By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Nov 27 2024A recent article published in Buildings proposed developing marine alkali paste (MAP) using seawater (SW), recyclable particles from paste specimens (RPPs), and alkali agents, including NaOH (NH) and Na2O·3SiO2 (NS). The physicochemical and mechanical properties of the MAP were investigated via uniaxial compression tests (UCTs), an energy-dispersive spectrometer (EDS), X-ray diffraction (XRD), and scanning electron microscopy (SEM)
Study: Novel Cement-Based Materials Using Seawater, Reused Construction Waste, and Alkali Agents. Image Credit: Parilov/Shutterstock.com
Background
Conventional cement-based materials (CCBMs) produced using natural minerals and freshwater (FW) are widely employed in global urbanization. However, this has proved to be detrimental to the ecological environment.
Reused construction waste (RCW) can partially or completely substitute CCBMs, which helps alleviate the ecological damage. RCW contains several active silicon and aluminum components that can be activated through mechanical, thermal, and chemical methods.
Additionally, SW can partially or completely substitute FW used in CCBMs, which helps mitigate tFW shortage, particularly in coastal and offshore regions that suffer from substantial availability gaps and exorbitant FW transportation expenses. Novel CBM (NCBM) production in marine environments using SW and RCW also shows advantages of economy, recyclability, applicability, and lower carbon emissions.
However, the successful application of such NCBM requires proper treatment of complicated ions, including Cl−, Na+, SO42−, CO32−, etc., which is possible through precise quantification of the physicochemical properties of RCW and its products.
Methods
Thirty parental specimens (PSs) were designed with a mix ratio of FW/cement =1:0.458. The properly mixed pastes were used to prepare 70.7×70.7×70.7 mm3 cured for 40 days. Subsequently, the PSs were crushed using a hammer and a portable crusher to prepare RPPs with a size below 0.075 mm for optimal physical activity.
MAP was synthesized via alkali activation using SW, RPPs, and alkali agents, NH and NS. Notably, seven mix ratios, including 4 NH and 3 NS ones, were prepared. After preparing the MAP pastes according to the mix ratios developed at the School of Marine Engineering Equipment, Zhejiang Ocean University, China, 70.7×70.7×70.7 mm3 specimens were molded and cured for 24 hours at controlled temperature and relative humidity.
Later, the MAP specimens in the standard curing room were completely submerged in SW tanks to simulate a marine environment for 8 and 49 days. The performance of the MAP specimens was evaluated via UCTs, EDS, SEM, and XRD analyses.
Additionally, sub-specimens taken from the fracture zones and intact fragments of the MAP specimens destructed in UCTs were investigated with EDS, SEM, and XRD. The locations of the broken zones and intact ones, where the sub-specimens were taken from, were defined as BZs and nBZs, respectively.
Results and Discussion
The bi-stage crushing process considerably improved the physical reactivity of RPPs by enhancing their specific surface area, which increased their reactivity during subsequent alkali activation and enabled the formation of other hydration products in MAP.
The primary hydration products in MAP were determined as xCaO·SiO2·nH2O (C-S-H), CaO·Al2O3·2SiO2·4H2O (C-A-S-H), Na2O·Al2O3·xSiO2·2H2O (N-A-S-H), Friedel’s salt, monosulfate (FS), and CaCO3. These products, specifically the N-A-S-H gel, enhanced the structural integrity of MAP and corrosion resistance to ions such as Cl− and SO42−, particularly in NS-activated systems.
The compressive strength of MAP samples varied with curing age. Notably, MAP prepared using NS had higher early-age strength (8.3 MPa at 8 days) than that using NH (5.59 MPa at 8 days). By 49 days, MAP prepared using NS continued to show higher strength. Additionally, some MAP specimens exhibited strength gains with longer curing time. Therefore, proper mix ratios could further improve the mechanical properties of MAP.
MAP specimens prepared using NS demonstrated a more compact and consistent microstructure with well-formed C-A-S-H and C-S-H gels, which reduced their porosity. Alternatively, NH-prepared MAP specimens had a more agglomerated structure, in which CO32− complexes increased their porosity. However, the gradual carbonation of the specimens enhanced their strength and compactness over time in marine conditions.
The MAP formulations adapted well to marine environments. The development of N-A-S-H, FS, and reduced ettringite prevented sulfate and chloride ion intrusion, ensuring the durability of MAP. These results underscored the potential of MAP as a sustainable construction alternative in marine and coastal environments.
Conclusion and Future Prospects
Overall, the researchers successfully developed MAP using RPPs, SW, and alkali agents, NH and NS. They comprehensively examined MAP's mechanical and physicochemical properties, demonstrating its excellent adaptability in marine environments.
However, this study has certain limitations. It primarily relied on SEM imaging, which provided only two-dimensional images of the material cross-section. This lack of three-dimensional structural information might lead to misinterpretations of MAP’s internal structure and the resulting mechanical performance.
Additionally, laboratory-scale testing limited this study's findings. MAP’s performance in diverse harsh environments and long-term durability need further validation. Thus, the researchers plan to optimize MAP mix designs for different marine exposure levels, perform field tests in marine structures, and investigate applications in other harsh environments.
Journal Reference
Bai, Y., Wang, Y., Yang, T., & Chen, X. (2024). Novel Cement-Based Materials Using Seawater, Reused Construction Waste, and Alkali Agents. Buildings, 14(11), 3696. DOI: 10.3390/buildings14113696, https://www.mdpi.com/2075-5309/14/11/3696
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.