Role of Adsorption Phenomena in Cubic Tricalcium Aluminate Dissolution

Abstract

The workability of fresh Portland cement (PC) concrete critically depends on the reaction of the cubic tricalcium aluminate (CA) phase in Ca- and S-rich pH >12 aqueous solution, yet its rate-controlling mechanism is poorly understood. In this article, the role of adsorption phenomena in CA dissolution in aqueous Ca-, S-, and polynaphthalene sulfonate (PNS)-containing solutions is analyzed. The zeta potential and pH results are consistent with the isoelectric point of CA occurring at pH ∼12 and do not show an inversion of its electric double layer potential as a function of S or Ca concentration, and PNS adsorbs onto CA, reducing its zeta potential to negative values at pH >12. The S and Ca K-edge X-ray absorption spectroscopy (XAS) data obtained do not indicate the structural incorporation or specific adsorption of SO on the partially dissolved CA solids analyzed. Together with supporting X-ray ptychography and scanning electron microscopy results, a model for CA dissolution inhibition in hydrated PC systems is proposed whereby the formation of an Al-rich leached layer and the complexation of Ca-S ion pairs onto this leached layer provide the key inhibiting effect(s). This model reconciles the results obtained here with the existing literature, including the inhibiting action of macromolecules such as PNS and polyphosphonic acids upon CA dissolution. Therefore, this article advances the understanding of the rate-controlling mechanism in hydrated CA and thus PC systems, which is important to better controlling the workability of fresh PC concrete.

Topological controls on the dissolution kinetics of glassy aluminosilicates

“…17 By directly tracking the evolution of the surface topography with time, VSI accesses the true dissolution rate of a dissolving solid. [23][24][25] The dissolution rates were quantified using a so-called raindrop procedure*, 17 by which the solution's pH and composition remain essentially constant over the course of the experiment. [23][24][25] The dissolution rates were quantified using a so-called raindrop procedure*, 17 by which the solution's pH and composition remain essentially constant over the course of the experiment.…”

“…[17][18][19][20] The technique features significant advantages over methodologies that are based on analysis of solution compositions because it is unaffected by complexities including: (i) inaccurate knowledge of the solid's surface area, or the presence of particles that are distributed over a wide sizerange, 20 (ii) metastable barrier formation that may be relevant to glassy systems, 21 (iii) incongruent dissolution, 22 and/or (iv) ion adsorption on dissolving or reacting surfaces. [23][24][25] The dissolution rates were quantified using a so-called raindrop procedure*, 17 by which the solution's pH and composition remain essentially constant over the course of the experiment. The typical l/s (liquid-to-solid mass ratio) used was on the order of 50 000 for a contact time of approximately 5 minutes per raindrop cycle for a minimum 74 -*It should be noted that in the rain-drop procedure, while the "bulk" undersaturation of the solution with respect to the dissolving solids will change negligibly, the reduction in undersaturation of the solution may be more substantial in close proximity (i.e., within the first 10 nm) of the solid surface.…”

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.…”