- Email: [email protected]
cement interactions
Waste Management 26 (2006) 687–688 www.elsevier.com/locate/wasman
Editorial
Mechanisms and modeling of waste/cement interactions International Workshop, May 8 to 12, 2005, Meiringen, Switzerland
Traditionally cement research has primarily focused on the development of safe and improved construction materials, centered on the use of Portland cement or one of its permitted variants. Cement chemists furthered our understanding of the hydration process, the kinetics, the minerals formed, the thermodynamic stability of the mineral assemblages, interactions with aggregate materials and long-term changes in mineralogy. Recent developments in the cement industry have increased the complexity of cementitious materials. Supplementary cementing materials, including coal combustion fly ash, iron blast furnace slag, silica fume, metakaolin, etc., greatly modify the bulk chemistry, and hence the mineralogy, of the solid matrix. Using wastes to replace coal, oil or gas normally used to pyroprocess the cement, saves energy and is a convenient way of eliminating wastes: examples include solvents, contaminated/used oil, rubber, paper or plastics. Contaminated soils may also be added to the kilns as a raw material. While organic contaminants are destroyed in the kiln, an increase in metal concentrations is possible. Stakeholders need to be able to assess the impact of elevated metal concentrations on cement properties and the environment. The last few decades have also seen much use of cement to immobilize hazardous and nuclear wastes and to chemically bind inorganic species in the cement matrix. Also, residues from incineration, for example, municipal solid waste or sewage sludge that are landfilled, can have chemical properties very similar to cementitious materials but at the same time have elevated heavy metal concentrations. For all these waste types, the long-term release of heavy metals or radionuclides to the environment is an essential element of risk assessment. For this, an understanding of heavy metal and radionuclide binding is essential. Like most chemical reactions, the binding mechanism is species- and concentration-dependent. The species-dependent nature of reactions, as well as the mechanisms and kinetics of competing reactions, has been a challenge to elucidate. Scientists have also to assess the duration of 0956-053X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2006.04.003
the performance of cement barriers. Specific questions that are equally applicable to conventional and nuclear waste management include the following: • how is a species bound in the matrix? • how soluble is it as a consequence of binding? • how and under what conditions does diffusion out of the cementitious matrix occur? • how does its solubility change as the cementitious matrix ages and degrades? • how does the near-field interact with the cementitious matrix and affect its binding properties? Three basic binding mechanisms can be discerned: precipitation, adsorption to the surfaces of minerals either by specific binding or electrostatically by ion exchange, and, lastly, incorporation in cement hydrate minerals. Solubility is a limiting factor with regard to the second and third mechanisms listed above, so only ions that are sufficiently soluble in basic media will be incorporated in or sorbed to hydrated cement minerals to a significant degree. Where concentrations are low, use can be made of partition coefficients in the assumption that a particular species is bound in an unspecified way. However, long-term assessments, or the assessment of concentration-dependent behavior requires an exact understanding of the binding mechanism and it is here that much research effort has to be made. Efforts to extend our understanding of waste–cement interactions are largely related to the need for and, as a consequence, the funding of fundamental research. Industrial countries are investing a great deal of money and time to assess the safety of planned nuclear repositories, of which predicting long-term stability of cementitious materials and release of radionuclides are key issues. Waste disposal is traditionally at the ‘‘cheap’’ end of investment and consequently has the weaker science base. Performance tends to rely heavily on rapid tests of product properties – mainly developed for non-cementitious materials – that involve equilibration at low pH values, usually achieved
688
Editorial / Waste Management 26 (2006) 687–688
by adding acid. These tests could never represent the field situation. However, present trends in European legislation that encourage the re-use of waste materials after a scenario-based assessment of environmental impact call for a more process-based understanding of the leaching of contaminants. Reactions between cement and waste may be dominated at early ages by metastable phenomena. In the longer term, as the system drifts towards thermodynamic equilibrium, waste species solubility may decrease and the performance of the system actually improve. Thermodynamic modeling is desirable if not essential for both shortand long-term performance calculations. However, data base deficiencies often limit the scope of the calculations that can be undertaken in support of performance assessment. This meeting gave researchers from the different fields the opportunity to meet and discover the potential of working together. The cement chemists presented the latest developments in our understanding and modeling of the hydration process and of the principles of waste–cement interactions. The ‘‘waste’’ researchers presented some excellent papers on these interactions and on mechanistic studies on the binding of specific ions to cement minerals. Despite differences in historic development, we have learnt that substantial synergy exists between the three types of activity and that we have much to learn from each other. The conference members recommended that a follow-up meeting be organized in France in 2008.
The workshop organizers would like to thank the following institutions for their financial support and encouragement: • Association of the Swiss Cement Industry (Cemsuisse) • National Cooperative for the Disposal of Radioactive Waste (NAGRA) • Swiss Federal Agency for Environment, Forest and Landscapes (SAEFL) • Paul Scherrer Institute (PSI) • Swiss Federal Water Research Institute (Eawag) • Swiss Federal laboratories for Materials testing and Research (Empa) Finally, we would like to thank all of the participants for their excellent contributions. Guest Editor Fred Glasser Aberdeen University, UK Workshop organizers and Guest Co-Editors Annette Johnson Eawag E-mail address: [email protected] Barbara Lothenbach Frank Winnefeld Empa Erich Wieland Alexander Wa¨llisch PSI
Editorial
Mechanisms and modeling of waste/cement interactions International Workshop, May 8 to 12, 2005, Meiringen, Switzerland
Traditionally cement research has primarily focused on the development of safe and improved construction materials, centered on the use of Portland cement or one of its permitted variants. Cement chemists furthered our understanding of the hydration process, the kinetics, the minerals formed, the thermodynamic stability of the mineral assemblages, interactions with aggregate materials and long-term changes in mineralogy. Recent developments in the cement industry have increased the complexity of cementitious materials. Supplementary cementing materials, including coal combustion fly ash, iron blast furnace slag, silica fume, metakaolin, etc., greatly modify the bulk chemistry, and hence the mineralogy, of the solid matrix. Using wastes to replace coal, oil or gas normally used to pyroprocess the cement, saves energy and is a convenient way of eliminating wastes: examples include solvents, contaminated/used oil, rubber, paper or plastics. Contaminated soils may also be added to the kilns as a raw material. While organic contaminants are destroyed in the kiln, an increase in metal concentrations is possible. Stakeholders need to be able to assess the impact of elevated metal concentrations on cement properties and the environment. The last few decades have also seen much use of cement to immobilize hazardous and nuclear wastes and to chemically bind inorganic species in the cement matrix. Also, residues from incineration, for example, municipal solid waste or sewage sludge that are landfilled, can have chemical properties very similar to cementitious materials but at the same time have elevated heavy metal concentrations. For all these waste types, the long-term release of heavy metals or radionuclides to the environment is an essential element of risk assessment. For this, an understanding of heavy metal and radionuclide binding is essential. Like most chemical reactions, the binding mechanism is species- and concentration-dependent. The species-dependent nature of reactions, as well as the mechanisms and kinetics of competing reactions, has been a challenge to elucidate. Scientists have also to assess the duration of 0956-053X/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2006.04.003
the performance of cement barriers. Specific questions that are equally applicable to conventional and nuclear waste management include the following: • how is a species bound in the matrix? • how soluble is it as a consequence of binding? • how and under what conditions does diffusion out of the cementitious matrix occur? • how does its solubility change as the cementitious matrix ages and degrades? • how does the near-field interact with the cementitious matrix and affect its binding properties? Three basic binding mechanisms can be discerned: precipitation, adsorption to the surfaces of minerals either by specific binding or electrostatically by ion exchange, and, lastly, incorporation in cement hydrate minerals. Solubility is a limiting factor with regard to the second and third mechanisms listed above, so only ions that are sufficiently soluble in basic media will be incorporated in or sorbed to hydrated cement minerals to a significant degree. Where concentrations are low, use can be made of partition coefficients in the assumption that a particular species is bound in an unspecified way. However, long-term assessments, or the assessment of concentration-dependent behavior requires an exact understanding of the binding mechanism and it is here that much research effort has to be made. Efforts to extend our understanding of waste–cement interactions are largely related to the need for and, as a consequence, the funding of fundamental research. Industrial countries are investing a great deal of money and time to assess the safety of planned nuclear repositories, of which predicting long-term stability of cementitious materials and release of radionuclides are key issues. Waste disposal is traditionally at the ‘‘cheap’’ end of investment and consequently has the weaker science base. Performance tends to rely heavily on rapid tests of product properties – mainly developed for non-cementitious materials – that involve equilibration at low pH values, usually achieved
688
Editorial / Waste Management 26 (2006) 687–688
by adding acid. These tests could never represent the field situation. However, present trends in European legislation that encourage the re-use of waste materials after a scenario-based assessment of environmental impact call for a more process-based understanding of the leaching of contaminants. Reactions between cement and waste may be dominated at early ages by metastable phenomena. In the longer term, as the system drifts towards thermodynamic equilibrium, waste species solubility may decrease and the performance of the system actually improve. Thermodynamic modeling is desirable if not essential for both shortand long-term performance calculations. However, data base deficiencies often limit the scope of the calculations that can be undertaken in support of performance assessment. This meeting gave researchers from the different fields the opportunity to meet and discover the potential of working together. The cement chemists presented the latest developments in our understanding and modeling of the hydration process and of the principles of waste–cement interactions. The ‘‘waste’’ researchers presented some excellent papers on these interactions and on mechanistic studies on the binding of specific ions to cement minerals. Despite differences in historic development, we have learnt that substantial synergy exists between the three types of activity and that we have much to learn from each other. The conference members recommended that a follow-up meeting be organized in France in 2008.
The workshop organizers would like to thank the following institutions for their financial support and encouragement: • Association of the Swiss Cement Industry (Cemsuisse) • National Cooperative for the Disposal of Radioactive Waste (NAGRA) • Swiss Federal Agency for Environment, Forest and Landscapes (SAEFL) • Paul Scherrer Institute (PSI) • Swiss Federal Water Research Institute (Eawag) • Swiss Federal laboratories for Materials testing and Research (Empa) Finally, we would like to thank all of the participants for their excellent contributions. Guest Editor Fred Glasser Aberdeen University, UK Workshop organizers and Guest Co-Editors Annette Johnson Eawag E-mail address: [email protected] Barbara Lothenbach Frank Winnefeld Empa Erich Wieland Alexander Wa¨llisch PSI