The hydration of synthetic brownmillerite in presence of low Ca-sulfate content and calcite monitored by quantitative in-situ-XRD and heat flow calorimetry

•In low Ca-sulfate environment ettringite is not stable but AFm-phases.•A three-step reaction of C4AF dissolution is observed.•Hydration of C4AF proceeds rapidly after depletion of sulfate in pore solution.

We present here experimental data of the hydration of C4AF in a Portland cement environment with low sulfate content. Calorimetric data indicate a distinct two-step main reaction of C4AF, which was successfully deconvoluted by laboratory scale in-situ-XRD in combination with the G-factor method and a quantitative approach by mass balance estimation with normalized peak areas of hydrate phases. The presented data here suggest a rapid unhindered dissolution of C4AF after depletion of sulfate in the pore solution, which is then rapidly leading to an oversaturation with respect to sulfate-AFm-16 (C4A$H16). Since the sulfate content of the mixture is low, a second acceleration of C4AF dissolution, due to precipitation of hydroxy-AFm-19 (C4AH19) was observable after the decline in ettringite (C6A$3H32) content and the subsequent conversion to sulfate-AFm-16 (C4A$H16).