An original test method to assess water absorption/desorption of lightweight aggregates in presence of cement paste

- This study proposes an original test method to assess absorption/desorption in cement paste at a given time during the dormant period.
- The test method allows to evaluate the effective water to cement ratio in cement paste.
- The results show that the water absorption by LWA decreases water to cement ratio and workability whereas the desorption increases them.
- Fresh paste absorption of 30 min seems to be a good value to accurately estimate the water demand of LWA in cement paste during mixing and placing.

The calculation of the effective water content in lightweight aggregates concrete is not easy because of possible water transfer from fresh cement paste to porous aggregates at an early stage (dormant period). A method to quantify and follow this phenomenon is proposed. The absorption method consists in mixing partially saturated lightweight aggregates with fresh cement paste of a given water/cement ratio. At a given time during the dormant period, lightweight aggregates and fresh cement paste are mechanically separated. Then a water balance is made through weighing procedures before and after thermal treatment to evaporate the physically bound water. For unsaturated lightweight aggregates, a transfer of water from the paste to aggregates is observed while for saturated aggregates the transfer is from the aggregates to the paste. The kinetic and the asymptotic absorption values are decreasing functions of the water/cement ratio. Fresh paste absorption of 30 min seems to be a good value to estimate the water demand of lightweight aggregates based on initial dried aggregates to obtain an efficient mixing and workability stability at early stages.