Study on the properties of geopolymer concrete modified with PDMS synergistic hydrophobic particle composites

Moisture penetration and aggressive ion intrusion are primary factors reducing concrete durability. To address this problem, this study utilizes polydimethylsiloxane (PDMS) to modify fly ash and recycled concrete micropowder to be prepared into the corresponding hydrophobic particles, and explores the effects of separately admixed PDMS and its composite admixed with two types of hydrophobic particles on the compressive strength, hydrophobicity, and microstructure of geopolymer concrete experiments. The results showed that adding PDMS decreased the 28 day compressive strength of concrete by 24.06%. when mixed with hydrophobic fly ash particles, the strength loss increased to 29.46%, whereas blending PDMS with hydrophobic recycled concrete micropowder reduced the loss to 20.14%. In terms of hydrophobity, PDMS significantly decreased concrete wettability. The 12-hour water absorption of the specimen mixed with hydrophobic recycled concrete micropowder was only 1.5%, representing a 78% reduction compared to control group. Droplet contact angle measurements revealed that PDMS changed the concrete surface from hydrophilic (contact angle <90°) to hydrophobic, with the angle further increasing to 136° when combined with hydrophobic fly ash particles. Microstructural analysis showed an increasing in average pore size for all PDMS modified samples. The smallest increase (1.015 nm) occurred with PDMS alone, while the largest increase (3.602 nm) was observed in the system combining PDMS and hydrophobic fly ash particles. Overall, the composite of PDMS and hydrophobic recycled concrete micropowder demonstrated the best performance, achieving strong hydrophobicity while maintaining superior mechanical properties.