Construction is an important segment of the economy that employs millions of people. Construction dust is an occupational health hazard to millions of construction workers worldwide. The hazards associated with respirable dust depend upon its particulate size distribution and chemical composition, as these determine the deposition pattern in the respiratory tract and reactivity, respectively. This study presents characterization of the size and composition of the dust from two key construction materials—cast cement and poured concrete. The dust was generated by cutting the cured cement and concrete blocks using an 18” hand-held circular saw as used in highway and building construction. Transmission electron microscopy, scanning electron microscopy, dynamic light scattering, and laser diffraction were performed for the size analysis of the particles. Energy dispersive spectroscopy and X-ray photoelectron spectroscopy were used for chemical analysis. X-ray diffraction was used for phase identification. Electron diffraction patterns were obtained to assess the crystallinity of individual particles. They confirm the crystallinity of particles of different size and shapes. With a particle size range between 0.5 μm and 10 μm, greater than 90% of particles fell below 2.5 μm, presenting a respirable health concern. Crystalline compounds including the metals Al, Ca, Fe, Mg, Na, and K were detected. The concrete particles were most enriched in crystalline silica with a concentration of more than 30% by weight. The presence of metals and high crystalline silica content pose a serious health concern to construction workers.

Originating from an IAE V2527 aircraft gas turbine engine running a series of reference and blended fuels, non-volatile particulate matter was collected upon transmission electron microscopy grids for analysis. A striking observation is the range of primary particle size with projected particle diameters ranging from 5 – 100 nm within the same aggregate. This range of particle size is interpreted as describing the corresponding ensemble of fuel rich pockets in terms of fuel-air-equivalence ratio (Φ) and duration. Some parcels are sufficiently diluted or short-lived by turbulent action and allow only small primary particles to form. Other parcels are larger, live longer or a combination allowing large primary particles to form. In this manner, the soot particles provide forensic insights into the turbulent mixing dynamics. Consequently, there is no single streamline or trajectory describing particle growth and no single condition in Φ or temperature for particle nucleation. There is also no observable correspondence between particle size and fuel aromatic or hydrogen content. Given the different trajectories of small/large particles, their growth histories and species origins, a difference in H/C ratio would be expected. This is reflected in the relative contributions of sp²/sp³ content within the particles as confirmed by electron energy loss spectroscopy analysis. Supporting evidence for the role of turbulent induced mixing upon primary particle growth is evidenced by varied particle size modes for XC-72, oxidation reactivity of R250, both commercial furnace blacks. Further confirmation is found by pulsed laser derivatization of XC-72. Graphical Abstract

Differences in lamellae length, stacking, and particularly a difference in the core-to-shell radial structure are observed for carbon blacks produced using different feedstocks. Carbon black (CB) produced using a coal tar (CT) feedstock formed particles with amorphous cores exhibiting a sharp transition to extended lamellae oriented about the periphery of the particle. In contrast, the carbon black produced from fluidized catalytic cracker (FCC) decant oil as feedstock formed particles with a single nucleated core possess a rather uniform radial transition—reflecting the presence of ordered, concentric lamellae across most of the particle radius. Minimal disorder was observed in the core while the undulations in perimeter lamellae were fewer. Our interpretation for these structural dissimilarities is premised on differences in fuel composition, specifically component classes as found by saturate, aromatic, resin, asphaltene (SARA) analysis. These in turn lead to variation in the relative rates of particle nucleation and particle growth by pyrolysis products, moderated by temperature. Electron energy loss spectroscopy reveals radial variation in the sp² content between the different feedstocks consistent with observed nanostructures. Collectively these results are interpreted in terms of an offset in nucleation and growth—dependent upon the relative contributions of feedstock aromatic content and pyrolysis processes to particle nucleation and growth. To further test the postulate of different formation conditions for the two carbon blacks pulsed laser annealing was applied. The high temperature heating accentuated the dissimilarities in nanostructure and chemistry—leading to stark dissimilarities. These differences were also manifested by comparing oxidative reactivity.

This study investigates and compares the efficacy of conventional thermal pyrolysis and microwave (MW) plasma pyrolysis in upgrading coal-derived precursors. Coal samples presenting a range of ranks were pyrolyzed under various reactive and nonreactive atmospheres using a pyroprobe, with the pyrolyzates analyzed by gas chromatography-mass spectrometry (GC-MS). Comparative MW plasma tests were conducted using a modified countertop MW unit, with condensed products similarly analyzed by GC-MS. A predominant coal devolatilization product-benzene was selected for analyzing the reactive MW plasma upgradation. Results demonstrate that conventional thermal pyrolysis lacks effectiveness in upgrading the precursors. To gain insight into the underlying reasons, chemical kinetic simulations were conducted. Oppositely, reactive MW plasma pyrolysis demonstrated remarkable precursor upgradation. These condensed MW plasma pyrolysis products were then subjected to a carbonization and graphitization heat-treatment with a comprehensive graphitic quality assessment conducted using X-ray diffraction and transmission electron microscopy. After graphitization, the MW plasma-upgraded precursor produced a carbon with a crystallite size several times greater than that of the initial benzene. By MW plasma processing, the poorly graphitizable benzene precursor was transformed into a highly graphitizable precursor comparable to coal tar pitch. The underlying reasons for this significant improvement were investigated by analyzing the compositional changes in the precursor under various reactive environments.