Synchrotron X-ray nanotomographic and spectromicroscopic study of the tricalcium aluminate hydration in the presence of gypsum

Abstract

The rheology of modern Portland cement (PC) concrete critically depends on the correct dosage of gypsum (calcium sulfate hydrate) to control the hydration of the most reactive phase-tricalcium aluminate (C 3 A). The underlying physio-chemical mechanism, however, remains unsolved mainly due to the lack of high-spatialresolved and chemistry-sensitive characterization of the C 3 A dissolution frontier. Here, we fill this gap by integrating synchrotron-radiation based crystallographic, photon-energy-dependent spectroscopic and high-resolution morphological studies of the C 3 A hydration product layer. We propose that ettringite (6CaO•Al 2 O 3 •SO 3 •32H 2 O) is the only hydration product after the initial reaction period and before complete gypsum dissolution. We quantify the 2D and 3D morphology of the ettringite network, e.g. the packing density of ettringite at various surface locations and the surface dissolution heterogeneity. Our results show no trace of a rate-controlling diffusion barrier. We expect our work to have significant impact on modeling the kinetics and morphological evolution of PC hydration.

Mechanistic insight into mineral carbonation and utilization in cement-based materials at solid–liquid interfaces

“…They found that, as soon as being in contact with sulfur-containing solution, C 3 A would undergo rapid dissolution owing to the quick growth of ettringite as well as the formation of AFm phases (but quickly vanish), and then followed by slow dissolution of C 3 A, which might probably be due to ion-complexation. 35 Similar observation was found in the case of C 3 A-gypsum system suggesting both the nucleation and growth of the s-AFm phases as the rate controlling mechanism. 15…”

“…Meantime, the deposited ettringite would react with C 3 A at the contact surface, as shown in eqn (10), to form calcium-aluminate-monosulfate (s-AFm). Geng et al 35 applied nanotomographic and spectromicroscopic techniques to evaluate the C 3 A hydration in the presence of gypsum (CaSO 4 ). They found that, as soon as being in contact with sulfur-containing solution, C 3 A would undergo rapid dissolution owing to the quick growth of ettringite as well as the formation of AFm phases (but quickly vanish), and then followed by slow dissolution of C 3 A, which might probably be due to ion-complexation.…”

Influence of water activity on hydration of tricalcium aluminate‐calcium sulfate systems

“…Though, there is still no consensus on which chemical species or surface complex is responsible for passivation of the C 3 A surface. Some studies have argued that the passivation layer comprises of SO 2− 4 ions 12,34 ; although, results contradicting this argument have been presented in a study conducted by Collepardi et al 41 Other studies [37][38][39] , have reported that Ca-Al-SO 3 complexes, that preferentially adsorb onto the Alrich C 3 A surface, are responsible for passivating it.…”

“…Notably, X‐ray synchrotron analysis and other examinations of C 3 A particulates' surfaces have shown that a metastable calcium aluminate hydrate phase does form on C 3 A surface; however, this phase does not appear to inhibit dissolution of C 3 A or the transport of ions from C 3 A surface to the contiguous solution. In other studies, a surface passivation mechanism has been theorized to be at the origin of retardation of C 3 A hydration during the period when ettringite crystals grow at a slow rate. Though, there is still no consensus on which chemical species or surface complex is responsible for passivation of the C 3 A surface.…”

“…12,34 The aforesaid barrier-layer hypothesis, however, appears implausible in light of recent findings. Notably, X-ray synchrotron analysis 37 and other examinations of C 3 A particulates' surfaces 38,39 have shown that a metastable calcium aluminate hydrate phase does form on C 3 A surface; however, this phase does not appear to inhibit dissolution of C 3 A or the transport of ions from C 3 A surface to the contiguous solution. In other studies, 13,34,37-40 a surface passivation mechanism has been theorized to be at the origin of retardation of C 3 A hydration during the period when ettringite crystals grow at a slow rate.…”