Hydrophobic treatment on hollow glass microspheres for enhancing the flowability of lightweight high-performance cementitious composites

The use of hollow glass microspheres (HGMs) in a low water-to-binder matrix generally hinders the flowability of the composite due to the high water absorption capacity of HGMs. This study investigated the feasibility of improving the flow behavior of lightweight high-performance cementitious composites (L-HPC) by adding microspheres with hydrophobic treatment. The hydrophobic HGMs were prepared with a ball milling method using low-cost stearic acid. The successfully modified hydrophobic HGMs were then used for the preparation of L-HPC. The results show that the production, including milling for 2-8 h with a 4 wt% addition of stearic acid, can convert hydrophilic HGMs into hydrophobic ones, with the water contact angle increasing from 0 degrees to 102-154 degrees (as a function of processing time). The diffraction peaks and phase transition latent heat results also proved the modification of the surface of HGMs with a hydrophobic coating layer. Cement hydration monitoring demonstrated that the hydrophobic HGMs hardly absorbed water and did not show significant wetting behavior during mixing. Since more water was available for cement hydration due to the hydrophobic effect of HGMs, the formation of hydration products and flowability were significantly increased. The presence of stearic acid delayed the contact time between some cement particles and water, thereby slowing down the heat release rate, resulting in the prevention of potential early cracking with the maintenance of high strength exhibition even with the enhanced flowability.