Multi-scale Study of Calcium Leaching in Cement Pastes with Silica Nanoparticles J.J. Gaitero, W. Zhu, and I. Campillo * Abstract. Calcium leaching is a degradation process consisting in the progressive dissolution of the cement paste as a consequence of the migration of the calcium ions to the aggressive solution. Although the most important changes take place at the nano- and micro-scale, their consequences are observed at every length scale. Within this work, a multi-scale approach combining a wide variety of experimental techniques was used to study such phenomenon in cement pastes with silica nanoparticles. The experimental results proved that the pozzolanic reaction induced by the nanoparticles resulted in a C-S-H gel more stable chemically and with longer silicate chains. In addition, the reduction of the amount of portlandite gave place to pastes with improved microstructure. As a consequence, the performances of such pastes were greatly enhanced both before and during the degradation process. 1 Introduction Calcium leaching is a degradation process consisting in the progressive dissolution of the cement paste as a consequence of the migration of calcium ions to the aggressive solution. The kinetics of the process depend of several parameters being the most important ones the porosity (the natural path for chemical attack), the composition of the paste (each phase degrades at a different rate), and the nature of the aggressive solution (generally soft water). These are the reasons for using nanosilica in order to reduce calcium leaching. Silica, generally in the form of silica fume, has long been used to improve the performance of the cement paste in terms of strength and refinement/reduction of the porosity [1, 2, 6, 7, 15, 17]. Furthermore, this takes place by mean of a pozzolanic reaction that results in the reduction of the amount of portlandite, the hydrous phase most severely affected by J.J. Gaitero Labein-Tecnalia, Parque Tecnologico de Bizkaia, Derio, Spain e-mail: jjgaitero@labein.es W. Zhu University of the West of Scotland, Paisley, Scotland I. Campillo CIC nanogune Consolider, Donostia, Spain
194 J.J. Gaitero et al. calcium leaching. The use of silica nanoparticles has several advantages in comparison to other types of silica [3, 13]. As a consequence of their reduced size the nanoparticles act as nucleation site for the growth of hydration products, accelerating the hydration rate. Furthermore, their large surface area and purity (up to 99 %), altogether with their amorphous nature, provides them a great reactivity. 2 Materials and Methods The samples were prepared at a water-cement ratio w/c=0.4 using an Ordinary Type I 52.5R Portland cement and a 6 wt.% of four different types of commercial silica nanoparticles. The mixing process varied depending on whether the nanoparticles were in the form of colloidal dispersion (CS1, CS2, CS3) or dry powder (ADS). While in the first case, the dispersion was mixed with the water and stirred for 5 minutes before adding the solution to the cement, in the other one, the powder was mixed with the cement and agitated for a minute before pouring the water on it. One set without silica nanoparticles (REF) was also prepared for comparison. The samples were cast into 1 1 6 cm 3 prisms using steel moulds, where they stayed for 24 hours in a chamber at 20 ºC and 100 % humidity. After such time, they were unmoulded and introduced in a saturated lime solution at room temperature for another 27 days of curing. At the end of this period (t 0 ), they were moved into a bath containing a 6 M amonium nitrate solution where they stayed for 9, 21, 42 or 63 days for the accelerated calcium leaching (t 1, t 2, t 3 and t 4 respectively). In order to prevent the samples from carbonation, the aggressive bath was maintained all the time in contact with a nitrogen atmosphere. Furthermore, the solution was renewed whenever its ph reached a value of 9.2 to ensure its aggressiveness. 3 Results and Discussion The multi-scale approach used in this work consisted in the combination of several experimental techniques that provided information about every length scale of the material. The discussion of the obtained results will be made in order attending to the length scale of the phenomena involved; i.e. it will begin with the macroscopic techniques and finish with the atomistic ones. From a macroscopic point of view the addition of silica nanoparticles to the cement paste increased the overall strength of the paste and helped retaining it along the degradation process, see Fig. 1(a). The improvement in performance was about 30 % in the cured specimens (t 0 ) and 100-700 % in the degraded ones (t 4 ). Comparison between the different types of additions used revealed that the colloidal dispersions were much more effective reducing the effects of the degradation than the agglomerated dry silica (ADS). There was a very good correlation between the macroscopic strength and the porosity evolution, see Fig. 1(a)-(b), which followed exactly the opposite trend. At t 0 the reference specimen (REF) was the one with the highest porosity, followed by ADS and then the pastes with colloidal silica. As soon as the degradation began
Multi-scale Study of Calcium Leaching in Cement Pastes 195 Fig. 1 (a) Compressive strength and (b) total pore volume, measured by mercury intrusion porosimetry, as a function of the degradation time (t 1 ), the total pore volume increased dramatically in REF followed by ADS, being the rest of the specimens barely affected until t 2. This difference of behavior was attributed to the better dispersion of colloidal silica throughout the paste which resulted in a more homogeneous porosity and portlandite distribution compared to ADS. After such time, the increase in porosity was more progressive in all the pastes. X-ray powder diffraction spectra proved that the great changes in porosity and strength undergone by the specimens during the early stages of the degradation process (t 1 for REF and ADS, and t 2 for CS1, CS2, and CS3) could be attributed almost entirely to the dissolution of portlandite, see Fig. 2. Therefore, the reduction in the amount of portlandite during the curing process because of the reaction with the silica nanoparticles contributed significantly to reduce the negative consequences of calcium leaching. Portlandite dissolution and the consequent porosity increase were also accompanied by a complete hydration of all the anhydrous cement present at the paste. The slower degradation observed afterwards was a consequence of the more progressive dissolution of other phases like ettringite or the C-S-H gel itself. Fig. 2 X-ray powder diffraction spectra after 9 days of degradation (t 1 ). C: Clinker, C: Calcite, P: Portlandite
196 J.J. Gaitero et al. The elemental composition obtained by x-ray fluorescence confirmed these findings, see Fig. 3(a)-(b). The complete dissolution of portlandite at t 1 in REF and ADS and t 2 in CS1, CS2, and CS3 were clearly reflected in the evolution of C/S. The same happened with the dissolution of ettringite at t 3 in REF and t 4 in ADS and CS3. However, such dissolution was not complete in the latter because it was accompanied by the apparition of gypsum which, having a smaller calcium content than ettringite, was considered as an intermediate state in the degradation process. Fig. 3 (a) C/S and (b) sulphur content measured by x-ray fluorescence at different stages during the degradation process Depth sensing nanoindentation [4, 5, 11, 12, 16, 18] was used to study the detrimental effects of the calcium loss in the mechanical properties of the C-S-H gel, see Table 1. Previous to the immersion of the specimens in the aggressive solution, no variation of the indentation modulus between the different pastes was observed. On the contrary, after 42 days of accelerated calcium leaching REF and ADS had undergone a severe and homogeneous degradation, while CS1 was almost intact at its centre with increasing loss of performance the closer to its outer surface. Table 1 Indentation modulus Sample REF ADS CS1 Degradation Time (Days) Indentation Modulus (GPa) LD-CSH HD-CSH 0 19±3 27±4 42 2.3±0.5 3.6±0.6 0 23±2 27±3 42 3.5±0.7 5.0±0.9 0 20±3 26±3 42 4.6±1-14±2 7.1±0.7-24±3
Multi-scale Study of Calcium Leaching in Cement Pastes 197 Fig. 4 Time evolution of the average segment length and the average polymeration calculated from the areas of the peaks of the 29 Si MAS-NMR spectra 29 Si MAS-NMR provided information about the changes taking place in the atomic structure of the C-S-H gel [8, 14]. Here the discussion will be made attending to two parameters calculated from the NMR spectra and defined elsewhere [9, 10]: the average polymerization, and the average segment length. According to Fig. 4, the addition of silica nanoparticles resulted in an increase in the average segment length. As soon as the degradation began, such average segment length was sharply reduced because of the hydration of the unreacted cement, which gave place to the apparition of an important number of new short silicate chains. Furthermore, as a consequence of the calcium loss, chains began to merge together resulting in an increase of the degree of polymerization. As calcium loss went on the average polymerization continued growing because of the progressive merging of the silicate chains. However, the fact that the average segment length barely varied could only be explained if such merging took place only at the chains ends. Therefore, it was concluded that longer chains improved calcium stability. 4 Conclusions The addition of silica nanoparticles improved considerably the performance of the cement paste both before and during the degradation by calcium leaching. The multi-scale approach used for the characterization of the samples showed that the consequences of the addition of only 6 wt.% of silica nanoparticles affected to all aspects of the material. At the macroscopic level, they increased the macroscopic strength of the cured pastes and limited its reduction along the degradation process. This was a consequence of the improved microstructure (less porosity and portlandite) and the changes undergone by the C-S-H gel which made it more resistant to the decalcification. References 1. Asgeorsson, H., Gudmundsson, G.: Pozolanic activity of silica dust. Cem. Concr. Res. 9, 249 252 (1979) 2. Bentur, A., Cohen, M.D.: Effect of condensed silica fume on the microstructure of the interfacial zone in Portland cement mortars. Am. Ceram. Soc. 70, 738 743 (1987)
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