Effect of hydrophobic surface-modified fine aggregates on efflorescence control in geopolymer

The geopolymer concrete (GPC) is susceptible to ion leaching and efflorescence formation and subsequently, affecting the mechanical and durability properties. This study assesses the effect of hydrophobic surface modification of aggregates on controlling the efflorescence in GPC. Two different hydrophobic agents, namely hydrophobic fumed silica and silane crème, were considered for coating sand particles at a varying dosage, and the efflorescence of GPC was measured. The visual observation and the leachate ions measurements indicated that the fumed silica performs better than silane crème in preventing the efflorescence at a significantly lower dosage. This behaviour is attributed to the hydrophobicity of GPC with the hydrophobic agents, where contact angles of 110°, 107° and 52° were observed for 2% of fumed silica, 20% of silane crème and control mix, respectively. Integrating fumed silica also improves the compressive strength while the silane crème showed a detrimental effect, possibly due to retarding the geopolymerization process as observed from the microstructural study. Meanwhile, both surface modification techniques showed good chemical compatibility between hydrophobic coatings and geopolymer products, as revealed from the FT-IR and XRD analyses.