This project is co-funded by the Office of International Science and Engineering (OISE), the Division of Civil, Mechanical and Manufacturing Innovation (CMMI) and the Division of Materials Research (DMR).
NON-TECHNICAL DESCRIPTION: Geopolymers are attracting considerable attention in the engineering community as a potential replacement for portland cement in making concrete, which is commonly used to make structures such as buildings and highways. Geopolymers are formed by reaction between clay, which has first been heated to make it more reactive, and an aqueous alkaline solution. The main reason that geopolymers are attracting attention is that their use in concrete may reduce considerably the amount of greenhouse gases associated with concrete production. The purpose of this research project is to explore whether geopolymers can be made using fly ash instead of clay. Fly ash is a waste material produced during coal combustion, for example to generate electricity, and is often used as a replacement for some of the Portland cement in concrete. Some types of fly ash have been used successfully in production of geopolymer, but Class C fly ash, common in the middle and western US, have not. The key intellectual objective of this research is to understand how the chemical composition and molecular structure of the fly ash control behavior of the geopolymer. This research involves collaboration with Prof. Lauren Gómez-Zamorano of the Universidad Autónoma de Nuevo León (UANL) in Mexico. Prof. Gómez's work is supported through the Consejo Nacional de Ciencia y TecnologÃa (CONACYT). A broad objective is to demonstrate whether geopolymer is effective as a binding material in concrete. Public support of such projects is vital to the development and implementation of new construction material systems, and new systems are critical to the nation?s progress towards controlling greenhouse gas emissions without sacrificing construction of buildings and highways. Finally, civil engineering students are seeking knowledge about new materials for sustainable construction, and research experience in such systems will enhance our ability to provide that knowledge.
TECHNICAL DETAILS: Geopolymer is the name given to synthetic aluminosilicate polymers formed by chemical reaction between a solid precursor, such as metakaolin, and an alkali solution. Geopolymers are attracting considerable attention as a replacement for Portland cement in concrete for civil engineering applications, in part because they much reduce the production of greenhouse gas associated with concrete. The chemistry of geopolymer formation is similar to the chemistry of zeolite synthesis, but the geopolymers are amorphous. Most of the research on synthesis of geopolymers has used metakaolin as the precursor. However, additional environmental benefits would be realized by producing geopolymers from waste materials, so this research is focused on synthesis of geopolymers using Class C fly ash as the precursor. Fly ash is a by-product of coal combustion, and Class C fly ash is derived from subbituminous and lignite coals, commonly found in the middle and western US. The key intellectual merit of this research is to understand how the chemical and molecular structures of the precursor control behavior of the geopolymer. To meet this objective, composition and structure of the precursor and the geopolymer are being studied using XRD, XRF, DSC, MAS-NMR, TEM, and SEM/EDX, and therefore graduate students in engineering are trained to use these fundamental characterization techniques. A related objective is to demonstrate whether geopolymer is effective as a binding material in concrete, and engineering behavior of concretes made using geopolymers is also of interest. This research has important impact on the cement and concrete industries; these industries are under considerable pressure to reduce greenhouse gas emissions and so they are looking at geopolymers as a possible strategy.
Portland cement, composed of calcium and silica, is the most important ingredient in concrete, which is used in numerous structures—for example highways, high-rise buildings, and foundations for houses. Unfortunately, the manufacture of portland cement also produces large amounts of the greenhouse gas carbon dioxide. Currently there is much interest in switching to a more sustainable and environmentally friendly alternative. One of the leading candidates is a class of materials called geopolymers, which are produced by reaction of an aluminosilicate powder with a concentrated alkali hydroxide solution. Geopolymers produce significantly less greenhouse gases, and the aluminosilicate powder may be a waste material—often fly ash, the powder left over when coal is burned to produce electricity. But the behavior of geopolymers is poorly understood, so they are receiving much attention but so far very little use. The main constituent in geopolymers is an aluminosilicate that is similar to zeolite (a common material both manufactured and found in nature). Because this product is not crystalline, it is called a gel. This gel is what gives the geopolymers their important engineering properties such as strength. Calcium is often found in the aluminosilicate powder used to produce geopolymers; and when calcium is present, we postulated when we began this research that the reaction would form calcium silicate hydrate, the main material found in hydrated portland cement, as well as geopolymer gel. Geopolymer mixtures start out as a liquid containing both powder and solution. As the reaction proceeds, they gradually set (meaning they become solid) and develop considerable strength. They are stronger than hydrated portland cements and gain strength more quickly. Their strength is known to be influenced by the chemical composition of the geopolymer mixture, especially the amounts of silica and alumina. In order to use geopolymers in place of portland cement, it is necessary to tune the composition so as to achieve targeted behavior, especially setting and strength. To understand the relationship between composition and strength, it is necessary to determine the composition. Because the geopolymer gel is not crystalline, most analytical methods do not provide useful information. Until now, it has not been possible to determine the amounts of geopolymer gel and any calcium silicate hydrate when they are found together. One of the important outcomes of this research project has been the successful use of nuclear magnetic resonance spectroscopy (NMR) to differentiate these two phases—more broadly, to determine the amount and composition of any unreacted aluminosilicate, the geopolymer gel, and any calcium silicate hydrate. For example, we find that replacement of 15% of the aluminosilicate powder with calcium hydroxide produces about 50% geopolymer gel, with a somewhat lower silica/alumina composition than is seen without the calcium, and about 50% calcium silicate hydrate. This capability represents an important advancement in the characterization of geopolymers, one that will greatly facilitate their use as a binder in concrete. With this advancement, the research group has been able to directly relate the geopolymer composition to its engineering properties. Increasing silica in the mixture increases its strength, and we see that increasing silica increases the amount of geopolymer gel and increases its silica content. Adding calcium to the mixture also increases strength. We see that adding calcium decreases the amount of gel and reduces its silica content. Thus the effect of calcium on strength cannot be explained by changes in the amount and composition of the gel and must therefore be attributed to the calcium silicate hydrate. The effect of composition on strength is important, but the effect of composition on setting behavior is even more striking. We have seen that setting may occur within minutes or may require many hours, a much wider range in behavior than seen with portland cement. We observed faster setting with lower silica contents, and we observed much faster setting with addition of calcium. We do not know yet what causes this behavior, but we plan to study it further. The setting behavior can only be controlled in concrete if we know what reactions are responsible for it.
