سیلیس آموروف در بتن با کارایی فوق العاده بالا: در مدت زمان اولیه هیدراتاسیون Amorphous silica in ultra-high performance concrete: First hour of hydration

۴٫ Conclusions In this study, the effects of different types of silica (Stoeber particles, silica fume and pyrogenic silica) on the hydration of UHPC pastes were investigated within the first hour. Two different reaction sequences for silica (paths I and II) can be observed in a cementitious environment: either silica dissolves followed by a pozzolanic reaction (path I) or C–S– H phases from the hydration of alite nucleate at its surface (path II). The silica reactivity was discussed considering different properties of silica. Surface silanol group densities, total contents of silanol groups and solubilities in alkaline suspensions indicate that Stoeber particles should be by far the most reactive, followed by pyrogenic silica and the less reactive silica fume. A different reactivity ranking is postulated (pyrogenic silica N silica fume ≈ Stoeber particles) if the specific surface area is considered which is twice as high for pyrogenic silica as for silica fume or Stoeber particles. Silica reactions were further traced in UHPC pastes within the first hour of hydration by investigating the solid phases and the pore solution. The results give no indication for the reactivity ranking assumed from the specific surface area or for any reaction following path I or II. Instead, silica particles seem to attract cations (Na+, K+ and Ca2+) from the pore solutions and form alkali silicate oligomers and calcium silicate oligomers. These oligomers might be held as a layer around the silica particles and form an aqueous, amorphous gel phase. The extent of the assumed oligomerization depends on the silica reactivity postulated from the surface silanol group density and the total content of silanol groups. Indeed, it seems to be high enough in pastes with Stoeber particles to bind almost all alkali ions in alkali silicate oligomers. The corresponding decline of alkali ions in the pore solution may have an impact on the subsequent processes because alkali ions accelerate the hydration of alite [69,70]. Therefore, the alite hydration in UHPC pastes containing Stoeber particles might not be as fast as in UHPC pastes containing other silica. Reaction path I seems to be more likely for Stoeber particles than for the less reactive pyrogenic silica and silica fume. Further studies should investigate the effect of silica with different reactivities for an extended observation period to fully clarify the influence on the hydration (path I, II or both) and the resulting properties of UHPC.