BRE EP104 PDF

INTRODUCTION

Calcium aluminates have many application usage qualities, but rapidity is probably the most widely used. Rapidity is brought by rapid hydration of CAC based systems. For most CAC products the main hydraulic phase is CA. These CAC are not rapid setting but they are rapid hardening. Their setting time is close to OPC, typically around 3 hours, but their hardening rate is in the range of 10 to 20 MPa (compression) per hour from setting. This rapidity is compatible with applications that need compressive strength from 10 MPa to 30 MPa after 4h to 6h, for example industrial floor repairs. For applications requiring a higher rapidity, the hydration has to be accelerated. The most common way to accelerate hydration of CAC is to add lithium salts (Li2CO3 or Li2SO4 for example) ^{[1, 2].} Increasing the content of the C12A7 phase of the CAC is another way to accelerate, this phase being more rapid to hydrate than CA. Although these levers are efficient ways to meet the requirement of most of the usual rapid applications, they can find limits when ultra rapid hardening is necessary, where typically a very high mechanical strength is required after just a few minutes.

This study investigates the feasibility to develop CAC based systems delivering ultra-fast hardening. The first part focuses on mechanical strength by comparing three different levers to accelerate hardening: (i) the use of lithium salt (ii) the use of C12A7 rich CAC instead of CA (iii) the use of a quenched CAC with composition close to C12A7. The second part of the study deals with the characterisation of the solid and liquid phases in order to understand the mechanism of hydration allowing ultra-rapidity of quenched CAC used within this study.