Communication pubs.acs.org/crystal
Direct Imaging of Nucleation Mechanisms by Synchrotron Diffraction Micro-Tomography: Superplasticizer-Induced Change of C−S−H Nucleation in Cement Gilberto Artioli,§ Luca Valentini,*,§ Marco Voltolini,† Maria C. Dalconi,§ Giorgio Ferrari,‡ and Vincenzo Russo‡ §
Department of Geosciences and CIRCe Center, University of Padua, I-35131 Padua, Italy Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡ Research & Development Department, Mapei SpA, I-20158 Milan, Italy †
S Supporting Information *
ABSTRACT: The properties of cementitious materials are related to the microstructure of their binder matrix, which develops, during cement hydration, by a sequence of dissolution−precipitation reactions. Here, microstructural development is monitored during hydration by synchrotron X-ray diffraction-enhanced computed microtomography (XRD-CT). This innovative, noninvasive technique yields images of the crystallographic phases present in the hydrating cement paste at different stages, which are combined to map the sites where dissolution and precipitation occur. The results indicate that the nucleation mechanism of the main hydration product (a calcium-silicate hydrate commonly referred to as C−S−H) changes in the presence of polycarboxylate ether (PCE) superplasticizers. The observed change is essential to understand the development of the cement microstructure and to provide a direct link between the reaction kinetics and the physicomechanical properties of the system.
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charged backbone with grafted uncharged side chains. The dispersing effect of these polymers stems from their conformation in solutions having high ionic strength, with the backbone adsorbing onto the surface of the cement particles and the nonadsorbing side chains inducing steric hindrance between cement particles.3 However, the details of the modes of interaction between cement phases and rheological modifiers such as PCEs are still debated, mainly because of the remarkable chemical and physical complexity of the system. Consequently, no agreed explanation has been given for the observed modification of the hydration reaction kinetics occurring in the presence of PCEs. Postulated mechanisms include hindered dissolution due to interference between surface dissolution sites and PCE adsorption;4 reduced activity of Ca ions in solution because of complexation by the PCEs;5 or change in the morphology and growth of the hydration products.6,7 Here, the kinetics of cement hydration and the microstructural changes induced by PCE admixtures are assessed based on the analysis of phase-selective maps (here and throughout the paper, “phase” is used with the meaning of “crystallographic phase”) displaying the spatial distribution of selected crystallographic phases present in the cement paste at
oncrete is the world’s most widely used manufactured material and an essential commodity for modern societies. The binding matrix of concrete consists of a hardened cement paste, which develops upon hydration of cement. Cement hydration is the result of a sequence of dissolution− precipitation reactions, by which the combination of phases present in the cement powder react with water to give a series of hydrated reaction products.1 As a result of hydration, cement is converted from a viscous suspension to a rigid solid. The time interval during which stiffening of the cement slurry occurs is defined as “setting”. From a microscopic point of view, setting occurs when percolation of the isolated particles is achieved. As hydration further proceeds, mechanical strength is developed. The need for designing high performance cement binders for innovative, durable, and sustainable building solutions requires appropriate knowledge of the fundamental processes of cement hydration and how these are modified by the use of admixtures, which are universally employed to control and enhance the rheological and mechanical properties of cementitious materials, with the aim of obtaining high performance concrete structures.2 Polycarboxylate ether (PCE) superplasticizers are an established important class of chemical admixtures, used to produce low-porosity binders characterized by an optimal workability of the cement paste and high strength of the final product. PCEs are comb copolymers consisting of a negatively © XXXX American Chemical Society
Received: October 1, 2014 Revised: November 4, 2014
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relationship of, e.g., the sites in which cement has dissolved and C−S−H has precipitated, in the time interval Δt, can be directly visualized (Figure 2).
different hydration times (Figure 1). The phase-resolved maps obtained by synchrotron X-ray diffraction-enhanced computed
Figure 1. Phase maps displaying the space distribution of the unhydrated cement particles, C−S−H and the combination of the two (unhydrated particles in red and C−S−H in green) within a virtual slice through an ordinary portland cement (OPC) paste sample hydrating in pure water and in the presence of a PCE superplasticizer. The color intensity is proportional to the volume fraction of the given phases in each voxel. The white circle represents the enclosing glass capillary (internal diameter 400 μm) and is added as a visualization aid.
microtomography (XRD-CT) represent virtual slices through a sample of a cement paste hydrating within a capillary, composed by an m × n grid of voxels having a size of 4 μm. The voxel size is essentially controlled by the size of the incident X-ray beam. Each voxel has a value 0 ≤ Gi (t, x, y) ≤ 1, corresponding to the volume fraction of the selected phase i at the voxel location (x, y) and time t since the beginning of hydration. More than one phase can be present within a single voxel, with 0 ≤ ∑Ni=1 Gi (t, x, y) ≤ 1, where N is the number of detected phases. By means of XRD-CT, the different phases present in the hydrating cement paste are unambiguously discriminated based on their characteristic Bragg peaks, rather than on X-ray attenuation, as in conventional tomography. The combination of phase maps relative to different phases gives a multiphase map (e.g., third column in Figure 1) by which the spatial relationships among different phases at a given time can be visually interpreted and then analyzed. When phase selective maps are available for exactly the same slice in the sample at different times, it is in fact possible to analyze quantitatively the evolution of the nucleation and growth sites of a single phase (i) in time, by calculating a “difference phase map” GΔi (tΔ, x, y) = |Gi (t2, x, y) − Gi (t1, x, y)| obtained by subtraction of the phase maps collected at times t1 and t2. The combination of difference maps relative to diverse phases gives a “multiphase difference map” by which the spatial
Figure 2. Difference GΔi maps for OPC cement with PCE, displaying the locations where dissolution of the unhydrated cement particles and precipitation of C−S−H occur, and their combination (cement dissolution in red and C−S−H precipitation in green) in the time interval 7−17 h. The color intensity is proportional to the amount of phases precipitating or dissolving in the considered time interval. The white circle marks the perimeter of the capillary, and it is only added as a visualization aid.
In the case of Portland cement, the reactions of interest are (i) the dissolution of the starting unhydrous phases (tricalcium silicate: Ca3SiO5; dicalcium silicate: Ca2SiO4; tricalcium aluminate: Ca3Al2O6; calcium aluminoferrite: Ca4Al2Fe2O10), (ii) the dissolution of gypsum, added to delay setting, (iii) the precipitation of the hydration products.8 The main hydration product of ordinary Portland cement is a calcium-silicate hydrate (commonly referred to as C−S−H) having a pseudotobermoritic crystal chemistry. C−S−H is a poorly crystalline, nanoscale phase, formed from the reaction of the calcium-silicate phases with water. Its colloidal-like behavior B
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and self-organization into an interconnected network conveys cohesive properties to the cement paste.9−11 The other hydration products are Ca(OH)2 (portlandite) and an hydrous sulfo-aluminate (ettringite). Ettringite has a major influence on the early stage rheology, but it has a negligible effect in determining the mechanical properties of the final product. The generally accepted model for C−S−H formation is based on the heterogeneous nucleation of C−S−H nuclei onto the surface of the dissolving cement particles.9 The branching outward of the C−S−H aggregates forms a percolated network and induces the development of mechanical properties in the hardened material.12−15 Any chemical or physical modification of the starting mixture induces substantial changes in the hydration reactions and consequently significant modifications of the microstructure of the hydrating cement paste, originating from the modified spatial relationships among the phases. For instance, the addition of nucleation seeds, especially in the form of synthetic C−S−H nanoparticles, seems to trigger the autocatalytic precipitation of C−S−H throughout the pore solution.16−18 Changes in both habit and nucleation density of the product phases have also been observed in cement pastes hydrating in the presence of superplasticizing polymers.14,15 Here, the precipitation of C−S−H in ordinary Portland cement with and without the presence of a PCE superplasticizer is assessed quantitatively by the analysis of XRD-CT maps acquired at different times. The phase maps displayed in Figures 1 and 2 are analyzed quantitatively by means of radial distribution functions19 (RDF). The RDF is obtained as the ratio of the average value of the C−S−H voxels calculated at a given distance r from the unhydrated cement particles, taken as reference points, and the average of the value of all the C−S−H voxels within a distance r = rmax from the reference points. The deviation of the value of RDF from unity provides a measure of the degree of spatial correlation between phases, here between C−S−H and unhydrated cement particles. Therefore, values of RDF larger than unity at small r values indicate that C−S−H precipitation occurred in proximity of the dissolving particle surfaces, whereas values of RDF close to unity in the whole probed r range are indicative of C−S−H precipitation occurring randomly throughout the available pore space. The results show that at any given time the C−S−H precipitating in the sample hydrated in the absence of superplasticizer is correlated with the unhydrated cement particles; that is, C−S−H is mainly located in proximity to the cement particle surfaces. Here we show that the distribution of C−S−H in the cement paste is still biased by surface nucleation effects even at long maturation times after nucleation (7 days, Figure 3a). On the other hand, the C−S−H difference map in the cement sample hydrated in the presence of superplasticizer shows no spatial correlation with the unhydrated particle surfaces, both at long (7 days, Figure 3b) and short (7−17 h, Figure 3c) hydration times. The remarkable spatial correlation between C−S−H and unhydrated cement particle surfaces indicates conclusively that, in the absence of PCE superplasticizers, C−S−H forms by a process of heterogeneous nucleation, on the surface of the dissolving cement particles. On the other hand, the lack of significant spatial correlation between C−S−H and the surface of unhydrated particles, when PCE is added to the system, indicates that C−S−H precipitates randomly throughout the available space in the paste.
Figure 3. Radial distribution functions relative to the spatial distribution of C−S−H with respect to the position of the unhydrated particle surfaces, as calculated for: (a) the phase maps of the OPC sample without PCE, at 7 days of hydration; (b) the phase maps of the OPC sample with PCE, at 7 days of hydration; (c) the difference phase map of the OPC sample with PCE at shorter times (7−17 h).
On the basis of the evidence that, when PCE is added to the system, the spatial distribution of C−S−H has no correlation with the unhydrated cement particles, it is suggested that a change in nucleation mechanism, from heterogeneous to homogeneous, occurs in the case of cement hydrating in the presence of PCE superplasticizers. A similar observation on the distribution of hydrates (ettringite) in the presence of PCE has been observed using cryo-microscopy.20 Also, results of ultracentrifugation experiments and analysis of the organic content of cement pastes hydrated in the presence of superplasticizers revealed the presence of C−S−H in the aqueous phase, when superplasticizers were added to the system. These results were interpreted as a switch to homogeneous nucleation, or even heterogeneous nucleation on the superplasticizer particles.21 A switch to homogeneous nucleation may be due to the inhibiting effect on C−S−H heterogeneous nucleation of the PCE adsorbed onto the surface of the cement particles. The higher degree of supersaturation required for C−S−H homogeneous nucleation to occur, compared to heterogeneous nucleation,9,22 may contribute to slowing the rate of dissolution down, by reducing the driving force for the dissolution of the C
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(14) Ridi, F.; Fratini, E.; Baglioni, P. J. Colloid Interface Sci. 2011, 357, 255−264. (15) Masoero, E.; Del Gado, E.; Pellenq, R. J. M.; Ulm, F. J.; Yip, S. Phys. Rev. Lett. 2012, 109, 155503. (16) Thomas, J. J.; Jennings, H. H.; Chen, J. J. J. Phys. Chem. C 2009, 113, 4327−4334. (17) Nicoleau, L. Transp. Res. Rec. 2010, 2142, 42−51. (18) Artioli, G.; Valentini, L.; Dalconi, M. C.; Parisatto, M.; Voltolini, M.; Russo, V.; Ferrari, G. Int. J. Mater. Res. 2014, 105, 628−631. (19) Torquato, S. Random Heterogeneous Materials; Springer, New York, 2002; p 701. (20) Zingg, A.; Holzer, L.; Kaech, A.; Winnefeld, F.; Pakusch, J.; Becker, S.; Gauckler, L. Cem. Concr. Res. 2008, 38, 522−529. (21) Sowoidnich, T.; Rößler, C.; Ludwig, H. M.; Völkel, A.Proceedings of the 10th International Conference on Superplasticizers and Other Chemical Admixtures in Concrete; Prague, Czech Republic, October 28−31, 2012, pp 418−425 (22) Garrault, S.; Finot, E.; Lesniewska, E.; Nonat, A. Mater. Struct. 2005, 38, 435−442.
calcium-silicate phases. Specific interaction between C−S−H and PCE occurring in the pore solution may also inhibit C−S− H nucleation and, subsequently, further reduce the overall rate of hydration. Further experiments, specifically designed for the assessment of C−S−H nucleation in the presence of PCE superplasticizers, will likely contribute to clarifying the above points. The reported experiments prove that XRD-CT provides a powerful noninvasive method for the characterization of the microstructure of complex polyphasic materials, such as the hydrating cement pastes. These findings represent a step forward in the understanding of the interaction between cement and organic polymers, which is of fundamental importance for the design of more efficient chemical admixtures in the building industry.
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ASSOCIATED CONTENT
S Supporting Information *
A detailed description of experimental measurements at ESRF ID22 and data analysis procedures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel. 00390498279169. Fax: 00390498279134. E-mail: luca.
[email protected]. Web: http://geo.geoscienze.unipd.it/ personal/Valentini%20Luca. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Mapei S.p.A. supports the research through the Mapei-UNIPD agreement.
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ACKNOWLEDGMENTS ESRF is acknowledged for beam time through the Long Term project MA-1063. Rémi Tucoulou greatly helped during data collection at ESRF.
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