A comparative life cycle assessment of recycling waste concrete powder into CO2-Capture products.

Waste concrete powder (WCP), a byproduct of construction and demolition (C&D), currently has a low degree of recycling despite its potential for environmentally friendly applications. WCP can serve as a valuable substitute for cement, offering advantages for resource conservation and carbon sequestration. However, there are very few studies that quantitatively assess the environmental impact of incorporating WCP into the circular economy as a secondary material instead of disposing of it. The energy-intensive processing of WCP raises questions about the optimal carbonation time using available equipment. This study aims to fill this knowledge gap by employing carbon footprint and life cycle assessments (LCA) to optimize WCP recycling. Three recycling WCP scenarios are analyzed. The first scenario involved the conversion of WCP into compacts that absorb CO2 during the carbonation process. The results of the first scenario revealed that the optimal carbonation time for WCP compacts was 8 h, during which 42.7 kg CO2-e per tonne of WCP compacts was sequestered. The total global warming potential (GWP) was -4.22 kgCO2-e, indicating a carbon-negative recycling process. In the second and third scenarios, LCA was conducted to compare the use of carbonated and uncarbonated WCP as a partial replacement for cement in concrete. In these scenarios, it was found that uncarbonated WCP is a more effective solution for reducing the carbon footprint of traditional concrete mixes, achieving a significant 16% reduction of GWP when 20% of cement is replaced. Conversely, using carbonated WCP as a partial cement replacement in concrete mixtures shows limited potential for CO2 uptake. The sensitivity analysis reveals that the carbon footprint of the WCP compacts production process is strongly influenced by the electricity supplier used.

Exploring greenhouse gas emissions pathways and stakeholder perspectives: In search of circular economy policy innovation for waste paper management and carbon neutrality in Hong Kong.

Waste paper disposed in landfills notably contributes to greenhouse gas (GHG) emissions and impedes more sustainable, circular alternatives, such as recycling. In Hong Kong, this unsustainable approach is currently dominant as 68% of waste paper products are treated in landfills in 2020. To contextualize the impact of local waste paper management and explore mitigation potentials of circular alternatives, this paper develops a quantitative assessment framework around GHG emissions development trajectories. Combining guidelines of the Intergovernmental Panel on Climate Change (IPCC), national GHG inventories, and local parameters from life cycle analysis, five GHG emissions projections were simulated along the Shared Socioeconomic Pathways (SSPs) until 2060. Most recent baselines indicate that Hong Kong's current waste paper treatment generated 638,360 tons CO2-eq in 2020, comprising 1,821,040 tons CO2-eq from landfill and 671,320 tons CO2-eq from recycling, and -1,854,000 tons CO2-eq from primary material replacement. Proceeding along a Business-as-Usual scenario under SSP5, GHG emissions will dramatically increase to a net 1,072,270 tons CO2-eq by 2060, whereas a recycling-intensive scenario will lead to a net saving of -4,323,190 tons CO2-eq. To complement the quantitative evidence on the benefits of waste paper recycling, field research was conducted to explore the feasibility of circular policy innovation from the perspective of recycling stakeholders. These empirical qualitative and quantitative findings from stakeholders' business routines and material transactions provide crucial indications for policy and institutional innovation: Essentially, for Hong Kong to improve waste paper recycling capacities and facilitate a circular economy (CE), local stakeholders require support via fiscal policy measures (financial subsidies or tax reductions) and infrastructure improvements (delivery access and material storage). In sum, this study employs a novel analytical framework combining original qualitative and quantitative evidence to provide policy innovation towards circular, GHG emission-saving waste paper management.

Hygroscopic and phase transition properties of alkyl aminium sulfates at low relative humidities.

Alkyl aminium sulfates (AASs) can affect the physicochemical properties of atmospheric aerosols such as hygroscopicity. Previous laboratory experiments have shown that the water content in AAS bulk solutions is higher than in aqueous ammonium sulfate solution in the range of 60-95% relative humidity (RH). Furthermore, amine was found to evaporate from the solution during the preparation of AASs from the parent amine and sulfuric acid solutions. Here we report the hygroscopicities of deposited particles of four AASs at different aminium-to-sulfate molar ratios (A/Ss) in the range of <3-90% RH using air-flow cells coupled with in situ micro-Raman spectroscopy. Normalized integrated areas of O-H stretching peaks in the Raman spectra were converted to water-to-solute molar ratios (WSRs) at various RH values. Evaporation of amine was also observed in most cases and the exact A/Ss of sample particles or solutions were determined by ion chromatography. Mono-methylaminium sulfate (MMAS) and mono-ethylaminium sulfate (MEAS) particles were stable at A/S = 2.0, but di-methylaminium sulfate (DMAS) and tri-methylaminium sulfate (TMAS) suffered from DMA and TMA evaporation and eventually equilibrated to the A/S of 1.5 and 1.0, respectively. At these stable compositions MMAS and MEAS exhibited phase transitions in the super-saturation region, while DMAS and TMAS showed a continuous and reversible water uptake. Besides, an approach to estimate the hygroscopicities of DMAS and TMAS particles at an initial A/S larger than that of the stable compositions was presented. In the range of 60-95% RH, the WSRs of all the studied AAS particles were consistent with a previous study based on experimental values and the extended Zdanovskii-Stokes-Robinson equation. In general, all the studied AASs were more hygroscopic than their corresponding ammonium counterparts within the studied RH range and evaporation of amine needs to be corrected in studying unstable AAS particles.