Proceedings of the International rilem conference Materials, Systems and Structures in Civil Engineering 2016
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25 International RILEM Conference on Materials, Systems and Structures in Civil Engineering Conference segment on Service Life of Cement-Based Materials and Structures 22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
mechanical interactions between particles (e.g. U 1rx ). The proposed scheme therefore combines chemistry and mechanics over the large timescales of chemical kinetics.
3. Results
The KMC approach presented here has been used to study the effect of the interaction potential and initial system configuration on the mechanism of growth-by-aggregation of the cement HP. Here we are interested only in the qualitative trends of rates as a function of time, and how these relate to the mechanisms of precipitation that are triggered. Converting the qualitative results into quantitative rates would require a model calibration to define the r 0 *
terms in Equation 1; this is left to another manuscript currently under review [21] The results for three scenarios are shown in Figure 2:
a)
very large in Equation 2, taken from ref. [18] where it was shown that such a large captures well the mechanics of the C—S—H; in this first case the interaction between cement surface particles and cement HP particles is taken to be as strong as the interaction between two HP particles; b)
particles is smaller than the that between two HP particles (25% of it to be precise) except for a small region in the middle of the cement surface where a full 100% of the HP-HP interaction is maintained; this small region serves as a preferential site for nucleation of the mesoscale HP domain; c)
[30].
Figure 2: Results from our simulations with different initial conditions (a) strong interactions between nanoparticles and homogeneous cement surface; (b) strong interaction and surface with one preferential site for HP particle formation; (c) weak interactions.
300 nm 26 International RILEM Conference on Materials, Systems and Structures in Civil Engineering Conference segment on Service Life of Cement-Based Materials and Structures 22-24 August 2016, Technical University of Denmark, Lyngby, Denmark
The snapshots in Figure 2 show that the first scenario leads to a layered growth of the HP domain, formed by aggregated nanoparticles. For this scenario we kept constant in time and very large, in order to avoid particle deletion. The layered growth results into a constant rate, very different from what is expected during early hydration (see simulation and experimental values in Figure). Notice that the size of the HP domain simulated here is in the same order as that observed experimentally during early hydration, i.e. ~500 nm, so it makes sense to aim for a rate curve that is similar to the one measured experimentally from calorimetry).
The second scenario, with the favourable nucleation site, gives a much better result, with a roughly hemispherical domain growing radially. The increasing surface of the hemispherical domain is at the origin of the acceleration in the rate curve. Indeed, in the KMC algorithm the total rate is the sum of all particle insertion rates, and an increasing surface of the HP domain implies that more trial nuclei can find a favourable spot that leads to a high rate. When the HP domain crosses the periodic boundary on the cement surface, the lateral impingement causes the rate peak, similar to the calorimetry experiments. The continuous decrease of rate after the peak is due to the fact that, for this simulation, we took a supersaturation that decreases with time, as known for the cement solution and as quantified in ref. [31].
Finally, the third scenario with much weaker strength of the interactions causes the mechanisms of HP formation to change from heterogeneous on the cement surface to homogeneous in the bulk. This also leads to a rate curve that agrees qualitatively with the experiments. In this case however the peak occurs when the space gets filled, which implies a quantitative dependence of the area under the rate curve (number of HP particles inserted) and the amount of water in the cement mix (viz. the size of the simulation box perpendicular to the cement surface). Such dependence would contradict the experiments [8], but the problem can be avoided assuming that the homogeneous nucleation is limited to a small region near the surface of the cement grain, as proposed in ref. [20]. This constraint might indeed originate from a gradient of supersaturation in the simulation box, sustained by a diffusive process of the ions in solution, which move from the cement surface toward the bulk solution. To explore this possibility, future work should couple the simulations presented here with the type or reaction-transport simulations mentioned in the introduction of this manuscript [7,8].
We have presented a new approach to simulate the precipitation of nanoparticle of cement hydrates from aqueous solution and their aggregation to form mesoscale domains. The approach is based on new coarse-grained expressions for the rate of particle insertion and deletion, which are derived from the theories of classical nucleation and crystal growth. The rate expression involve one kinetic parameter r 0 *
to measure. r 0 * determines a linear scaling of time, which means that even if it is not quantified, all the nonlinearities in the rate of HP formation obtained from the simulations are still a true reflection of the predicted mesoscale mechanism only. Another important feature of our coarse-grained rates is that they combine chemistry and the mechanical interactions between nanoparticles, creating new opportunities to study technologically important phenomena where chemistry and mechanics play together, e.g. crystallization pressure and Yüklə 8,6 Mb. Dostları ilə paylaş:
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