Resistance of cements against sulfate attacks is an important issue since the beginning of the history of cements. Sulfates are responsible of swelling and breaking in concrete. Their presence in many environments, such as sea water and gypsum-rich soil, is a problem for the durability of Portland cement. Studies have shown the important role of tricalcium aluminate in the lowering of cement resistance in sulfated environments. However, the role of brownmillerite, not well known, is the subject of this present study.
Brownmillerite, the so-called ferrite phase, is the iron bearing phase of Portland cement with typically 7-8 wt.% of the total mass of ordinary Portland cement. It is a wide solid solution between Ca₂Fe₂O₅ and Ca₂Al_1.3Fe_0.7O₅, often written Ca₂Al_xFe_2-xO₅. In usual Portland cement, it is well established that brownmillerite crystallizes in space group Ibm2, with Al/Fe ratio in the range 1 to 2. At the present time, only few studies have analyzed brownmillerite phase of the more specific sulfate resisting Portland cements (SRPC). There is a lack of recent and accurate crystal structure parameters and reactivity studies on industrial brownmillerite, which are of particular importance for the study of the cement durability.
Brownmillerite is a minor phase of Portland cement with micronic needle-like crystals (~1 μm) mixed with other cement phases. Studying crystal chemistry and reactivity directly from the cement leads to an overly complex interpretation due to the other phases. In order to overcome this drawback, the proposed approach is to extract brownmillerite by selective chemical dissolution.
Brownmillerite was extracted from four SRPC industrial samples, then systematically studied by powder X-ray diffraction, electron microprobe (EPMA), X-ray fluorescence spectroscopy and ⁵⁷Fe Mössbauer spectroscopy. Two extraction steps were used: the first is the salicylic acid/methanol protocol that leaves a residue rich in brownmillerite, tricalcium aluminate and sulfate phases. The second, as developed in this study, uses acetic acid to dissolve tricalcium aluminate and sulfates phases. Once extracted, the reactivity of brownmillerite was studied with and without sulfates/lime by conductimetry in diluted solutions and induced coupled plasma spectroscopy. The reactivity of extracted brownmillerite was compared to eight synthetic brownmillerite samples with Al/Fe ratio between 0 and 2.
The selective extraction clearly appears of particular importance to obtain a correct estimation of Al/Fe content in ferrite phase, consistent with EPMA analysis. All the industrial brownmillerite samples crystallize in space group Ibm2. Two families of brownmillerite can be distinguished looking either at their cell parameters and their aluminium content: the Al/Fe ratio stands within 0.8-0.9 when tricalcium aluminate is present and around 0.7 without tricalcium aluminate [1-2].
Even within such a narrow Al/Fe ratio, a strong dependence of ferrite reactivity is found either on the Al/Fe ratio and the environment. Threshold values depend on the solution. This work clearly shows that ferrite can no longer be ignored concerning its role in the sulfated attack.