ROLE OF CHEMICAL BINDING IN DIFFUSION AND MASS TRANSPORT

摘要

Mass transport of species through Portland cement is influenced by species binding to cement solids. The strength and nature of the binding forces depends greatly on the chemical nature of the species as well as on cement composition. A classification of binding types is presented: the main mechanisms, in approximate order of increasing strength of binding, are: sorption, ion exchange and precipitation. The spectrum of binding mechanisms is also influenced by concentration, with sorption often dominant at low concentrations. The presentation concentrates on developing examples, including alkali and chloride binding and the influence of binding on reactions with cement of salt solutions, including magnesium sulfate and sea water. Freeze-thaw is introduced as an example of how spontaneous physical changes can influence mass transport processes. Many of the features induced by binding in cement and concrete can be anticipated from thermodynamics but, in the present state of knowledge, mineral zonation and the balance between precipitation and dissolution are best inferred from real exposures and laboratory simulations.