Laila Raki

Cement and Concrete Nanoscience and Nanotechnology

Concrete science is a multidisciplinary area of research where nanotechnology potentially offers the opportunity to enhance the understanding of concrete behavior, to engineer its properties and to lower production and ecological cost of construction materials. Recent work at the National Research Council Canada in the area of concrete materials research has shown the potential of improving concrete properties by modifying the structure of cement hydrates, addition of nanoparticles and nanotubes and controlling the delivery of admixtures. This article will focus on a review of these innovative achievements.

Hydration of tricalcium silicate in the presence of synthetic calcium–silicate–hydrate

The early age-hydration of tricalcium silicate, the main chemical compound in Portland cement, was studied in the presence of synthetic calcium silicate hydrate (C–S–H) addition having C/S ratios = 0.8 and 1.2. Isothermal conduction calorimetry, scanning electron microscopy, differential scanning calorimetry and 29Si MAS NMR were employed in order to investigate events occurring during various stages of the hydration. The results that were analyzed using novel methods in cement chemistry showed that the addition of seeds of synthetic C–S–H significantly accelerated the hydration of C3S. The extent of the acceleration was dependant on the amount and chemical composition of the C–S–H seeds. It was suggested that the synthetic C–S–H significantly increased the rate and degree of dissolution of the C3S particles. It was also found that the nucleation and silicate polymerization of the C–S–H that formed during the hydration of the C3S phase was promoted. Direct evidence of the seeding effect was provided. The properties of the resulting C–S–H hydration products seemed to be dependant on the lime-to-silica ratio of the synthetic C–S–H. It was suggested that the silicate polymerization and chemical composition of the hydration products of silicate phases may be manipulated through C–S–H seeding. As the chemical and mechanical properties of C–S–H are largely controlled by its C/S ratio, this method should provide a unique tool for tailoring the nanostructure of the hydration products of Portland cement through the addition of selective C–S–H seeds for optimum engineering and durability performance.

Formation and characterization of calcium silicate hydrate–hexadecyltrimethylammonium nanostructure

Results of an investigation of the interaction potential of synthetic and pre-treated calcium silicate hydrate (C-S-H) [with hexadecyltrimethylammonium (HDTMA)] are reported. The effective and strong interaction of these molecules with the C-S-H surface was shown using 13 C and 29 Si cross polarization magic angle spinning (CP MAS) nuclear magnetic resonance, x-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and Fourier transform infrared spectroscopy analysis. The HDTMA–C-S-H interaction is influenced by the poorly crystallized layered structure of C-S-H. An indefinite number of layers and an irregular arrangement are confirmed by the SEM images. The position and shape of the 002 reflection of C-S-H are affected by drying procedures, chemical pre-treatment, and reaction temperature. Recovery of the initial 002 peak position after severe drying and rewetting with distilled water or interaction with HDTMA is incomplete but accompanied by an increase in intensity. It is inferred that the stability of C-S-H binders in concrete can be affected by a variation in nanostructure resulting from engineering variables such as curing temperature and use of chemical admixtures.

Nanotechnology Applications for Sustainable Cement-Based Products

Concrete is a macro-material strongly influenced by the properties of its components and hydrates at the nanoscale. Progress at this level will engender new opportunities for improvement of strength and durability of concrete materials. This article will focus on recent research work in the field of nanoscience applications to cement and concrete at the NRC-IRC. A particular attention will be given to nanoparticles and cement-based nanocomposites.

A comparative study of the volume stability of C–S–H (I) and Portland cement paste in aqueous salt solutions

Ordinary Portland cement (OPC) paste specimens and compacted C–S–H (I) powders were immersed in distilled water and aqueous salt solutions of varying concentration to study their volume change behavior. Immersion resulted in ionic interaction leading to various degrees of expansion, leaching, softening and dissolution of the test samples. In all cases relatively rapid expansion occurred. The expansion of C–S–H (I) in aggressive media was found to be large and fast in MgCl2, MgSO4, LiOH, LiNO3 and calcium (magnesium) acetate (CaMgAc) solutions. LiCl, CaCl2 and NaCl were moderately aggressive towards C–S–H (I) depending on the solution concentration. Trends in the length change behavior of OPC paste are similar to that of synthesized C–S–H (I). Similarities were observed between the length change behavior of compacted C–S–H (I) and the swelling of smectite clays in contact with these osmotic media. The similarities are compatible with explanations of expansion provided by both the osmotic and the electrical double layer (EDL) theories. The relationship between the expansive behavior of C–S–H and both Na and Ca Montmorillonite in contact with aqueous salt solutions is discussed extensively in the context of its significance in cement science.

Volume Stability of Calcium-Silicate-Hydrate/Polyaniline Nanocomposites in Aqueous Salt Solutions

The volume stability of phase pure calcium-silicate-hydrates (C-S-H) and C-S-H/polyaniline nanocomposites prepared with two CaO-SiO₂ molar ratio (C/S) variations (0.8 and 1.2) was assessed in MgSO₄, MgCl₂, LiCl, and NaCl aqueous solutions. The change in the crystalline structure of the samples with the time of immersion was also explored using X-ray diffraction, scanning electron microscopy, and thermal gravimetric analysis techniques. It was observed that the modification of the C-S-H samples with polyaniline significantly enhanced their volume stability and durability in all the salt solutions. The beneficial effect of the polyaniline modification was more pronounced in the C-S-H host with higher C/S (C/S = 1.2). The longitudinal expansion of the C-S-H/polyaniline nanocomposites with C/S = 1.2 in the salt solutions was about 30% of that of the phase pure C-S-H with a similar C/S ratio. In addition, the polyaniline modification of C-S-H samples reduced the rate of formation of gypsum, brucite, and other reaction products in the samples.