Modifikacija veziva v cementnih kompozitih z vgrajevanjem nanomaterialov : doktorska disertacija

Štefančič, Mateja (author), Mladenovič, Ana (mentor)

Alternativna trajnostna veziva na osnovi portlandskega cementa morajo imeti enake funkcionalne lastnosti in enako trajnost kot konvencionalna. Eno od perspektivnih področij so veziva, v katerih je del cementa nadomeščen z nanodelci. Motiv za raziskovalno delo v okviru doktorske disertacije je slabo poznavanje vpliva vgradnje kemijsko inertnih nanodelcev na reološke lastnosti, kinetiko in mehanizme hidratacije cementa, na razvoj mikrostrukture in na obstojnost takšnih cementnih kompozitov. V raziskavi so bili kot kemijsko inertni fini delci uporabljeni nanodelci α-Al2O3. Aglomeracija teh delcev v postopku vmešavanja s cementom je bila preprečena z ultrazvočnim tretiranjem in z dodatkom superplastifikatorja polikarboksilatnega tipa. Reološke meritve viskoznosti cementnih suspenzij so pokazale, da se vrednosti maksimalnega prostorninskega deleža trdnih delcev z naraščanjem deleža nanodelcev α-Al2O3 neenakomerno zmanjšujejo, kar je posledica učinka specifične površine trdnih delcev in učinka polnila, pri čemer je bil slednji manj izražen. In-situ fazna analiza cementnih past v srednjem obdobju hidratacije, izvedena z izotermno kalorimetrijo in elektronsko mikroskopijoje pokazala, da ima dodajanje superplastifikatorja polikarboksilatnega tipa močan vpliv na mehanizem in kinetiko hidratacije cementa. Dodatek superplastifikator je preprečil pojav nukleacijskega učinka, ki se sicer pojavi pri vgradnji kemijsko inertnih finih delcev, kamor spadajo tudi nanodelci α-Al2O3. Učinek dodatka superplastifikatorja je tudi pomembno vplival na nadaljnji potek hidratacije cementa in s tem na končne lastnosti cementnega kompozita. Tretji delujoči učinek je bil učinek redčenja cementa, kateri se je povečeval z večanjem deleža nadomestka cementa z nanodelci α-Al2O3 v cementnih kompozitih, ker nukleacijski učinek ni bil izražen. Malte in betoni z delnim nadomestkom cementa z nanodelci α-Al2O3 so zaradi učinka redčenja cementa izkazovale nižje trdnosti, povečano celokupno poroznost in zmanjšano obstojnost na procese degradacije v primerjavi z referenčnimi maltami in betoni. Zmanjšanje trdnosti je bilo pripisano tudi večjemu deležu ujetih por, ki pa so po drugi strani izboljšale obstojnost betona na cikle zmrzovanja/tajanja.

• Language: Slovenian
• Keywords: grajeno okolje, cementni kompoziti, hidratacija, nanodelci, superplastifikator, pakiranje delcev, nukleacijski učinek, in-situ XRD, mikrostruktura, obstojnost
• Work type: Dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FGG - Faculty of Civil and Geodetic Engineering
• Publisher: [M. Štefančič]
• Year: 2017
• PID: 20.500.12556/RUL-92399
• UDC: 55
• COBISS.SI-ID: 1338462
• Publication date in RUL: 02.06.2017

Secondary language

• Language: English
• Title: Modification of the matrix microstructure of cement composites with the incorporation of nanomaterials : doctoral thesis

Abstract

Alternative binders based on the use of portland cement should provide the same functionality and durability as conventional cement binders. One area of growing interest involves binders in which cement is partially replaced by nanoparticles, which are known to possess unique physico-chemical properties. A good reason to perform the research described in this doctoral dissertation was the lack of knowledge about how to modify the nanoscale microstructure of the cement binder in such a way that cement composites made using such a binder would have better-performing characteristics. The chemically inert nanoparticles selected were α-Al2O3. The agglomeration of such particles always causes considerable problem; in this case they were resolved by mixing the components by means of a two-step procedure through a combination ultrasonication and the addition of a surface active agent, a polycarboxylate type superplasticizer was selected. Taking into account the semi-empirical Krieger-Dougherty relationship, the maximum solid volume fraction of the cement suspensions, was calculated. The results showed a uneven reduction in the measured values, with an increasing replacement level of cement by α-Al2O3 nanoparticles. This observed reduction was attributed to a combination of two opposing effects, the filler effect and the specific surface area effect – the latter effect was somewhat stronger than the former effect. The results obtained during the middle period of cement hydration of the cement pastes, showed that the used polycarboxylate-type superplasticizer had a stronger influence on the kinetics of cement hydration than the used α-Al2O3 nanoparticles. This confirmed that the nucleation effect of the α-Al2O3 had been significantly reduced. This reduction had a subsequent effect on the further hydration and development of the hardened microstructure. With regard to the so-called cement dilution effect, it was found that this effect increased when the nucleation effect was reduced. Hardened samples of mortar and concrete with binders incorporating nanoparticles of α-Al2O3 were characterized by lower strengths, increased overall porosity, and reduced resistance to degradation processes, as compared to the reference mortars and concretes. However, due to the increased proportion of entrapped pores, the modified samples showed improved frost resistance.

• Keywords: building environment, cement composites, hydration, nanoparticles, superplasticizer, particle packing, nucleation effect, in-situ XRD, microstructures, durability