Effects of nanosilica addition on workability and compressive strength of Portland cement pastes

Characterization of morphology and texture of several amorphous nano-silica particles used in concrete

“…In general, at any given replacement level, the workability of cement pastes would decrease as the surface area of the nano-silica increases. In addition, it was demonstrated that the CASAH gel precipitated in the surface of the nano-silica aggregates has less favorable mechanical properties (cannot function as a binder) and higher porosity than the gel produced by well dispersed silica nano-particles [47,48]. The presence of an interparticle transition zone (ITZ) between the large reacted agglomerates and the bulk paste has been reported, which acts as a weak zone that decreases the final mechanical properties of concrete.…”

“…The authors suggested that the lower water absorption capacity of micro-silica (pyrogenic type) causes the higher hydration. As was explained in Section 3.3, the CASAH gel precipitated on the surface of the nano-silica aggregates, may, moreover, have lower mechanical properties (cannot function as a binder) and higher porosity than the gel produced by well dispersed silica nano-particles [47,48]. The presence of an ITZ between the large agglomerates with a modified surface after reaction and the bulk paste, which act as weak zone may decrease the final mechanical properties of the mortars.…”

“…The remarkable decrease in workability produced by addition of nano-silica to cement pastes, which are observed also after the addition of a superplasticizer (SP), suggest that the high specific surface area is not the only controlling parameter in producing the significant increase in the yield stress and in the plastic viscosity. Some researchers reported [46][47][48] a strong and ''instantaneous'' interaction between silica nano-particles (no matter its state: colloidal, powder or in slurry) and some chemical species dissolved in the liquid phase of fresh cement pastes, leading to formation a destabilizing gel with high water retention capacity (in form of bound water). The results of slump-flow tests on the pastes made with the micro-and nano-silica produced by different routes support this explanation.…”

Comparative study of the characteristics of nano silica - , silica fume - and fly ash - incorporated cement mortars

“…One important difficulty is dispersing NS particles in a cement mixture. The previous studies showed that the dispersion method of the NS particles affects the workability of cement mortar, the rate and extent of cement hydration as well as the structural, physical and mechanical properties of the fresh and hardened cement composites 11,[30][31][32][33] . Some authors have concluded that the appropriate percentage of NS must be small (1-5 wt%) to avoid agglomeration of particles during mixing [12][13][14][15][16] , while others have indicated that properties can also be improved with higher dosages up to approximately 10 wt% if these nanoparticles are successfully dispersed in the initial cementious mixture 7,12,17,29 .…”

“…Some authors have concluded that the appropriate percentage of NS must be small (1-5 wt%) to avoid agglomeration of particles during mixing [12][13][14][15][16] , while others have indicated that properties can also be improved with higher dosages up to approximately 10 wt% if these nanoparticles are successfully dispersed in the initial cementious mixture 7,12,17,29 . Second problem related with using NS in a cement mixture is the decrease in fluidity, due to its high surface area and increased water demand [29][30][31][32][33][34] . Some research groups have studied the interaction of superplasticizers with NS added cements, and reported that the type and concentration of superplasticizer, along with the particle size and surface area of NS, affect the rheological properties of fresh cement mixtures [29][30][31][32][33][34] .…”