Thermal desorption/pyrolysis gas chromatography-mass spectrometry of Portland cement and concrete for the determination of grinding aids and chemical admixtures.

摘要

The increasing demand for improved quality of various construction materials such as Portland cement and Portland cement-based concrete mixtures has placed a burden on material scientists to characterize these materials in order to address deviations from expected performance. This research project was conducted to advance the art of determining the identity and concentration of organic additions to cement and concrete by evaluating the feasibility of using thermal extraction and pyrolysis gas chromatography-mass spectrometry.; Current methods of analyses are limited to a handful of techniques, all of which involve solvent extraction of the cement or concrete, followed by an attempt to spectroscopically identify the extracted compounds. The major difficulties for determining admixtures in cement and concrete include: (a) dosage levels can vary from 5 to 2000 ppm; (b) the additives are normally highly polar mixtures of surface active agents, which can be strongly adsorbed onto the cement and mineral admixtures such as coal derived fly ash, blast furnace slag, and silica fume; (c) additives can react to form other compounds; (d) the extraction process can convert the additives to other compounds; and (e) concrete often has three or more additives which cause cross interference during measurement.; To address the many challenges associated with determining additives in cement and concrete, the adaptation of thermal desorption and pyrolysis—coupled with gas chromatography-mass spectrometry (GC-MS)—was explored by fabricating a unique oven capable of handling up to 2 grams of powdered material, and developing various in-situ pyrolytic derivatization reactions to determine strongly adsorbed additives.; Among the many cement additives and concrete admixtures evaluated over the course of this research project, only phenol-based cement grinding aids and glycol-ether type shrinkage reducing admixtures could be quantitatively measured by simple thermal desorption. Admixed naphthalene sulfonate formaldehyde condensate and neutralized rosin acids could be quantitatively measured by pyrolysis of the concrete to form naphthalene and phenanthrene, respectively. In-situ pyrolytic derivatization using BSTFA-TMCS [N,O-bis(trimethylsilyl)trifluoroacetamide - trimethylchlorosilane] reagent enabled the qualitative determination of glycols and alkanolamines in Portland cement. A more complex nitrosation/hydrolysis reaction system was developed for the quantitative determination of a nitrite-based corrosion inhibitor in concrete. In situ pyrolytic methylation of tricyclic and fatty acid salts in a powdered concrete sample was accomplished using TMAH (tetramethyammonium hydroxide) reagent.; The feasibility of thermally extracting (or pyrolyzing) lignosulfonate and corn syrup, two common dispersing/retarding agents for concrete, was inconclusive. Pyrolysis of the neat lignosulfonate polymer provides a rich GC-MS profile indicating characteristic phenylpropyl fragments. However, using programmed pyrolysis from 200 to 800°C, only trace level molecular fragments characteristic of lignosulfonate and corn syrup could be detected in an admixed concrete sample. (Abstract shortened by UMI.)