Nanocomposites and nanomaterials

Evaluation of seawater as a mixing water in cementitious composites incorporated with nanosilica
Sikora P.1,2, Cendrowski K.3, Augustyniak A.4, Mijowska E.3, Stephan D.2
1 Faculty of Civil Engineering and Architecture, West Pomeranian University of Technology, Szczecin. Al. Piastow 50, 70-311 Szczecin, Poland
E-mail: pawel.sikora@zut.edu.pl
2 Building Materials and Construction Chemistry, Technische Universität Berlin, Gustav-Meyer-Allee 25, 13355 Berlin, Germany
3 Nanomaterials Physicochemistry Department, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology, Szczecin. Al. Piastow 45, 70-311 Szczecin, Poland
4 Department of Immunology, Microbiology and Physiological Chemistry, Faculty of Biotechnology and Animal Husbandry, West Pomeranian University of Technology, Szczecin. Al. Piastow 45, 70-311 Szczecin, Poland

The dramatically increasing water consumption in concrete industry, as well as the shortage in freshwater in many parts of the world is a severe environmental issue. Therefore, the search for alternative sources of concrete components including new water sources, e.g., recycled (from concrete production) or seawater, has gathered the attention of researchers and authorities in recent years. Particular interest was put to using seawater in order to develop a durable and strong concrete.
Studies related to the effect of seawater on the properties of cement-based composites have shown that the salt quantity in seawater can improve the early strength of concrete, while later, the strength remains similar or is slightly higher in comparison to concrete made with tap water. However, due to a high chloride content in seawater, its usage in concrete increases the corrosion risk of the embedded reinforcing steel, which makes the utilization of this type of water controversial. According to EN 1008, saltwater can be used for a non-reinforced concrete or embedded metal, although it is generally considered not suitable for the production of reinforced or prestressed concrete. Therefore, to suppress the corrosion induced by seawater, various methods including mixing with pozzolan materials (e.g. nanosilica), corrosion inhibitors, or the use of stainless steel reinforcement, as well as corrosion resistant reinforcement, were proposed by researchers. In addition, nanosilica is used as an admixture noticeably improving mechanical properties and durability of cementitious composites. 
The goal of this study was to evaluate the suitability of seawater for production of cement-based composites modified with nanosilica. The interaction of nanosilica and seawater was evaluated by isothermal calorimetry, as well as compressive strength development tests (1, 2, 7, 14, 28 days). In addition, nanoparticles were suspended in seawater and analysed with transmission electron microscopy (TEM), energy dispersive spectrum (EDS), and X-ray diffraction (XRD).
The study has shown that nanosilica dissolves during mixing with seawater due to the formation of unstable silicic acid in the presence of NaCl. This phenomenon cause the uncontrolled formation of partially dissolved and agglomerated structures. Furthermore, such agglomeration of silica nanoparticles is likely to influence the performance of cement-based composites by hindering its hydration kinetics, thus decreasing its compressive strength development at early ages. It was shown that the incorporation of seawater as potential mixing water for the production of cement-based composites modified with nanosilica exhibited certain threats related to the dissolution of nanosilica and the creation of agglomerates. 

Acknowledgement: This research was supported by the National Science Centre within the project No. 2016/21/N/ST8/00095 (PRELUDIUM 11).