Juan J. Gaitero

Comparative Study of the Effects of Microsilica and Nanosilica in Concrete

It is well recognized that the use of mineral admixtures such as silica fume enhances the strength and durability of concrete. This research compares the effects of adding silica fume and nanosilica to concrete and provides a better understanding of the changes in the concrete nanostructure. Nanoindentation with scanning probe microscopy imaging was used to measure the local mechanical properties of cement pastes with 0% and 15% replacement of cement with silica fume. A reduction in the volume fraction of calcium hydroxide in a sample with silica fume provides evidence of pozzolanic reaction. Furthermore, replacing 15% cement by silica fume increased the volume fraction of the high-stiffness calcium silicate hydrate (C-S-H) by a small percentage that was comparable with the decrease in the volume fraction of calcium hydroxide. A parallel study of cement pastes with nanosilica showed that nanosilica significantly improves durability of concrete. This research provides insight into the effects of nanosilica on cement paste nanostructure and explains its effect on durability of concrete. The nanoindentation study showed that the volume fraction of the high-stiffness C-S-H gel increased significantly with addition of nanosilica. Nanoindentation results of cement paste samples with similar percentages of silica fume and nanosilica were compared. Samples with nanosilica had almost twice the amount of high-stiffness C-S-H as the samples with silica fume.

Small Changes Can Make a Great Difference

Four different types of commercially available silica nanoparticles were added to ordinary portland cement pastes to study their effects. The subsequent multiscale characterization of the material revealed that the addition of the nanoparticles induced a pozzolanic reaction that increased the amount of calcium silicate hydrate (C-S-H) gel in the paste to the detriment of portlandite. This had important implications for the hydration kinetics and the microstructure of the paste, including an increase in the initial hydration rate. A reduction of the overall porosity was also observed. The C-S-H gel of the pastes with nanosilica also showed some particular features, such as greater aluminum content and longer silicate chains. This was especially relevant because nanoindentation measurements and atomistic calculations showed that this was bound to an improvement in the mechanical properties of the C-S-H gel itself. Finally, the sum of all these factors resulted in pastes with 30% more compressive strength, which proved that, effectively, small changes can make a great difference.

Effect of hydration on the dielectric properties of C-S-H gel

The behavior of water dynamics confined in hydrated calcium silicate hydrate (C-S-H) gel has been investigated using broadband dielectric spectroscopy (BDS; 10(-2)-10(6) Hz) in the low-temperature range (110-250 K). Different water contents in C-S-H gel were explored (from 6 to 15 wt%) where water remains amorphous for all the studied temperatures. Three relaxation processes were found by BDS (labeled 1 to 3 from the fastest to the slowest), two of them reported here for the first time. We show that a strong change in the dielectric relaxation of C-S-H gel occurs with increasing hydration, especially at a hydration level in which a monolayer of water around the basic units of cement materials is predicted by different structural models. Below this hydration level both processes 2 and 3 have an Arrhenius temperature dependence. However, at higher hydration level, a non-Arrhenius behavior temperature dependence for process 3 over the whole accessible temperature range and, a crossover from low-temperature Arrhenius to high-temperature non-Arrhenius behavior for process 2 are observed. Characteristics of these processes will be discussed in this work.

Effect of addition of silica- and amine functionalized silica-nanoparticles on the microstructure of calcium silicate hydrate (C-S-H) gel

In this work we study the influence of adding nano-silica (SiO2, Nyasil™) and aminopropyl (-(CH2)3-NH2,) functionalized silica nanoparticles (Stoga) during the synthesis of calcium-silicate-hydrate (C-S-H gel). Characterization by solid state (29)Si NMR and ATR-FTIR spectroscopy showed that the addition of both particle types increases the average length of the silicate chains in C-S-H gel being this effect slightly more important in the case of Stoga particles. In addition, (13)C NMR and XPS confirmed that the aminopropyl chain remains in the final product cleaved to silicon atoms at the end of the silicate chain of C-S-H gel whereas XRD measurements showed that this result in an increment in the basal distance compared with ordinary CSH. In addition, the dynamics of water within the pores of C-S-H gel was analyzed by broadband dielectric spectroscopy. We observed that water confined in C-S-H formed with the addition of nanoparticles is faster than that in plain C-S-H which can be related to a different porous structure in these materials.

Cause of the fragile-to-strong transition observed in water confined in C-S-H gel

In this study, the rotational dynamics of hydration water confined in calcium-silicate-hydrate (C-S-H) gel with a water content of 22 wt.% was studied by broadband dielectric spectroscopy in broad temperature (110-300 K) and frequency (10(-1)-10(8) Hz) ranges. The C-S-H gel was used as a 3D confining system for investigating the possible existence of a fragile-to-strong transition for water around 220 K. Such transition was observed at 220 K in a previous study [Y. Zhang, M. Lagi, F. Ridi, E. Fratini, P. Baglioni, E. Mamontov and S. H. Chen, J. Phys.: Condens. Matter 20, 502101 (2008)] on a similar system, and it was there associated with a hidden critical point of bulk water. However, based on the experimental results presented here, there is no sign of a fragile-to-strong transition for water confined in C-S-H gel. Instead, the fragile-to-strong transition can be explained by a merging of two different relaxation processes at about 220 K.

Structural characterization of C-S-H gel through an improved deconvolution analysis of NMR spectra

The structure of the C-S-H gel in hydrated cement samples prepared with and without the addition of silica nanoparticles is studied in this work through the deconvolution of 29Si MAS-NMR spectra. X-ray diffraction and energy dispersive X-ray microanalysis are also considered for the analysis. A method with improved sensitivity and reliability is proposed for the deconvolution of the spectra into elementary components involving a good fitting of both the spectral curve and its second derivative. The increased sensitivity of the deconvolution proves to be especially interesting for the characterization of low concentration components in the NMR spectra, and the high reliability allows properly defining the effect of nanosilica addition on the C-S-H gel nanostructure. In fact, the addition of nanosilica is found to produce a pozzolanic reaction that results in an increase of the mean silicate chain length and an enhancement of the aluminum incorporation into the C-S-H gel phase.

Nanotechnology and Concrete: Concepts and Approach

The structure of cement derivatives (cement-based materials) is complex and not fully understood. They are formed by self-assembly of different chemical species in a specific way to form basic building blocks (or units); the size of these units is on the order of a few nanometers. After they are formed, these basic units arrange themselves into larger units, still nano-metric in size, which constitute the cementitious matrix. The distinctive macroscopic properties of cementitious materials arise from their unique structural arrangement. This paper is an introduction to the results obtained by the Centre for Nanomaterials Applications in Construction over the past 5 years. The approach used in this study is a combination of experimental and numerical (multiscale simulation) techniques. On the one hand, the study shows how the addition of nanosilica particles can modify the mechanical properties of the cement matrix as well as its resistance to the calcium leaching process. On the other hand, the study illustrates how computational modeling is a valuable tool for gaining insight into the complex calcium silicate hydrate nanostructure and for designing improved cementitious materials.

Effect of Chemical Environment on the Dynamics of Water Confined in Calcium Silicate Minerals: Natural and Synthetic Tobermorite

Confined water in the slit mesopores of the mineral tobermorite provides an excellent model system for analyzing the dynamic properties of water confined in cement-like materials. In this work, we use broadband dielectric spectroscopy (BDS) to analyze the dynamic of water entrapped in this crystalline material. Two samples, one natural and one synthetic, were analyzed, and despite their similar structure, the motion of confined water in their zeolitic cavity displays considerably different behavior. The water dynamics splits into two different behaviors depending on the chemical nature of the otherwise identical structural environment: water molecules located in areas where the primary building units are SiO4 relax slowly compared to water molecules located in cavities built with both AlO4 and SiO4. Compared to water confined in regular porous systems, water restricted in tobermorite is slower, indicating that the mesopore structure induces high disorder in the water structure. A comparison with water confined in the C-S-H gel is also discussed in this work. The strong dynamical changes in water due to the presence of aluminum might have important implications in the chemical transport of ions within hydrated calcium silicates, a process that governs the leaching and chemical degradation of cement.

An Innovative Self-Healing System in Ultra-high Strength Concrete Under Freeze-Thaw Cycles

The development of cementitious materials with self-healing properties is currently a very active research line in order to increase the service life of structures and reduce the costs associated to maintenance and repair. A combination of silica microcapsules containing an epoxy sealing compound and nanosilica particles functionalized with amine groups has been developed to confer self-healing properties to an ultra-high performance concrete. The durability of the self-healing system under severe climatic conditions is considered in this work. Cylindrical concrete specimens were made with and without the self-healing additions. After a curing time of 28 days, micro-cracks with a controlled width were created in the specimens that were subsequently subjected to a freeze-thaw test consisting of four cycles per day between −20 and +20 °C. The results from capillary water absorption test after 150 cycles confirm the efficiency of the sealing system even under the aggressive conditions considered in this study.

A multi-scale approach for percolation transition and its application to cement setting

Shortly after mixing cement grains with water, a cementitious fluid paste is formed that immediately transforms into a solid form by a phenomenon known as setting. Setting actually corresponds to the percolation of emergent network structures consisting of dissolving cement grains glued together by nanoscale hydration products, mainly calcium-silicate-hydrates. As happens in many percolation phenomena problems, the theoretical identification of the percolation threshold (i.e. the cement setting) is still challenging, since the length scale where percolation becomes apparent (typically the length of the cement grains, microns) is many times larger than the nanoscale hydrates forming the growing spanning network. Up to now, the long-lasting gap of knowledge on the establishment of a seamless handshake between both scales has been an unsurmountable obstacle for the development of a predictive theory of setting. Herein we present a true multi-scale model which concurrently provides information at the scale of cement grains (microns) and at the scale of the nano-hydrates that emerge during cement hydration. A key feature of the model is the recognition of cement setting as an off-lattice bond percolation process between cement grains. Inasmuch as this is so, the macroscopic probability of forming bonds between cement grains can be statistically analysed in smaller local observation windows containing fewer cement grains, where the nucleation and growth of the nano-hydrates can be explicitly described using a kinetic Monte Carlo Nucleation and Growth model. The most striking result of the model is the finding that only a few links (~12%) between cement grains are needed to reach setting. This directly unveils the importance of explicitly including nano-texture on the description of setting and explains why so low amount of nano-hydrates is needed for forming a spanning network. From the simulations, it becomes evident that this low amount is least affected by processing variables like the water-to-cement ratio and the presence of large quantities of nonreactive fillers. These counter-intuitive predictions were verified by ex-professo experiments that we have carried out to check the validity of our model.