Influence of organosilane additive content and mixing method on the properties of geopolymer composites

Developing innovative building materials is one of the current fields of research, especially in view of mitigating the negative environmental impacts of production of traditional Portland cement. Geopolymers offer excellent physical and mechanical properties, a wide range of applications and the potential to utilise waste materials in their production. In this work, geopolymer samples were prepared and modified with a hydrophobic silane-based additive (n-octyltriethoxy silane (OTES)) to enhance physical (hydrophobicity, thermal performance) and mechanical properties (bending and compressive strengths). Different concentrations of OTES were applied to prepare testing samples without fillers and with silica-based fillers of one or two different grain sizes. Mixing method was found to have a crucial influence resulted geopolymers properties. The pre-mixing method resulted in better incorporation of OTES, as confirmed by gas chromatography-mass spectrometry (GC-MS) analysis. In the comparison with post-mixing method, samples prepared using pre-mixing method showed water contact angle (WCA) and bending strengths higher for approximately 15 and 40%, respectively. The maximum WCA was detected to be 132°. The obtained result may be utilized for the production of geopolymer materials for applications with increased requirements for limiting water transport. • The effect of organosilane additive (OTES) on geopolymers (GP) was studied. • OTES (0–4 wt% relative to the matrix) was incorporated using two mixing methods. • The pre-mixing method enabled more efficient and stable OTES incorporation. • Pre-mixed samples showed ∼15% increase in water contact angle. • OTES content, mixing method and filler type simultaneously affected final GP properties.