From selective dissolution to crystal chemistry of brownmillerite in sulfate resisting cement

The aim of this study is to extract brownmillerite from sulfate resisting Portland cement (SRPC) in order to determine its crystal chemistry and the effects of dissolution protocols. Brownmillerites (C₄AF) from four SRPC were extracted and systematically studied by X-ray diffraction (XRD), electron probe microanalysis (EPMA), and X-ray fluorescence spectroscopy. Two extraction steps were used. The first is the salicylic acid/methanol (SAM) protocol that leaves a residue rich in ferrite, C₃A and sulfates. Precipitated hydrated sulfate minerals are also observed for clinker with high molar SO₃/Na₂O_eq ratio. The second, developed in this paper, uses acetic acid (AcA) extraction to dissolve C₃A and sulfates. The Rietveld refinements showed that all brownmillerites of this study crystallize in Ibm2 space group. Two families of brownmillerite were identified by their aluminum content and cell parameters, related to the presence of C₃A in the clinker. EPMA indicated that ferrites from SR0 and SR3 cements have Al/Fe ratios about 0.7 and 0.8–1.0, respectively. XRD allows to predict the (Al+Mg+Si)/Fe ratio in brownmillerite in good agreement with EPMA. The SAM protocol is the best way to study crystal chemistry of brownmillerite and the AcA protocol is recommended to dissolve sulfates and C₃A for further reactivity studies.