Colloidal Nano-SiO2 Migration in Hardened Mortars and Its Chemical Interaction with Cementitious Matrix Ismael Diaz, Mercedes Sanchez Moreno, Ma Cruz Alonso, Rodrigo González, and Antonio Zaldivar Abstract Concrete performance can be improved by using colloidal nano-SiO2 (NS) by two different methods: direct addition at the fresh stage to improve mechanical and durability properties; and superficial application, such as sealing treatment to hinder further penetration of aggressive ions. Chemical interaction of NS with the hardened cement mortar (HCM) was analyzed in both situations. NS chemical reactivity was confirmed by SEM and TGA tests. Mortar specimens with nanosilica additions (up to 2.5 % cement weight) were prepared and cured for 7 days before characterization. A superficial treatment by migration of colloidal NS dispersion (30 % weight concentration) under a 12 V was applied on mortar samples after 7 days of curing. Reactivity of NS with the HCM was confirmed by energy dispersion spectroscopy analysis (EDS) coupled to scanning electron microscopy in backscattering (SEM-BS). The new silicon enriched C-S-H gels with C/S ratio around 1.0 were determined by TGA. Keywords Nanosilica • Surface treatment • Migration I. Diaz Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, Pedro de Alba S/N, San Nicolás de los Garza, Nuevo León, Mexico e-mail: [email protected] M. Sanchez Moreno • M.C. Alonso Instituto “Eduardo Torroja” de Ciencias de la Construcción (IETCC-CSIC), Serrano Galvache 4, 28033 Madrid, Spain R. González (*) • A. Zaldivar Facultad de Ingeniería Civil, Universidad Autónoma de Nuevo León, Pedro de Alba S/N, San Nicolás de los Garza, Nuevo León, Mexico e-mail: [email protected] © Springer International Publishing Switzerland 2015 K. Sobolev, S.P. Shah (eds.), Nanotechnology in Construction, DOI 10.1007/978-3-319-17088-6_21 173 174 I. Diaz et al. 1 Introduction Nano-SiO2 (NS) additions have shown a great capacity to improve some concrete properties such as reducing pore system, which is the medium through aggressive ions (e.g. Ca+, Cl− ions) come into the concrete and reach the steel rebar to promote corrosion . The relation between corrosion mechanisms and the pore system is based on the principle that external ions move into the concrete matrix through the connectivity and water content of the pore system . NS additions can work as a filler material, due to its dimensions and the feasibility to be clogged in the pore channels and create discontinuities in the pore connectivity. A second possible approach is pozzolanic activity, when it comes in contact with calcium hydroxide (Ca(OH)2). The reaction of NS with byproducts of cement hydration produce new C-S-H gel which will increase matrix density [3, 4]. The conventional way of adding NS to the mix is during fresh concrete elaboration. NS are normally added along with mix water. Colloidal NS used in this work was slightly negative charged, whereby an electric field was set in order to migrate nanoparticles from the surface to the interior of the hardened mortar as a non-conventional way of adding NS [5, 6]. A cell was designed in order to perform the migration of NS through the HCM. This cell consisted of an acrylic container with two electrodes in both sides with an HCM sample assembled in the middle. Then, an electric field was applied through the cell. An analysis was performed by scanning electron microscopy with an energy dispersive x-ray spectroscopy detector to compare HCM with NS addition in fresh state and a HCM after migration treatment. Microstructure and Ca/Si ratios measurements could indicate a reaction of NS with the HCM after migration, where lower values than common reported values show an enrichment of Si in the hydrated phases. Thus, it can be inferred that the movement of NS was effectively carried out under the presence of an electric field. 2 2.1 Materials and Methods NS Addition Samples production was performed using Ordinary Portland Cement (OPC, 42.5 R/ SR sulphate resistant, with low tricalcium aluminate content of 3.8 % and low alkali content). In Fig. 1a a commercial colloidal NS aqueous dispersion (nano particle size around 7 nm) with an amorphous nature was mixed with water before mortar fabrication (water/cement: 0.5 and aggregate/cement 3:1) according to the ASTM C109 standard procedure. The colloidal NS was used as a replacement material at 2.5 % in weight, and an adjustment in the amount of water was made as well. Samples were cured in a moist curing room with a relative humidity of 95 ± 5 % and 22 ± 2 °C. Samples were eventually fractured in order to perform a future analysis according to the experimental schedule. Colloidal Nano-SiO2 Migration in Hardened Mortars… 175 a b 900 800 Lin (counts) 700 600 500 400 300 200 100 0 0 10 20 30 40 50 60 2-Theta-Scale Fig. 1 Amorphous characteristic of NS (a) and general configuration of the electrical circuit of migration cells (b) 2.2 NS Migration Cylindrical samples were produced (length 15 cm, diameter 7.5 cm) with a siliceous aggregate in a ratio of 3:1 with the cement and a water/cement ratio of 0.5. Samples were removed from the mold after a day of fabrication and they were stored for curing under controlled conditions in a room with a temperature of 22 ± 2 °C and a relative humidity of 95 ± 5 % for 56 days before testing. After curing, the top and the bottom sides of the samples were cut. From the remaining part, slices of 1.3 cm of thickness were obtained and conditioned for migration cells. A day before the treatment with colloidal NS, the HCM samples were saturated in water under vacuum conditions. This process consisted in introducing the HCM samples in a container with a vacuum pressure of 133 Pa during 3 h. Then, water was poured into the container to cover the samples and the vacuum conditions were maintained for 1 more hour. Finally, the samples remained immersed in water for 18 h more to guarantee the total saturation of the mortar pores. The assembly of the migration cells was performed according to electrical configuration from which was possible analyze if an ionic transport exist from the surface to the inside of a mortar sample (see Fig. 1b). This electrical circuit was determined from slightly negative charge of the NS. The main aim of this arrangement is only to study the ionic transport and its effect on the microstructure. 3 3.1 Results Microstructure and Chemical Composition: Addition Versus Migration It could be seen by SEM-EDS analysis that samples with NS by addition, presented a mostly conventional microstructure and chemical composition at early ages (2 and 7 days). Anhydride phases were mainly found, where EDS analysis indicated 176 I. Diaz et al. Fig. 2 Ca/Si values of cement mortar with 2.5 % NS addition; (a) 2 days and (b) 7 days Fig. 3 Image of HCM after (a) and before (b) NS migration with Ca/Si values the presence of Ca/Si values corresponding to C3S and C2S typical from cement particles. Figure 2 shows the sample with 2.5 % NS addition and indicates Ca/Si values of different areas. In Fig. 2a it can be seen that there are more similar values to those from a conventional cement paste fabricated under standard conditions, and due to its early age (2 days), hydrated products could not be easily detected. Figure 2b shows the sample at 7 days, and it indicates a value lower than reported C-S-H gel, this could be interpreted as NS agglomeration in the microstructure. In the case of NS migration, images were taken from the surface in contact with the NS at the end of the 7 days of treatment. In some points (Fig. 3b), the sample showed values which are under those of conventional CSH gel (1.5–1.8) and other corresponding phases of a reference sample: C3S: 3, C2S: 2 (Fig. 3a) . This lower values of Ca/Si showed in the microstructure, could be related to the transport of NS to the interior of the HCM. This transport of NS seems to result in a deposition of Si in the conventional phases and in a further enrichment of the matrix. This deposition could lead to a later reaction with sub products of the normal hydration process, like Ca(OH)2, in a pozzolanic reaction or just acting as a filler or sealing 177 Colloidal Nano-SiO2 Migration in Hardened Mortars… Table 1 Loss weight % by TGA of reference and NS treated sample Range 100–350 350–450 Ref 1.95 1.43 NS 1.89 1.09 material. In any of these cases, it would be a process to densify the microstructure. From the surface in contact with the NS, a depth distance of these low Ca/Si values was measured up to 1.5 mm approximately. 3.2 Termogravimetric Analysis Equipment of thermogravimetric and differential thermal analysis was used to determine the level of reactivity of NS in HCM. As reported by recent works, NS is well known to react and improve the hydration process, but its effect in HCM is not clear. In order to study this effect, an analysis of powered HCM after NS migration was carried out and it is shown in Table 1. The thermogravimetric analysis of the HCM sample was carried out with rate of 4 °C/min from 25 °C to 1,000 °C. In the results shown in Table 1 it can be seen that the weight loss in the range of temperature between 350 and 450 °C (which is related to the decomposition of Ca(OH)2) in the sample after NS treatment is lower than the reference sample. It could be related to a reaction of the NS with the byproduct of hydration and a subsequent formation of new CSH gel. The difference between the weight loss in the range of C-S-H gel (100–350 °C) could be explained with the different composition of these gels; and also the effect of the electric field in HCM could be leading a leach of some phases. 4 Conclusions The migration of NS through the HCM under an electric field has shown that it is possible to modify the chemical composition of an HCM with the increment of Si in the matrix. 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