The structural stability and compression behavior of hydrated magnesium carbonates are critical for understanding their role in permanent CO2 sequestration and for optimizing low-carbon cementitious materials. This work investigates the compressibility, anisotropy, and structural evolution of Mg5(CO3)4(OH)2·4H2O hydromagnesite mineral under high-pressure conditions using a combination of single-crystal and powder X-ray diffraction, high-pressure Raman spectroscopy, and density functional theory calculations. The results reveal a previously unreported pressure-induced order–disorder–order phase-transition sequence occurring at 0.6(1) and 7.7(1) GPa, driven by positional disorder of a carbonate group along the direction of maximum compressibility. No changes in structural topology or space-group symmetry are observed throughout compression. Quantitative Euler-angle analysis demonstrates that the structural evolution is governed by progressive carbonate tilting and rigid-body rotations of [MgO6] octahedra. A third-order Birch–Murnaghan equation of state yields V0 = 687.71(14) Å3, K0 = 40.7(6) GPa, and K′0 = 5.18(14). Raman spectroscopy confirms the vibrational stability of hydromagnesite over the investigated pressure range and reveals an initial stiffening of OH stretching vibrations, followed by progressive strengthening of hydrogen bonding above ~7.2 GPa. These findings provide a comprehensive crystallographic, elastic, and spectroscopic description of hydromagnesite under compression and offer new insights into the behavior of hydrated magnesium carbonates relevant to carbon storage, geochemical processes, and MgO-based cement technologies.
Modal correlation techniques are widely used to compare the dynamic behaviour of different structural models. However, classical mode-by-mode indicators such as the Modal Assurance Criterion (MAC) may become misleading when repeated or closely spaced modes are present, especially in systems with complex mode shapes. In these cases, small changes in mass, stiffness, damping, modelling assumptions, or experimental conditions, may produce large rotations of the modal basis within an almost invariant subspace, without necessarily indicating a relevant change in the underlying dynamic behaviour. This paper addresses this limitation by extending the concept of rotated MAC (ROTMAC) to systems with complex modes and by introducing subspace-based correlation indicators for complex modal bases. Structural dynamic modification theory is used to express the perturbed complex modes as linear combinations of the unperturbed modes, providing a direct interpretation of modal rotations and modal mixing. Then, Euclidean and mass-weighted ROTMAC formulations are developed by estimating an optimal unitary transformation that aligns the modal bases before mode-by-mode comparison. In addition, Euclidean and mass-weighted subspace MAC formulations are proposed based on orthonormal bases and principal angles between modal subspaces. Practical guidelines are provided to clarify the use of MAC, ROTMAC, or subspace MAC. The proposed methodology is assessed using an analytical four-degree-of-freedom system with repeated modes and an experimental symmetric steel cantilever beam. The results show that ROTMAC improves mode-by-mode correlation when discrepancies are mainly caused by modal-basis rotations, whereas subspace MAC provides a more robust and physically meaningful assessment when modal uniqueness is lost.
This paper evaluates how the availability of high-speed rail (HSR) affects tourists' transport mode choices for medium-distance domestic trips. Exploiting the opening of a new HSR line connecting Madrid with Northern Spain in late 2023 as a quasi-natural experiment, we apply a non-linear difference-in-differences design within a multinomial logit framework to representative microdata on more than 3200 domestic trips over a 36-month period. Results show a significant increase in rail use, accompanied by reductions in the use of competing transport modes, particularly the private car. The substitution of air travel by rail appears to be limited and is only documented among the leisure segment. By providing causal evidence of HSR's impact on modal substitution patterns in tourism, this study contributes to the literature by showing the potential of HSR investments to reduce the carbon footprint of tourism while maintaining regional accessibility.
This work studies the use of modified bio-oil derived from Chlorella vulgaris microalgae (MBOC) as lubricity improver of diesel. Microalgae-based modified bio-oils (MBOs) offer a sustainable alternative to first and secondgeneration sources, providing superior lubricity through their rich content of polar fatty acid methyl esters (FAMEs). The microalgae bio-oil was produced by alkaline transesterification, subsequently purified using bentonite to remove pigments and residual impurities, and finally epoxidized to enhance its properties prior to blending with diesel. High-frequency reciprocating rig tribological tests and surface analysis by confocal and scanning electron microscopies were used to evaluate the effect of various MBOC concentrations (5-20% v/v) in diesel fuel on tribological response (friction, wear scar, and wear mechanism). The results revealed a consistent friction reduction with an increase in MBOC content. This phenomenon may be attributed to the adsorption of FAME, which could foster the formation of a robust tribolayer on metallic surfaces. However, the wear exhibited distinctly non-linear behavior. Optimal tribological performance occurred at intermediate MBOC concentrations (5-10%), yielding minimal wear scar diameter (WSD), shallower wear tracks, and smoother morphology which is indicative of effective surface protection. Conversely, a higher concentration (20%) of MBOC resulted in increased wear severity, despite the ongoing reduction in friction. The SEM observation showed that abrasion was the predominant wear mechanism. These findings position MBOC from Chlorella vulgaris as a potential renewable lubricity improver for diesel, suggesting that a blending ratio between 5% and 10% could achieve an optimal balance between friction control and wear mitigation.
