A reaction-diffusion model for the hydration/setting of cement

We propose a heterogeneous reaction-diffusion model for the hydration and setting of cement. The model is based on diffusional ion transport and on cement specific chemical dissolution/precipitation reactions under spatial heterogeneous solid/liquid conditions. We simulate the spatial and temporal evolution of precipitated micro structures starting from initial random configurations of anhydrous cement particles. Though the simulations have been performed for two dimensional systems, we are able to reproduce qualitatively basic features of the cement hydration problem. The proposed model is also applicable to general water/mineral systems.Comment: REVTeX (12 pages), 4 postscript figures, tarred, gzipped, uuencoded using `uufiles', coming with separate file(s). Figure 1 consists of 6 color plates; if you have no color printer try to send it to a black&white postscript-plotte

On the global hydration kinetics of tricalcium silicate cement

We reconsider a number of measurements for the overall hydration kinetics of tricalcium silicate pastes having an initial water to cement weight ratio close to 0.5. We find that the time dependent ratio of hydrated and unhydrated silica mole numbers can be well characterized by two power-laws in time, $x/(1-x)\sim (t/t_x)^\psi$. For early times $t < t_x$ we find an `accelerated' hydration ($\psi = 5/2$) and for later times $t > t_x$ a `deaccelerated' behavior ($\psi = 1/2$). The crossover time is estimated as $t_x \approx 16 hours$. We interpret these results in terms of a global second order rate equation indicating that (a) hydrates catalyse the hydration process for $t<t_x$, (b) they inhibit further hydration for $t > t_x$ and (c) the value of the associated second order rate constant is of magnitude 6x10^{-7} - 7x10^{-6} liter mol^{-1} s^{-1}. We argue, by considering the hydration process actually being furnished as a diffusion limited precipitation that the exponents $\psi = 5/2$ and $\psi = 1/2$ directly indicate a preferentially `plate' like hydrate microstructure. This is essentially in agreement with experimental observations of cellular hydrate microstructures for this class of materials.Comment: RevTeX macros, 6 pages, 4 postscript figure

Avalanches in Breakdown and Fracture Processes

We investigate the breakdown of disordered networks under the action of an increasing external---mechanical or electrical---force. We perform a mean-field analysis and estimate scaling exponents for the approach to the instability. By simulating two-dimensional models of electric breakdown and fracture we observe that the breakdown is preceded by avalanche events. The avalanches can be described by scaling laws, and the estimated values of the exponents are consistent with those found in mean-field theory. The breakdown point is characterized by a discontinuity in the macroscopic properties of the material, such as conductivity or elasticity, indicative of a first order transition. The scaling laws suggest an analogy with the behavior expected in spinodal nucleation.Comment: 15 pages, 12 figures, submitted to Phys. Rev. E, corrected typo in authors name, no changes to the pape

Flight Demonstration of Non-Cooperative Rendezvous using Optical Navigation

The ultimate goal of this work is to demonstrate the capability of a maneuverable servicer spacecraft to rendezvous with a non-cooperative space resident object from far-range distance using optical angles-only measurements. To this end, the Advanced Rendezvous experiment using GPS and Optical Navigation (ARGON) has been executed during the extended phase of the PRISMA formation-flying mission in April 2012. This paper addresses the experiment design, the developed flight dynamics system, the obtained flight results, and the lessons learned. Furthermore the evaluation of the rendezvous tracking, navigation and control accuracy is performed by means of GPS-based precise relative orbit determination products. The presented results give a clear demonstration of the high readiness level reached by key technologies which are needed by future on-orbit servicing and debris-removal missions