Abstract

High alumina refractory cements usually contain CA as main hydraulic phase next to the more refractory but less reactive CA2. In a study by Klaus et al. [1] the influence of particle fineness on the kinetics of CA hydration was investigated. Surprisingly, a shift to later times was observed with increasing fineness. However, the grinding times were relatively short and no X-ray amorphous phase was detected in their investigated powders.

In the presented study the properties of CA and CA2, both synthesised by solid state reaction from CaO and Al2O3, were compared after equal but longer grinding times. For enabling a good workability of the pastes fine alumina was added as inert filler. The heat flows during the hydration of pastes containing pure CA or CA2 in combination with alumina filler were precisely recorded from the time of water addition over a period of at least 72 h using a TAM Air calorimeter equipped with tools which allowed a reproducible internal water injection and paste mixing (InMixEr). The dissolution of CA or CA2 and precipitation of different hydrate phases during the hydration of the investigated pastes was analysed by highly time resolved in-situ XRD and in-situ 1H time domain NMR. This enabled a quantitative determination of the phase contents of crystalline and amorphous hydrate phases at defined points in time of hydration.

CA2 showed a higher susceptibility to grinding which was indicated by higher BET surface areas and higher amounts of X-ray amorphous phase after equal grinding times in comparison with CA. Although CA2 is indeed less reactive than CA its hydraulic activity could be significantly increased by grinding. Especially, the increasing amounts of X-ray amorphous phase and the higher BET surface areas led to an acceleration of the hydration and to an unusual well defined heat flow peak prior to the main hydration reaction. Additional thermal treatment of the ground samples led to a reduction of the BET surface area and to a transformation of the X-ray amorphous fractions into completely crystalline CA or CA2. The thermally treated samples showed significantly different hydration kinetics. Finally, the hydraulic behaviour of the investigated samples could successfully be influenced in a targeted way by different grinding and tempering procedures.

[1] S.R. Klaus, J. Neubauer, F. Goetz-Neunhoeffer, How to increase the hydration degree of CA – The influence of CA particle fineness, Cem. Concr. Res. 67 (2015) 11-20.