Impact of calcium aluminates technology on drying capacity on self levelling underlayment

Rapid drying is a key function for a fast covering with Self Levelling Underlayments (SLU). In order to prevent issues such as debonding a well-accepted value is 3wt% for the residual moisture. Three principle drying mechanisms are involved: two are linked to environmental conditions (evaporation and substrate absorption) and the last one is the self-desiccation capacity. It can be defined as the trapping of free water by a chemical reaction. One of the most powerful means to accelerate drying in cementitious material is via ettringite formation. The first part of this paper investigates the determination of the self-desiccation capacity and the hydration reactions associated for different kind of binders under endogenous conditions. Three ettringitic systems based on different Calcium Aluminate Technologies, C12A7 and CA with various amount of Portland cement are investigated and compared to a calcium sulfate system. Different model formulas are designed to be similar especially with respect to the same binder amount (30wt %) and a mixing water fixed at 20wt%. Thermogravimetric analysis (TGA) is used to determine bound water and then the residual moisture is calculated. More than 60wt% of mixing water is trapped by ettringite precipitation. After 6h, AH3 and AFm phases contribute also significantly to the self-desiccation capacity. After seven days, only the C12A7 based aluminate technology shows residual moisture lower than 3wt% under endogenous conditions. The second part of this paper then highlights the competition between hydration and evaporation (a non-absorbent substrate has been used) under drying conditions. It is focused on Portland cement rich systems associated with Aluminates based upon CA and C12A7 including one without Portland cement. Samples have been monitored by mass loss at 20°C and 50% relative humidity to determine evaporation and TGA to calculate the residual moisture. The microstructure has been investigated at different time intervals through the pore size distribution measured by Mercury Intrusion (MIP). Conclusions are drawn as to the impact of the different calcium aluminates technologies on the drying capacity of SLU. In Portland cement rich binder systems (with CA); residual water is mainly managed by evaporation whereas in C12A7 binder system, the hydration is the main lever. Drying conditions can have an unfavorable impact on the microstructure of Portland cement rich systems. Speaker: Stéphane Berger, Kerneos (FR) Contact: stephane.berger@kerneos.com Co-Authors: P-A. Andreani, D. Tourlakis, D. Niepmann