparticle thin films are presented in this work. Each gas sensor was prepared by depositing CuFe2O4 thin film on a glass substrate by dc sputtering inside a high vacuum chamber. Argon inert gas was used to sputter the material from a composite sputtering target. Interdigitated metal electrodes were deposited on top of the thin films by thermal evaporation and shadow masking. The produced sensors were tested against hydrogen, hydrogen sulfide, and ethylene gases where they were found to be selective for hydrogen. The sensitivity of the produced sensors was maximum for hydrogen gas at 50 °C. In addition, the produced sensors exhibit linear response signal for hydrogen gas with concentrations up to 5%. Those sensors have potential to be used for industrial applications because of their low power requirement, functionality at low temperatures, and low production cost.Scopu

Low-carbon footprint cements incorporating high volumes of bauxite residue

The use of large quantities of bauxite residue (BR) to produce two types of low-carbon cement (ferrobelitic and aluminoferrite) was studied in this project. These types of cement are of great interest for the industry because of the lower energy demand and CO2 footprint as well as the higher possible incorporation of by-products, compared to ordinary Portland cement (OPC). To produce the clinkers, BR was combined with limestone, clays and reagent chemicals in order to achieve a suitable raw meal chemistry. The goal was to keep BR addition constant at 50 wt% in all mixtures. The mineralogical phase formation at different burning temperatures was estimatedby means of thermodynamic calculations. Clinkers were produced at different temperatures, 1200, 1250 and 1300 °C, followed by rapid cooling by air. The obtained clinkers were mineralogically quantified by the Rietveld method using X-Ray diffraction analysis. Additionally, microstructural characterisation was performed using SEM-EDS. Hydration kinetics were also studied by isothermal calorimetry. The results show that BR quantities as high as 50 wt% can be used to produce reactive, and environmental friendly cement clinkers.status: publishe

Structure–Superstructure Inter-Relations in Ca₂SiO₄ Belite Phase

Belite, the second most abundant mineralogical phase in Portland cement, presents five polymorphs which are formed at different temperatures. The increased interest in belite cement-based products is due to the lower environmental impact associated with the lower energy consumption. The importance of belite polymorphs formed at higher temperatures for cement industry applications is high, because they present better hydraulic properties. Thus, any study that helps to explore the structure relations of all belite polymorphs is of interest for both scientific and practical points of view. In the present work, a systematic structure–superstructure relation study is presented for all polymorphs, and it is based on the work of O’Keefe and Hyde (1985). In this pioneering work, generally, the structures of oxides are considered as having common characteristics with prototype structures of alloys. The basic result of the present work is the fact that all the polymorphs adopt a common architecture which is based on capped trigonal prisms of Ca cations, which host the Si one, and the oxygen anions occupy interstitial sites, i.e., an architecture in conformity with the model which considers the oxide structures as stuffed alloys. This result supports the displacive character of the transformation structural mechanism that links the five polymorphs based on the cation sites in their structures. However, based on the sites of oxygen anions, it could be considered as of diffusion character. The study of belite polymorphs is also of interest to products obtained by doping dicalcium silicate compounds, which present interesting luminescent properties

Influence of the Type of Cement on the Durability of Concrete Structures Exposed to Various Carbonation Environments in Greece: A Contribution to the Sustainability of Structures

The research objective of this paper is to investigate the effect of different types of cement and different climatic conditions on the durability of reinforced concrete structures to understand and address issues of durability and erosion. The types of cement used were CEM I 42.5N, CEM II/A-M (P-LL) 42.5N and CEM II/B-M (W-P-LL) 32.5N. Mixtures of three different cement mortars and six different concretes were prepared with these three types of cement. Cement mortars were produced according to the European standard EN 196-1. Concrete mixtures were of the strength classes C25/30 and C30/37. Concrete mixtures produced according to the specifications of the European standard EN 206 may have a shorter service life due to carbonation-induced corrosion if the choice of the cement type is not made carefully. The results indicate that the carbonation rate of concrete mixtures is significantly influenced by the type and strength class of the cement used. Using meteorological data from six regions of Greece, an empirical carbonation prediction model for these regions was obtained

Hybrid Porous Molybdenum Disulfide Monolith for Liquid Removal of Dibenzothiophene

Molybdenum disulfide (MoS₂) has been historically used as a hydrodesulfurization (HDS) catalyst and industrial lubricant. Because of its 2D nature and unique properties, MoS₂ is being considered for new applications in catalysis and electronics. In addition, there is great interest in designing new physical forms of MoS₂ that will allow for improved implementation of its properties, such as a continuous porous monolithic form. In this work, we report a new synthesis method to fabricate continuous, centimeter-sized, open cell hybrid foam (monolith) that consists of molybdenum sulfide and carbon as its main constituents. The hybrid foam was characterized using XRD, electron microscopy (SEM and TEM), Raman, FTIR, and EELS spectroscopy. Furthermore, liquid adsorption of dibenzothiophene (DBT) in toluene solvent was used to test the foam affinity to adsorb organosulfur compounds. The monolith is of low density and exhibits high specific adsorption capacity compared to existing materials reported in the literature

The Role of Titanium Dioxide on the Hydration of Portland Cement: A Combined NMR and Ultrasonic Study

Titanium dioxide (TiO2) is an excellent photocatalytic material that imparts biocidal, self-cleaning and smog-abating functionalities when added to cement-based materials. The presence of TiO2 influences the hydration process of cement and the development of its internal structure. In this article, the hydration process and development of a pore network of cement pastes containing different ratios of TiO2 were studied using two noninvasive techniques (ultrasonic and NMR). Ultrasonic results show that the addition of TiO2 enhances the mechanical properties of cement paste during early-age hydration, while an opposite behavior is observed at later hydration stages. Calorimetry and NMR spin–lattice relaxation time T1 results indicated an enhancement of the early hydration reaction. Two pore size distributions were identified to evolve separately from each other during hydration: small gel pores exhibiting short T1 values and large capillary pores with long T1 values. During early hydration times, TiO2 is shown to accelerate the formation of cement gel and reduce capillary porosity. At late hydration times, TiO2 appears to hamper hydration, presumably by hindering the transfer of water molecules to access unhydrated cement grains. The percolation thresholds were calculated from both NMR and ultrasonic data with a good agreement between both results

Bauxite residue as main raw material in the production of calcium sulfo-ferroaluminate clinker

status: publishe

Increasing the Fe2O3/Al2O3 ratio in ordinary Portland cement clinker, aiming to incorporate higher contents of bauxite residue

status: publishe

Ultra-Fine Ni2P Nanoparticles Decorated R-GO: Novel Phosphidation Approach and Dibenzothiophene Hydrodesulfurization

Nanostructured transition metal phosphides gathered last years an elevated scientific interest, due to their unique physical-chemical properties. Nickel phosphide nanoparticles, with the controllable crystal structure, from the metal rich tetragonal Ni12P5 to the phosphorous rich hexagonal Ni2P, and hcp Ni2P decorated r-GO (reduced graphene oxide), nano-hybrid materials have been synthesized via a novel one step organometallic approach in primary-tertiary aliphatic amines mixture. The nanoparticles are monodispersed, with spherical shape and controllable size in the sub-10 nm regime and decorate uniformly the surface of the r-GO, leading to the formation of Ni2P/r-GO hybrid materials. The materials were characterized by powder XRD, TEM and Raman spectroscopy and catalytically evaluated for the dibenzothiophene hydrodesulphurization (HDS) reaction. The results show that the role of the tertiary amine is crucial for the phosphidation process and the r-GO is an ideal alternative, to the traditional inorganic ones, support for the immobilization of the catalytically active component, preventing significantly sintering effects