Matrix wettability and mechanical properties of geopolymer cement-polydimethylsiloxane (PDMS) hybrids

Geopolymers have attracted attention as green building materials with low carbon dioxide emissions and excellent mechanical properties. Due to their hydrophilicity and porous structure, geopolymers can be infiltrated by water involving harmful ions. In this study, the effects of polydimethylsiloxane (PDMS) as an additive on the wettability, mechanical properties and microstructure of metakaolin-based geopolymer were investigated. The microstructure was characterized by MIP, μ-XCT, SEM, EDX, TIM, XRD, FTIR and 29Si NMR test. As the PDMS content increased, the geopolymer became hydrophobic, increasing its contact angle to approximately 140°. This hydrophobicity mainly results from the chemical modification and the increased roughness of the matrix. PDMS, with the content over 1.5 mL/L, can raise the porosity and pore size of the geopolymer, and promote the transformation of small gel pores into transitional and capillary pores. The fluffy pore structures and cured hybrids jointly created nano-scale and micron-scale roughness structures. No new crystalline phases will be generated by adding PDMS. The compressive strength increased with the addition of a small amount of PDMS, while it began to decrease when the PDMS content exceeded 10 mL/L. The effect of PDMS with different contents on the pore and silicate structure of geopolymers leads to changes of mechanical properties. The hybridization mechanism of PDMS molecules and N-A-S-H was summarized based on microstructure analysis.