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This study explores the influence of incorporating recycled aggregate (RA) and CO 2 -pretreated recycled aggregate (CRA) in reactive magnesia concrete (RMC) formulations. Through comprehensive microscopic characterizations, the phase composition, carbonation degree, pore structure, and interfacial transition zone (ITZ) characteristics are examined in both interior and exterior regions of specimens. The findings highlight that RA and CRA contribute to a 28 d strength in RMC formulations 14% and 25% higher than that with NA, respectively. This enhancement is attributed to internal curing and significantly increased channels for CO 2 diffusion within (C)RA specimens, leading to improvements in the modulus, hardness, and microstructures of the interior region—the traditionally weaker part in RMC-based concrete. Furthermore, a robust linear correlation is observed between ITZ characteristics, porosity, and compressive strength of CO 2 -cured specimens. Finally, utilizing different grades of RA improves the performance of RMC-based concrete to varying degrees while reducing costs, thus potentially expanding the application of RA in practical RMC-based concrete scenarios, regardless of RA quality. • The feasibility of using (C)RA in RMC system was verified, regardless of RA quality. • Interior region was strengthed by using (C)RA in RMC system. • ITZ characteristics and porosity indicated a strong linear correlation with strength. [ABSTRACT FROM AUTHOR]

Phosphate ions significantly enhance the water resistance of magnesium oxychloride cement (MOC). In this study, MOC was prepared with a MgO/MgCl 2 molar ratio of 7 and a H 2 O/MgCl 2 molar ratio of 12, while varying the dosage of sodium dihydrogen phosphate. The aim was to observe the microstructural changes in 5Mg(OH) 2 ·MgCl 2 ·8H 2 O (Phase 5) crystals and the macroscopic mechanical properties before and after immersion. The research found that phosphate ions increase the content of air-cured Phase 5 and reduce the formation of Mg(OH) 2 by lowering the required Mg2+ concentration for Phase 5 production. However, excessive phosphate ions can damage the crystal structure of Phase 5, leading to a decrease in MOC strength. An appropriate amount of phosphate ions increases the output of Phase 5, resulting in a highly dense microstructure, which significantly improves the water resistance of MOC. Additionally, this enhances the pore structure of MOC, making the overall structure more compact and resistant to water penetration. This research offers valuable theoretical insights into Phase 5 crystal variations during air curing and water-immersion damage under low Mg2+ concentrations. • MOC with a relatively low molar ratio of MgO/MgCl2 at 7. • Additional 1–2 % Phase 5 crystal formation. • Excessive sodium dihydrogen phosphate depletes Phase 5 quantity. • Dosage of sodium dihydrogen phosphate should be kept below 4 %. [ABSTRACT FROM AUTHOR]

The use of recycled aggregates (RCAs) could further enhance the sustainability of reactive magnesia cement (RMC) formulations, but the performances of RMC-RCA are still unknown to researchers. Therefore, in this study, the strength development of CO 2 -cured RMC-RCA was explored for the first time, and the strength variation among groups was further interpreted by microstructural analyses. The initial results show that compared with samples with natural aggregates (NAs), a 28 d strength boost (i.e., 37 vs. 15 MPa) in RMC-RCA can be observed. The strength improved in RMC-RCA could be related to its refined microstructures, ITZs and larger carbonation depths, which resulted from the easiness of CO 2 penetration within samples. Then the presence of porous ITZs in RCA contributes to better CO 2 diffusion, as described in the proposed revised CO 2 diffusion model. Overall, while enabling a pronounced CO 2 capture potential of RMC concrete, this preliminary study could pave the path for the upcycling of RCAs in RMC concrete on a large scale. • RMC with RCAs and NAs (various w/b ratios) in CO 2 and air curing is prepared. • Use of RCAs led to better strength improvement due to improved CO 2 diffusion and ITZs. • Porous ITZs in RCAs also promoted enhanced compacted structures and carbonation depths. • Pore structrues and carbonation degrees could be both critical for RMC samples' strength. • The revised CO 2 diffusion model provided enables better RMC-RCA preparation. [ABSTRACT FROM AUTHOR]

The properties of municipal solid waste incineration bottom ash (MSWIBA) relevant to its role as a potential cement replacement are not fully well understood. This study explores the efficacy of diverse physical and chemical pre-treatments in enhancing the physical properties and microstructural integrity of MSWIBA within mortar mixtures. Through extensive microscopic analysis, the obtained results revealed the effectiveness of ball milling MSWIBA for 0.5 h in optimizing the hydration response, evidenced by reduced porosity, increased hydration product formation, and improved crystallinity. Moreover, the addition of an alkaline chemical (Ca(OH) 2 , NaOH, and Na 2 CO 3) accelerated early hydration, albeit resulting in an irregular structure that impaired strength development. Notably, Ca(OH) 2 alkali solution facilitated a greater production of hydration products via the provision of additional nucleation sites. These findings established a foundation towards advancing the performance and broader utilization of MSWIBA in cement-based formulations. • Pre-treatment techniques were used to optimize MSWIBA's physicochemical properties. • Gas generation in MSWIBA was observed to affect the stability of its concrete. • Ball milling boosts MSWIBA reactivity, reduces porosity, and densifies the microstructure. • Alkali activation accelerates hydration but causes irregular product formation. [ABSTRACT FROM AUTHOR]

Magnesium potassium phosphate cement (MKPC) is susceptible to high-temperature damage when used as a fireproofing coating or structural repair material and as a nuclear waste encapsulation shell, and the strength loss and dimensional shrinkage at about 100 °C severely limit the application of MKPC. Therefore, in this paper, the macroscopic properties, physical phase composition and microscopic morphology of sucrose-modified MKPC at room and high temperatures (20°C, 50°C, 120°C) are investigated from the perspective of modified retarder, and the mechanism of sucrose's influence on MKPC at high temperatures and the optimal dosage are speculated, and the Pearson correlation analysis is performed on the various macroscopic and microscopic indexes. The results showed that 2.5 % sucrose doping could reduce dimensional shrinkage by 59.6 % and increase late strength by 21.8 % at high temperatures, significantly optimizing the macroscopic properties of MKPC. The inhibition of the high-temperature decomposition product MgKPO 4 ·H 2 O by sucrose and the reduction of pores formed by water evaporation are among the main reasons for the optimization of the properties. Additionally, the adsorption of amorphous products to form new polymers that fill the pores is also an important reason for the optimization of performance. However, Pearson's correlation analysis showed that too much sucrose contributed to the reduction of pulp strength, and its optimal dosage should be between 2 % and 3 %. • Experiments on the effect of sucrose on MKPC performance at high temperature. • Sucrose optimizes high-temperature-induced dimensional shrinkage of MKPC. • Sucrose at high temperature significantly reduced the late strength loss of MKPC. • Sucrose forms polymers to fill MKPC pores and improve densification. • The optimal range of sucrose dosing is 2∼3 %. [ABSTRACT FROM AUTHOR]

To mitigate land losses in the Mississippi River Delta, sediment diversions are being employed to enable the flow of river water and sediments into wetlands experiencing degradation. A two-dimensional coupled flow-wave Delft3D model was used in this study to explore the hydrodynamics and sediment transport in Fourleague Bay (FLB), Louisiana, USA, which has been considered an analog site for studying the efficiency of sediment diversion projects. In-situ measurements of sediment accretion and hydrodynamic characteristics from 2015 to 2016 were utilized to calibrate and validate the morphodynamic model. The validated model was then applied to quantify sediment transport in FLB and surrounding marshes between May 2015 and May 2016. The results show that more sediment could be deposited to the surrounding marshes with high river discharges and strong winds. Thus, by strategically aligning the timing of pulses of river water from the diversion with the seasonal intensification of atmospheric forcing, it is possible to sustain and promote the growth of the surrounding wetlands. Moreover, we found that multiple sediment transport processes occurred during the entire study period, including the deposition of riverine sediment into the bay floor, direct deposition of riverine sediment in the surrounding marshes, resuspension of bay floor sediment, and redistribution of resuspended sediment to adjacent marshes and the Gulf of Mexico (GoM). The results indicate that the riverine sediment tended to be directly deposited in the marshes when the river discharge was high. During calm weather conditions and normal river discharge, FLB acted as a reservoir, storing sediment from the upper river, and later acted as a sediment source to the nearby wetlands and the GoM during energetic atmospheric conditions. This suggests that using the bay floor as a reservoir can extend the distance over which wetlands can benefit from the sediment diversions, as the supply of sediment to the wetlands becomes a multi-step process. Thus, it is important to retain sediments from river diversions in shallow bays and allow storms to redistribute them to adjacent wetlands. • A morphodynamic model was developed to understand the fine sediment dispersal process from a river to coastal wetlands. • The model was calibrated and validated using unique measurements of hydrodynamics and sediment accretion rates. • More sediment could be deposited to the surrounding marshes with high river discharges and strong winds. • Sediment deposited on the surrounding marshes was primarily from the river and bay floor. • Riverine sediment tends to be directly deposited onto the marshes during high discharge stages. [ABSTRACT FROM AUTHOR]

Hurricanes can greatly modify the sedimentary record, but our coastal scientific community has rather limited capability to predict hurricane-induced sediment deposition. A three-dimensional sediment transport model was developed in the Regional Ocean Modeling System (ROMS) to study seabed erosion and deposition on the Louisiana shelf in response to Hurricanes Katrina and Rita in the year 2005. Sensitivity tests were performed on both erosional and depositional processes for a wide range of erosional rates and settling velocities, and uncertainty analysis was done on critical shear stresses using the polynomial chaos approximation method. A total of 22 model runs were performed in sensitivity and uncertainty tests. Estimated maximum erosional depths were sensitive to the inputs, but horizontal erosional patterns seemed to be controlled mainly by hurricane tracks, wave–current combined shear stresses, seabed grain sizes, and shelf bathymetry. During the passage of two hurricanes, local resuspension and deposition dominated the sediment transport mechanisms. Hurricane Katrina followed a shelf-perpendicular track before making landfall and its energy dissipated rapidly within about 48 h along the eastern Louisiana coast. In contrast, Hurricane Rita followed a more shelf-oblique track and disturbed the seabed extensively during its 84-h passage from the Alabama–Mississippi border to the Louisiana–Texas border. Conditions to either side of Hurricane Rita’s storm track differed substantially, with the region to the east having stronger winds, taller waves and thus deeper erosions. This study indicated that major hurricanes can disturb the shelf at centimeter to meter levels. Each of these two hurricanes suspended seabed sediment mass that far exceeded the annual sediment inputs from the Mississippi and Atchafalaya Rivers, but the net transport from shelves to estuaries is yet to be determined. Future studies should focus on the modeling of sediment exchange between estuaries and shelves and the field measurement of erosional rates and settling velocities. [ABSTRACT FROM AUTHOR]