Developing a Nondestructive Way to Characterize Cement Alternatives

Lynn M. Savage

The production of Portland cement — with which concrete is made — is responsible for about 5 percent of the world’s CO₂ emissions. Among the efforts to reduce anthropogenic carbon emissions is the scientific investigation of geopolymers made from fly ash as a replacement for traditional cements.

Geopolymers are aluminosilicates generally made from fly ash mixed with a solution containing alkali-metal ions and hydroxide ions. A by-product of coal production, fly ash is composed mostly of oxides of silica, aluminum and iron. Before geopolymers can make the grade, however, they must be fully characterized to assure they will harden, realize their full strength and then age — all in a predictable manner.

Scanning electron microscopy provided images of geopolymers activated with sodium silicate solutions with various concentrations, but researchers used attenuated total reflection Fourier transform IR spectroscopy to differentiate the molecular bonds that exist and change as the material cures. Geopolymers activated with 0.6 mol/l SiO₂ (top) and with 3.5 mol/l SiO₂(bottom) are shown. Reprinted with permission of the American Chemical Society.

During geopolymer formation, several phases of material exist within the mix: unreacted and partially reacted particles, the nascent aluminosilicate gel and some smaller aluminosilicate species liberated from it, dissolved alkali-metal hydroxides and water.

Now researchers at the University of Melbourne in Australia have reported their use of attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to study the formation of fly ash-based geopolymers for up to 200 days. They chose ATR-FTIR spectroscopy because it can differentiate among the various materials inside the solidifying geopolymer while not affecting the formative processes.

According to John L. Provis of the university’s chemical and biomolecular engineering department, nuclear magnetic resonance (NMR) and other spectroscopic methods are used to study geopolymers; however, certain information that is readily accessible using ATR-FTIR is difficult to obtain with these techniques.

“The fly ash used in this study contains a significant quantity of iron within particles that cannot easily be removed by magnetic separation. This makes NMR analysis of the reaction products very troublesome,” he said.

The investigators, led by doctoral student Catherine A. Rees, began by preparing 30 geopolymer formulations of varied alkali and silica content. They cured the samples, then acquired FTIR spectra from them at various intervals — starting at once per day, then slowing — for 200 days, using a spectrometer made by Varian Inc. of Palo Alto, Calif., and a single-reflectance diamond ATR attachment made by Specac Ltd. of Kent, UK. They chose diamond for the total reflectance aspect because it is resistant to alkalinity and to abrasion.

They analyzed the spectra for the position of the main Si-O-T stretching band (where T represents either tetrahedral aluminum or another silicon atom). “The Si-O-T stretching band is the most intense band in the mid-IR region of geopolymers,” Provis said. “It is highly sensitive to silicon/aluminum substitution or the presence of nonbridging oxygen atoms.”

The researchers found that the spectrum of the geopolymer resembled fly ash alone shortly after mixing but, up to two days later, they saw an increase in intensity in the 850- to 1000-cm^–1 region. Because that region corresponds with dissolved silicate, aluminosilicate species or both, the increased intensity indicated the dissolution of the fly ash as the geopolymer began to cure. Additional changes were noted between two and five days, and from nine days onward.

According to Provis, the researchers will try to use the technique to study related systems with more complex rheological features, and they hope to correlate the nanostructural properties of the geopolymers with their macroscale performance and to develop a way to predict durability and ultimate lifetimes of the materials.

July 17, 2007, pp. 8170-8179.