Chemical mapping of thaumasite formed in sulfate-attacked cement mortar using near-infrared Raman imaging microscopy

This work reports a new method for identifying and mapping the distribution s of both thaumasite and ettringite in cement using near-infrared Raman ima ging microscopy (NIRIM). This technique offers an important alternative to conventional X-ray diffraction (XRD) and scanning electron microscopy (SEM) , which cannot easily distinguish preexisting ettringite from thaumasite fo rmed as the result of sulfate attack. The NIRIM instrument used for these s tudies combines fiber-bundle image compression (FIC) hardware and multivari ate signal processing software to identify and map chemical species in ceme nt samples produced with and without extended exposure to sulfate. The NIRI M images clearly reveal thaumasite, etiringite, and gypsum microstructures. Cement paste formed in a carbon dioxide environment is found to contain bo th ettringite and gypsum species, while NIRIM images of cement exposed to s ulfate reveals only thaumasite and gypsum. The NIRIM results are shown to c orrelate well with combined SEM/X-ray microanalysis of the same samples. (C ) 2001 Elsevier Science Ltd. All rights reserved.