Effect of Gypsum on the Early Hydration of Cubic and Na-Doped Orthorhombic Tricalcium Aluminate.

Ana Paula Kirchheim,Luciano A Gobbo,Denise C C Dal Molin,Erich D Rodríguez,Rui B De Souza,Maria Alba Cincotto,Rupert J Myers,Paulo J M Monteiro

doi:10.3390/ma11040568

Abstract

The tricalcium aluminate (C3A) and sulfate content in cement influence the hydration chemistry, setting time and rheology of cement paste, mortar and concrete. Here, in situ experiments are performed to better understand the effect of gypsum on the early hydration of cubic (cub-)C3A and Na-doped orthorhombic (orth-)C3A. The isothermal calorimetry data show that the solid-phase assemblage produced by the hydration of C3A is greatly modified as a function of its crystal structure type and gypsum content, the latter of which induces non-linear changes in the heat release rate. These data are consistent with the in situ X-ray diffraction results, which show that a higher gypsum content accelerates the consumption of orth-C3A and the subsequent precipitation of ettringite, which is contrary to the cub-C3A system where gypsum retarded the hydration rate. These in situ results provide new insight into the relationship between the chemistry and early-age properties of cub- and orth-C3A hydration and corroborate the reported ex situ findings of these systems.

Highlights

Tricalcium aluminate (Ca3 Al2 O6 or known in cement chemistry notation as C3 A)—the most reactive phase in Portland cement (PC)—begins reacting essentially instantaneously once in contact with water to produce a hydroxy-AFm-type meta-stable product, which is subsequently converted to katoite (Ca3 Al2 (OH)12 or C3 AH6 ) and heat
The peaks related to C3 AH6 exhibited high intensities and appeared prior to two minutes of the reaction, which are attributed to the precipitation of this phase as a major hydration product in both cub- and orth-C3 A systems (Figure 2A,B)
This paper presented isothermal conduction calorimetry (IC) and in situ X-ray diffraction (XRD) analyses to follow the hydration of cub- and orth-C3 A

Summary

Introduction

Tricalcium aluminate (Ca3 Al2 O6 or known in cement chemistry notation as C3 A)—the most reactive phase in Portland cement (PC)—begins reacting essentially instantaneously once in contact with water to produce a hydroxy-AFm-type meta-stable product, which is subsequently converted to katoite (Ca3 Al2 (OH) or C3 AH6 ) and heat. Calcium sulfate (typically ~5 wt %) is normally milled with PC clinker to retard the C3 A hydration rate [4], which leads to longer setting times [5,6,7]. The most common type of calcium sulfate added to PC clinker is gypsum, but anhydrite and hemihydrate can be used. Previous research regarding the C3 A-CaSO4 ·H2 O system showed that the C3 A hydration rate is related to its crystal structure and specific surface area, the temperature, water/solid ratio [8], type and amount of CaSO4 (gypsum, hemihydrate, anhydrite) [6,9,10,11,12,13,14], the presence of other mineral or chemical admixtures [15,16,17,18,19], and the solution chemistry [20,21]

Methods

Results

Conclusion