Use of End-of-Waste Foamed Fibers and Aggregates into a Cementitious Mortar

2015 - 2016Durability and sustainability of cementitious materials are two important issues in the field of construction materials. Durability is defined as the ability of cementitious materials to resist weathering action, chemical attack, abrasion or any other process of deterioration. The use of fibers is a viable solution to partially overcome the brittle behavior of such materials. At the same time it is demonstrated that fibers, by reducing cracking phenomena, allow to face the durability related issues. Different fibers have been used according to the aims of composite materials: high strength fibers are generally used for structural purposes (toughness increase) while low modulus synthetic fibers are mainly used to avoid plastic shrinkage cracking. The effectiveness of fibers reinforcing action lies mainly on the fiber/matrix interactions. Three types of interactions can be recognized: i) physical and/or chemical adhesion; ii) friction and iii) mechanical anchorage induced by deformations on the fiber surface (e.g. crimps, hooks, twisted or deformed fibers in general). Sustainability can be identified according to the definition of sustainable development stated in 1987 by Brundtland et al.: “the development that meets the needs of the present without compromising the ability of future generations to meet their own needs”. Sustainable development should take into account economic growth, social equality and environmental protection. The construction industry involves all these fields: the main concerns are raw materials consumption and CO2 emissions during cement production. Moreover, also the plastic production and disposal present several environmental issues. Once again, raw materials consumption and the speed with which these materials became waste. Thus, seen the aforementioned drawbacks related to cementitious materials, this Ph.D. was aimed to study the possibility of using end-ofwaste materials (i.e when waste ceases to be waste and becomes a secondary raw material) for the production of synthetic fibers and aggregates characterized by improved mechanical interactions with the cementitious matrix. To this extent, fibers and aggregates with a rough and porous surface, able to offer interlocking positions for the cementitious matrix, were produced in laboratory by melt extrusion-foaming process. Moreover, some chemical treatments (alkaline hydrolysis and sol-gel deposition of nanosilica) were performed on fibers, to improve chemical adhesion with the cement paste. Finally, taking into account the need for reducing the consumption of raw materials, foamed fibers and aggregates were produced starting from a polymeric end-of-waste material made of a polyolefins blend (HDPE, LDPE and PP). Alkaline hydrolysis promoted the creation of interlocking positions on fiber surface but the best behavior was recognized for fibers with nano-silica particles on the surface. In this case, a denser ITZ and a great amount of hydration products were observed by SEM investigations. Pull-out tests confirmed the better performances of treated fibers: a higher pull-out peak load was achieved and an increase of pull-out energy was evident. Subsequently, a foam extrusion process was used to manufacture polymeric fibers (both virgin and recycled) with a rough surface, to improve mechanical friction with the cementitious matrix. Optimizing foaming agent quantity and processing parameters was possible to produce fibers having adequate surface texture and diameter to be used in fiber reinforced mortars. Although fiber reinforced mortars workability decreases at increasing fiber volume fraction, the results demonstrated that this happens to a lower extent for mortars containing foamed fibers. Fibers mechanical properties decreased at increasing fibers porosity but fiber reinforced mortars mechanical properties, flexural and compressive strength, were not influenced by fibers addition nor their morphology. The rougher surface gives rise to a better fiber/matrix adhesion, as confirmed by pull-out tests. Durability investigations on the fiber reinforced mortars reported good results for capillary water absorption, sulfate attack and plastic shrinkage cracking. In particular, fibers length and volume fraction are key parameters in controlling plastic shrinkage cracking. Moreover, mortar samples containing foamed fibers displayed a better control of shrinkage cracking: cracks opening was delayed and the improved fiber/matrix bond was able to reduce crack width, compared to mortars containing smooth fibers. Finally, lightweight artificial aggregates (LWAs) were produced, starting from foamed strands. At increasing LWAs substitution, a sharp decrease of density was achieved. Also workability and mechanical properties decreased, but a more ductile behaviour was recognizable. Thermal conductivity and water vapor resistance were proportional to mortars density which obviously decreased at increasing natural sand substitutions. Moreover, the use of aggregates porosity as reservoir of internal curing water showed promising preliminary results. In brief, the results of this study demonstrate that engineered fibers with improved fiber/matrix bond allow to optimize (i.e. to reduce) fibers volume fraction in cementitious mortars. Foamed fibers characteristics can be in turn optimized by changing the manufacturing process conditions. Benefits could be not only in the control of plastic shrinkage cracking but also in the workability of fresh mortars, mechanical strength and durability of the hardened composite. In addition, using end-of-waste materials a more sustainable product can be obtained. In particular, replacing natural aggregates with plastic aggregates, is possible to reduce raw materials consumption and improve mortar properties (mainly unit weight, thermal conductivity and water vapor permeability). [edited by author]La durabilità e la sostenibilità dei materiali cementizi sono argomenti estremamente importanti nell’ambito dei materiali da costruzione. La durabilità è definita come l’abilità dei materiali cementizi a resistere nel tempo alle azioni di degrado, di attacco chimico, abrasione o qualunque altro processo di deterioramento. L’uso delle fibre consente di ovviare, seppur in modo parziale, al problema del comportamento fragile di tali materiali. È inoltre ampiamente dimostrato che le fibre, contrastando i fenomeni fessurativi, consentono di far fronte anche ai problemi legati alla durabilità. In letteratura sono state utilizzate diverse fibre, a seconda del composito da realizzarsi: fibre ad elevato modulo elastico vengono utilizzate per scopi strutturali (incremento di duttilità) mentre fibre con basso modulo sono utilizzate nel contrasto alla fessurazione. Nell’efficacia dell’azione esplicata dalle fibre gioca un ruolo fondamentale l’interazione fibra/matrice. Tre diverse tipologie di interazione sono riscontrabili: i) adesione di tipo fisica e/o chimica; ii) frizione e iii) ancoraggio meccanico dovuto alle deformazioni presenti sulla superficie delle fibre (ad esempio rilievi, uncini, scanalature ecc.). La sostenibilità può essere definita per tramite del concetto di sviluppo sostenibile, espresso nel 1987 dal rapporto Brundtland, come lo “sviluppo che incontra i bisogni della generazione presente senza compromettere la possibilità delle generazioni future di soddisfare i propri”. Lo sviluppo sostenibile deve portare in conto la crescita economica, l’uguaglianza sociale e la protezione ambientale. L’industria delle costruzioni coinvolge tutti questi settori ed i principali problemi sono legati al consumo delle materie prime e all’emissione di CO2 durante la produzione del cemento. Inoltre, anche la produzione della plastica e la sua relativa dismissione, pone alcuni problemi ambientali. Detto ciò, viste le problematiche precedentemente esposte relative sia ai materiali cementizi che plastici, le ricerche di dottorato sono state dedicate allo studio della possibilità di utilizzare materiali end-of-waste (cioè materiali che hanno cessato di essere rifiuti e sono diventati materie prime seconde) per la produzione di fibre ed aggregati sintetici, caratterizzati da una migliore interazione di tipo meccanico con la matrice cementizia. A tale scopo, le fibre e gli aggregati sono state prodotti in laboratorio attraverso un processo di melt-extrusion foaming, per ottenere una superficie scabra e porosa, capace di offrire posizioni di incastro per la matrice cementizia. Inoltre, sono stati sperimentati sulle fibre anche due trattamenti chimici (idrolisi alcalina e deposizione tramite processo sol-gel di nano-silice), in maniera tale da incrementare l’affinità chimica con la pasta cementizia. Infine, considerando anche la necessità di ridurre il consumo delle materie prime, delle fibre e degli aggregati schiumati sono stati prodotti partendo da un materiale polimerico end-of-waste costituito da una miscela di poliolefine (HDPE, LDPE e PP). L’idrolisi alcalina ha promosso la creazione di posizioni di incastro sulla superfice delle fibre ma il miglior comportamento è stato riscontrato per le fibre ricoperte di nano-silice in superficie. In questo caso è riconoscibile una più densa ITZ e sono stati osservati anche un gran numero di prodotti di idratazione tramite SEM. Le prove di pull-out hanno confermato le migliori prestazioni delle fibre trattate poiché è stato raggiunto un carico di pull-out più elevato oltre ad un incremento dell’energia di pull-out. Successivamente, un processo di foam extrusion è stato utilizzato per produrre fibre polimeriche (sia vergini che riciclate) con una superficie ruvida, per incrementare la frizione meccanica con la malta cementizia. Ottimizzando la quantità di agente schiumante ed i parametri di processo è stato possibile produrre fibre con un’adeguata tessitura superficiale e diametro, tali da poter essere utilizzate per il rinforzo di una malta cementizia. Nonostante la riduzione di lavorabilità delle malte, all’aumentare della frazione volumetrica di fibre, le prove sperimentali hanno dimostrato che l’entità di tale riduzione è minore nel caso in cui vengano utilizzate fibre schiumate. Le proprietà meccaniche delle fibre diminuiscono all’aumentare della porosità delle fibre ma le proprietà meccaniche delle malte rinforzate con tali fibre (resistenza a compressione e flessione) non risultano influenzate dalla presenza delle fibre né dalla loro morfologia. La maggiore rugosità della superficie porta ad una migliore adesione fibra/matrice, come confermato dalle prove di pull-out. Lo studio della durabilità sulle malte fibro-rinforzate ha restituito buoni risultati nei confronti dell’assorbimento d’acqua per capillarità, attacco solfatico e fessurazione da ritiro plastico. In particolare, la lunghezza e la frazione volumetrica delle fibre sono dei parametri chiave nel controllo di tale fenomeno. Inoltre, i campioni di malta contenenti le fibre schiumate hanno mostrato un maggior controllo nei confronti della fessurazione da ritiro, ritardando l’apertura delle fessure e riducendo l’ampiezza delle stesse, grazie alla migliore adesione, rispetto alle malte contenenti fibre lisce. Infine, a partire dai filamenti schiumati sono stati prodotti degli aggregati artificiali alleggeriti (LWAs). All’aumentare della sostituzione della sabbia naturale con LWAs, è stata ottenuta una marcata riduzione di densità. Inoltre, anche la lavorabilità e le proprietà meccaniche sono diminuite, ma è stato riscontrato un comportamento più duttile. La conducibilità termica e la resistenza al passaggio di vapor d’acqua diminuiscono all’aumentare della sostituzione di sabbia silicea, in maniera proporzionale alla riduzione di densità. Inoltre, l’uso della porosità degli aggregati come serbatoio d’acqua per l’internal curing ha riportato risultati promettenti. In conclusione, i risultati di questo studio hanno dimostrato che l’utilizzo di fibre ingegnerizzate con una migliore adesione all’interfaccia fibra/matrice permette di ottimizzare (cioè di ridurre) la frazione volumetrica di fibre da utilizzare nelle malte cementizie. Le caratteristiche delle fibre schiumate possono di volta in volta essere cambiate ed ottimizzate, cambiando il processo di produzione. I vantaggi ottenuti sono sia in termini di controllo della fessurazione da ritiro plastico che nella lavorabilità allo stato fresco delle malte rinforzate, ma anche proprietà meccaniche e di durabilità del composito indurito. In aggiunta, utilizzando un materiale endof-waste, può essere ottenuto un materiale più sostenibile. In particolare, sostituendo gli aggregati naturali con aggregati plastici, è possibile ridurre il consumo di materie prime e migliorare alcune proprietà della malta: in particolare densità, conducibilità termica e permeabilità al vapor d’acqua. [a cura dell'autore]XV n.s. (XXIX

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Robocasting of Single and Multi-Functional Calcium Phosphate Scaffolds and Its Hybridization with Conventional Techniques: Design, Fabrication and Characterization [Allizond V. is the co-corresponding author]

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