Juraj Šipušić

Bio: Juraj Šipušić is an academic researcher from University of Zagreb. The author has contributed to research in topics: Aluminate & Cement. The author has an hindex of 12, co-authored 38 publications receiving 661 citations.

Thermophysical Comparison of Five Commercial Paraffin Waxes as Latent Heat Storage Materials

Abstract: Thermophysical properties of phase change materials (PCM) are of utmost importance in latent heat thermal energy storage (LHTES) applications. Therefore, an experimental study is conducted in order to determine thermophysical properties of five technical grade paraffin waxes produced by major Croatian oil company, INA d.d. Rijeka. The temperatures and enthalpies of melting and solidification (latent heat capacity) and specific heat capacities of solid and liquid paraffin waxes were measured by differential scanning calorimetry (DSC). The thermal diffusivity of paraffin waxes was determined utilizing transient method. The importance of eliminating phase transformation interferences to thermophysical properties determination is addressed. The densities and the coefficient of thermal expansion were measured using Archimedes methods. A self-adopted simple and inexpensive laboratory procedure for the determination of liquid density as a temperature function is presented. Finally, the thermal conductivities have been calculated from measured densities, heat capacities and diffusivities. Based on results obtained, the investigated paraffin waxes were evaluated in regard to their applicability as PCM for LHTES.

Dehydration of a layered double hydroxide – C2AH8

Abstract: Thermal dehydration of dicalcium aluminate hydrate, C2AH8, has been investigated by simultaneous differential thermal and thermo gravimetric analysis (DTA/TGA), powder X-ray diffraction (XRD), temperature-dependent infrared spectroscopy (FT-IR), and BET method of surface area measurement. The temperature-dependent infrared measurements were studied by two-dimensional infrared (2D-IR) correlation spectroscopy. The structure of aluminum-oxide polyhedron, characterized by 27Al solid state NMR spectrum method and FT-IR, shows tetrahedron and octahedron as the main forms of aluminum-oxide polyhedrons in C2AH8 sample. From the results obtained a variety of structural transformations observed are explained as a consequence of the removal of loosely held interlayer water molecules at lower temperatures, followed by grafting process of the interlayer [Al(OH)4]− anion. Structural model of a grafting process of the interlayer [Al(OH)4]− tetrahedron onto hydroxylated octahedrons of [Ca2Al(OH)6]+ layers has been proposed in order to explain observed loss of one water molecule, shrinkage of interlayer spacing and qualitative changes of FT-IR spectra. At higher temperatures the dehydroxylation of the lattice and decomposition of the interlayer species occurs, yielding amorphous material that crystallizes into C3A and C12A7 at 885 °C. Those findings provide improvement in the interpretation of thermo-analytical results of calcium aluminate cements (CAC) hydration products, and better understanding of CAC conversion process.

Porosity-strength relation in calcium aluminate cement pastes

Abstract: The compressive strength and the volume porosity of calcium aluminate cement pastes have been studied in order to connect their relationship. The influence of mass fraction of lithium carbonate on compressive strength and porosity of calcium aluminate cement (CAC) has been investigated at different water–cement (w/c) ratios. The functions proposed in the literature for different technical materials were tested on obtained strength and porosity data. Those functions have been a base for further development of more general functional dependence of strength and porosity for cement materials. Thus, we propose the following equation to relate the strength and porosity for CAC pastes: σ=σ P 0 1− P P 0 2

Synthesis and characterization of poly(styrene-co-methyl methacrylate)/layered double hydroxide nanocomposites via in situ polymerization

Abstract: Intercalated poly(styrene-co-methyl methacrylate)/layered double hydroxide nanocomposites (PS–PMMA/LDH–B) have been synthesized by the in situ bulk multistep polymerization of styrene and methyl methacrylate in the presence of the Ca–Al layered double hydroxide, previously modified by the incorporation of benzoate anions [Ca4Al2(OH)12(C6H5COO)2·xH2O, LDH–B]. Nanocomposites were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). XRD and TGA results pointed to the successful incorporation of the LDH–B within the copolymer matrix. XRD results indicated that the characteristic layered structure of the LDH–B had disappeared due to disordering. TEM analysis confirmed that LDH–B was partially dispersed within the matrix forming a structure with alternating matrix-particle regions, where particles appear in a form of intercalated nanocomposite structure. TGA results showed improved thermal properties in comparison to the neat PS–PMMA copolymer.

Influence of metal chloride salts on calcium aluminate cement hydration

Abstract: In this paper the influence of alkali, alkaline earth and transition metals in the form of chlorides salts on the hydration of commercial iron-rich calcium aluminate cement (CAC) is investigated. The effect on setting time and mechanical properties is studied. The results of the setting time obtained from the measured temperature evolution of cement pastes are compared to standard Vicat needle measurements. Addition of alkali metal salts both accelerates the setting time of CAC and deteriorates long-term strengths in the order of decreasing ionic radius of the metals. The effect of alkaline earth metal salts on setting behaviour depends on the amount of addition: low concentrations retard, but high concentrations accelerate setting behaviour. Calcium is found to have the strongest acceleration effect with the lowest deterioration in strength. An interesting regularity is observed when plotting the results of both retarded setting times and strengths against atomic number of added transition metal cation.

Influence of porosity on compressive and tensile strength of cement mortar

Abstract: The compressive, flexural and splitting tensile strength of cement mortar has been measured and interpreted in terms of its porosity The authors first reviewed the existing porosity–strength relationships (Ryshkewithch, Schiller, Balshin and Hasselman model) and assessed the suitability of existing relationships The Zheng model for porous materials has been used to evaluate the porosity–strength relationship of cement mortar Over the porosity ranges examined, the extended Zheng model is good representation of the experimental data on the strength of cement mortar Based on the generality of the assumptions used in the derivation of the extended Zheng model, this model for cement mortar can be applied for other cement-based materials The experimental data also show that the ratio between compressive strength and indirect tensile (splitting tensile and flexural) strength of cement mortar is not constant, but is porosity dependent The ratio decreases with increase porosity values of cement mortar

Polymer nanocomposites from modified clays: Recent advances and challenges

Abstract: Since the end of the last century, the discovery of polymer nanocomposites and their ever-expanding use in various applications has been the result of continuous developments in polymer science and nanotechnology. In that regard, progress in developments on the use of modified natural and synthetic clays for designing polymer nanocomposites is presented herein. The modified clays used in composite preparation include natural clays such as montmorrilonite, hectorite, sepiolite, laponite, saponite, rectorite, bentonite, vermiculite, biedellite, kaolinite, and chlorite, as well as synthetic clays including various layered double hydroxides, synthetic montmorrilonite, hectorite, etc. The preparation, structure and properties of polymer nanocomposites using the modified clays are discussed. Even at a low loading, these composites are endowed with remarkably enhanced mechanical, thermal, dynamic mechanical, adhesion and barrier properties, flame retardancy, etc. The properties of the nanocomposites depend significantly on the chemistry of polymer matrices, nature of clays, their modification and the preparation methods. The uniform dispersion of clays in polymer matrices is a general prerequisite for achieving improved mechanical and physical characteristics. Various theories and models used to design polymer/clay nanocomposites have also been highlighted. A synopsis of the applications of these advanced, high-performance polymer nanocomposites is presented, pointing out gaps to motivate potential research in this field.

Hydration and rheology control of concrete for digital fabrication: Potential admixtures and cement chemistry

Abstract: Concrete digital fabrication is an innovative construction approach where infrastructural elements can be built additively without using formwork. This represents a significant advantage, but also introduces materials engineering challenges, as the requirements normally fulfilled by the formwork are now imposed on the concrete. In this paper, it is discussed how admixtures can be employed to achieve the rheological and hydration properties necessary for printable concrete. An overview of various admixtures currently implemented in standard practice is presented. Then, the main required concrete states for extrusion and deposition processes are analyzed with respect to required performances and potential admixtures. Finally, possible side effects and incompatibilities are discussed, as well as how they could be unconventionally used for printable concrete purposes. The main objective is to demonstrate how admixtures will be critical in the development of concrete systems to realize digital fabrication, and to ultimately motivate investigation in the key areas discussed.

Acceleration of discharge process of clean energy storage unit with insertion of porous foam considering nanoparticle enhanced paraffin

Abstract: In current investigation, a clean energy storage unit has been designed aiming to decrease the energy consumption through the building. To increase the space between the passing air and the Phase change material (PCM), instead of a smooth channel, a sinusoidal channel has been considered for the airway passage. Furthermore, porous media and nanoparticles have been applied in this unit for the purpose of enhancing heat transfer. Considering previous research works, the simultaneous use of these three methods for augmenting the performance has not been scrutinized so far. In this study, three mechanisms for augmenting heat transfer and the interaction of these methods to augment the performance of the system have been evaluated. Based on comparison with experimental data, outputs are very excellent fit with maximum 9.3% deviation. For evaluate efficiency of unit, solidification duration was analyzed and outputs proved that for case with porous foam solidification finishes 21.4% faster than the base case. Slope of profile for second case is higher than first one and the process completed at t = 33 h. For case with porous foam, augmenting time up to 13 h leads to augmenting trend for heat release value and then opposite trend appears. Heat flux transfer to air reduces with rise of time which provides lower temperature of outlet air. Slope of profile for air outlet temperature augments at t = 33 h when all liquid convert to solid for second case. As porous foam utilizes, temperature of PCM reduces and no sensible difference appears in various positions in each section.