Lorenzo Fernández

Bio: Lorenzo Fernández is an academic researcher from University of Valencia. The author has contributed to research in topics: Mesoporous material & Density functional theory. The author has an hindex of 9, co-authored 15 publications receiving 461 citations.

Chemical changes and phase analysis of OPC pastes carbonated at different CO2 concentrations

Abstract: Chemical changes and phase analysis of OPC pastes exposed to accelerated carbonation using different concentrations of CO2 (3%, 10% and 100%) have been undertaken and compared with those of natural carbonation (≅0.03%). 29Si Magic Angle Spinning-Nuclear Magnetic Resonance (29Si M.A.S-N.M.R), Thermogravimetric analyses (TG) and X-Ray Diffraction (XRD) have been used for characterisation. The carbonation of the samples has resulted in a progressive polymerisation of CSH that leads to formation of a Ca-modified silica gel and calcium carbonate. The carbonation of CSH and portlandite occurs simultaneously and the polymerisation of the CSH after carbonation increases with the increase in concentration of CO2. When ≅0.03% and 3% CO2 are used, CSH gel with a lower Ca/Si than that of the uncarbonated sample, and quite similar for both samples remained. When carbonating at 10% and 100% of CO2, the CSH gel completely disappears. For every condition, a polymerised Ca-modified silica gel is formed, as a result of the decalcification of the CSH. From these results it can be deduced that among the different concentrations of CO2 tested, carbonation up to a 3% of CO2, (that is to say, by a factor of 100) results in a microstructure much more similar to those corresponding to natural carbonation at ≅0.03% CO2 than those at the 10% and 100% concentrations.

Microstructure development in mixes of calcium aluminate cement with silica fume or fly ash

Abstract: The most widely identified degradation process suffered by calcium aluminate cement (CAC) is the so-called conversion of hexagonal calcium aluminate hydrate to cubic form. This conversion is usually followed by an increase in porosity determined by the different densities of these hydrates and the subsequent loss of strength. Mixes of calcium aluminate cement (CAC) and silica fume (SF) or fly ash (FA) represent an interesting alternative for the stabilization of CAC hydrates, which might be attributed to a microstructure based mainly on aluminosilicates. This paper deals with the microstructure of cement pastes fabricated with mixtures CAC-SF and CAC-FA and its evolution over time. Thermal analysis (DTA/TG), X-ray diffraction (XRD) and mid-infrared spectroscopy (FTIR) have been used to assess the microstructure of these formulations.

Nanoparticulated silicas with bimodal porosity: chemical control of the pore sizes.

Abstract: Nanoparticulated bimodal porous silicas (NBSs) with pore systems structured at two length scales (meso- and large-meso-/macropores) have been prepared through a one-pot surfactant-assisted procedure by using a simple template agent and starting from silicon atrane complexes as hydrolytic inorganic precursors. The final bulk materials are constructed by an aggregation of pseudospherical mesoporous primary nanoparticles process, over the course of which the interparticle (textural) large pore system is generated. A fine-tuning of the procedural variables allows not only an adjustment of the processes of nucleation and growth of the primary nanoparticles but also a modulation of their subsequent aggregation. In this way, we achieve good control of the porosity of both the intra- and interparticle pore systems by managing independent variables. We analyze in particular the regulating role played by two physicochemical variables: the critical micelar concentration (cmc) of the surfactant and the dielectric constant of the reaction medium.

The interaction of magnesium in hydration of C3S and CSH formation using 29Si MAS-NMR

Abstract: Hydration of tricalcium silicate in hydrothermal conditions in the presence of magnesium oxide has shown changes in the formation of CSH gel structure (Calcium silicate hydrates). The new CSH incorporates magnesium ions, brucite, but a weak presence of portlandite. The magnesium oxide would hinder the precipitation of portlandite. The characterization of CSH gel by 29Si MAS-NMR with various CaO/SiO2 ratios would point out that: (1) A dreierketten structure of the CSH for low CaO/SiO2 1, magnesium ions would be incorporated in the silicate chains of the CSH gel in a tetrahedral coordination. Although, the low MgO/CaO ratios of CSH gels indicate that the magnesium incorporation in CSH chain is low.

Mesosynthesis of ZnO-SiO(2) porous nanocomposites with low-defect ZnO nanometric domains.

Abstract: Silica-based ZnO–MCM-41 mesoporous nanocomposites with high Zn content (5 Si/Zn 50) have been synthesized by a one-pot surfactant-assisted procedure from aqueous solution using a cationic surfactant (CTMABr = cetyltrimethylammonium bromide) as structure-directing agent, and starting from molecular atrane complexes as inorganic hydrolytic precursors. This preparative technique allows optimization of the dispersion of the ZnO nanodomains in the silica walls. The mesoporous nature of the final materials is confirmed by x-ray diffraction (XRD), transmission electron microscopy (TEM) and N2 adsorption–desorption isotherms. The ZnO–MCM-41 materials show unimodal pore size distributions without blocking of the pore system even for high Zn content. A careful optical spectroscopic study (using x-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and UV–visible spectroscopy) of these materials shows that, irrespective of the Si/Zn ratio, the Zn atoms are organized in well-dispersed, uniform low-defect ZnO nanodomains (radius about 1 nm) and are partially embedded within the silica walls.

Performance enhancement of recycled concrete aggregate – A review

Abstract: Recycled concrete aggregate differ from natural aggregate as the former contains hardened cement mortar. The adhered cement mortar on recycled concrete aggregate has higher porosity and water absorption and lower strength than natural aggregate do. It has negative effects on the mechanical properties and durability of fresh and hardened concrete made with recycled concrete aggregate. Therefore, it will facilitate the applications of recycled concrete aggregate if the adhered cement mortar can be enhanced. Removing and strengthening the adhered mortar are the two main methods for enhancing the properties of recycled concrete aggregate. This paper reviews the published enhancement methods for recycled concrete aggregate, and points out their advantages and disadvantages so as to facilitate the selection and further development of suitable enhancement methods for recycled concrete aggregate. It suggests that carbonation treatment is an efficient and feasible method for improving the mechanical properties and durability of recycled concrete aggregate. Carbonation treatment of recycled concrete aggregate is not only an efficient way for enhancing the properties of recycled concrete aggregate, but also an environmental friendly approach.

Investigation of the carbonation mechanism of \{CH\} and C-S-H in terms of kinetics, microstructure changes and moisture properties

Abstract: The purpose of this article is to investigate the carbonation mechanism of CH and C-S-H within type-I cement-based materials in terms of kinetics, microstructure changes and water released from hydrates during carbonation. Carbonation tests were performed under accelerated conditions (10% CO2, 25 °C and 65 ± 5% RH). Carbonation profiles were assessed by destructive and non-destructive methods such as phenolphthalein spray test, thermogravimetric analysis, and mercury intrusion porosimetry (destructive), as well as gamma-ray attenuation (non-destructive). Carbonation penetration was carried out at different ages from 1 to 16 weeks of CO2 exposure on cement pastes of 0.45 and 0.6 w/c, as well as on mortar specimens (w/c = 0.50 and s/c = 2). Combining experimental results allowed us to improve the understanding of C-S-H and CH carbonation mechanism. The variation of molar volume of C-S-H during carbonation was identified and a quantification of the amount of water released during CH and C-S-H carbonation was performed.

Carbonation of cement paste : Understanding, challenges, and opportunities

Abstract: Cement paste is known to react with atmospheric carbon dioxide. Carbonation of cement paste has long been recognized as one of the causes of reinforcement corrosion. On the other hand, carbonation causes numerous chemomechanical changes in the cement paste, most notably changes in strength, porosity, pore size distribution, and chemistry. Furthermore, it can cause shrinkage and cracking of the cementitious matrix. The present review summarises the state of the art regarding the understanding and consequences of carbonation of cement paste. Apart from the passive process of reaction of atmospheric CO2 with cement paste, carbonation is sometimes used on purpose in order to improve certain properties of cementitious materials. This review further summarises recent efforts regarding active use of carbonation as a tool for manipulating certain properties of cement based materials. Possible fields of application include accelerated curing, improvement of fibre reinforced cementitious composites, concrete recycling, and waste immobilization.

Microstructure of cement paste subject to early carbonation curing

Abstract: Microstructure of Ordinary Portland Cement paste subjected to early age carbonation curing was studied to examine the effect of early carbonation on performance of paste at different ages The study was intended to understand the mechanism of concrete carbonation at early age through the microstructure development of its cement paste Early carbonation was carried out after 18-hour initial controlled air curing The microstructure characterized by XRD, TGA, 29 Si NMR and SEM was correlated to strength gain, CO 2 uptake and pH change It was found that early carbonation could accelerate early strength while allowing subsequent hydration The short term carbonation created a microstructure with more strength-contributing solids than conventional hydration Calcium hydroxide was converted to calcium carbonates, and calcium–silicate–hydrate became intermingled with carbonates, generating an amorphous calcium–silicate–hydrocarbonate binding phase Carbonation modified C–S–H retained its original gel structure The re-hydration procedure applied after carbonation was essential in increasing late strength and durability

MgO content of slag controls phase evolution and structural changes induced by accelerated carbonation in alkali-activated binders

Abstract: The structural development and carbonation resistance of three silicate-activated slags (AAS) with varying MgO contents ( 5%), hydrotalcite is identified as the main secondary product in addition to C–A–S–H. Higher extent of reaction and reduced Al incorporation in the C–S–H product are observed with higher MgO content in the slag. These gel chemistry effects, and particularly the formation of hydrotalcite, seem to reduce the susceptibility to carbonation of AAS produced with higher MgO contents, as hydrotalcite appears to act as an internal CO2 sorbent. This is evidenced by an inverse relationship between natural carbonation depth and slag MgO content, for paste samples formulated at constant water/binder ratio. Thus, the carbonation performance of AAS can be enhanced by controlling the chemistry of the precursors.