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Effects of amorphous nano-silica additions on mechanical and durability performance of SCC mixtures
(2012)
In the recent years the application of nanotechnology in building materials has increased
exponentially. One of the most referred and used nano-materials is amorphous silica with
particles size in the nano-range, even though its application and effect in concrete has not been
fully understood yet. It has been reported that nano-silica (nS) addition increases the
compressive strength and reduces the overall permeability of hardened concrete due to the
pozzolanic properties which are resulting in finer hydrated phases (C-S-H gel) and densified
microstructure (nano-filler and anti leaching effects). These effects enhance the durability of
concrete structures such as bridges, quays or off-shore oil facilities in marine environments.
In this study two different types of nano-silica were applied in self-compacting concrete (SCC),
both having similar particle size distributions (PSD) but produced in two different processes
(fumed powder silica and precipitated silica in colloidal suspension). The influence of nanosilica
on SCC was investigated with respect to the properties of concrete in the fresh state
(workability) and hardened state (mechanical properties and durability). Additionally, the
densification of microstructure of the hardened concrete was verified by SEM and EDS analyses.
The obtained results demonstrate that an efficient use of nano-silica in SCC can improve its
mechanical properties and durability. Considering the reactivity of the two nano-silica studied,
colloidal type shown more reactivity at early age, which influenced all the final SCC properties.
Innovative admixture technology has significantly widened up the ränge of possibilities of concrete engineers. For many decades the water to cement ratio (w/c) was the major influencing factor for the performance of concrete. Due to the need to adjust a consistency, which still allowed reasonable workability, the w/c was typically significantly higher than technologically reasonable. Rheology modifying admixtures Support adjusting the concrete consistency largely independent of the w/c. It was only after the invention of the first superplasticizers that modern concrete technology significantly evolved in terms of flowability, strength, and durability, and only due to the steady evolution of the technology modern innovations, such as Self-Compacting Concrete, Ultra-High-Performance Concrete, or Engineered Cementitious Composites were made possible. Today’s superplasticizers are extremely versatile and can be adjusted to individual technologicalspecifications. However, the other side of the coin of versatility is that cementitious Systems incorporating superplasticizers have become more sensitive against environmental influences, such as the environmental temperature, which may cause unwanted effects or demand for supplementary admixture use such as stabilizing admixtures. Hence, concrete mixture composition with admixtures demands for a high level of expertise and offen there is lack of awareness about the mode of Operation of rheology modifying admixtures among concrete technologists. The paper gives a comprehensive overview about rheology modifying admixtures such as superplasticizers or stabilizing agents, and how they can be used depending upon the application in the most favourable way. Based on experiences with the sub-Saharan African concreting boundary conditions, which exhibit many challenges in terms of environmental boundary conditions and construction site logistics, conclusions are finally drawn, how admixtures can be used in the most beneficial way to improve the concrete casting Situation.