Elucidating the effects and mechanisms of OTES@silica nano capsules on water resistance and compressive strength of cement paste

The bulk modification of cementitious materials by hydrophobic agents is an effective strategy to reduce the water absorption. However, the decrease in compressive strength has been one of the most significant problems. We reported in this paper the synthesis of a sustained-release capsule (OTES@SC) with n-octyltriethoxy silane (OTES) as core material and silica shell. The capsule (containing 70.0 % of OTES) enabled the continuous release of OTES within days. The addition of OTES@SC reduced the water absorption while improved the compressive strength of hardened cement paste. Both the effects would be more pronounced at higher capsule dosages (~70.6 % water absorption reducing and 17.4 % increase of compressive strength at 0.5 wt% capsule dosage). The sustained release of OTES from OTES@SC enabled the continuous surface modification by OTES both already in cement paste and newly formed. Compared with the direct addition of OTES (average thickness of the hydrophobic layer decreased from ~10 nm to ~0 nm during hydration), the hydrophobic layer had reached a thickness of a few molecules layer at 28 d, exhibiting significantly improved uniformity on cement paste particle surfaces. The thinner layer and the normal hydration of part of reactive sites of clinkers greatly reduced the inhibition effect of OTES on hydration. Additionally, the pozzolanic effect and nano-size of silica further promoted hydration. Thus, OTES@SC improved hydration degree and refined pore structure of paste.