Experimental and Theoretical Study of Earth-Moist Concrete

No abstract

Analytical solutions for non-linear conversion of a porous solid particle in a gas–I. Isothermal conversion

Analytical description are presented for non-linear heterogeneous conversion of a porous solid particle reacting with a surrounding gas. Account has been taken of a reaction rate of general order with respect to gas concentration, intrinsic reaction surface area and pore diffusion, which change with solid conversion and external film transport. Results include expressions for the concentration distributions of the solid and gaseous reactant, the propagation velocity of the conversion zone inside the particle, the conversion time and the conversion rate. The complete analytical description of the non-linear conversion process is based on a combination of two asymptotic solutions. The asymptotic solutions are derived in closed form from the governing non-linear coupled partial differential equations pertaining to conservation of mass of solid and gaseous reactant, considering the limiting cases of a small and large Thiele modulus, respectively. For a small Thiele modulus, the solutions correspond to conversion dominated by reaction kinetics. For a large Thiele modulus, conversion is strongly influenced by internal and external transport processes and takes place in a narrow zone near the outer surface of the particle: solutions are derived by employing boundary layer theory. In Part II of this paper the analytical solutions are extended to non-isothermal conversion and are compared with results of numerical simulations

Analytical solutions for non-linear conversion of a porous solid particle in a gas–II. Non-isothermal conversion and numerical verification

In Part I, analytical solutions were given for the non-linear isothermal heterogeneous conversion of a porous solid particle. Account was taken of a reaction rate of general order with respect to the gas reactant, intrinsic reaction surface area and effective pore diffusion, which change with solid conversion and external film transport. In this part, the analytical solutions are extended to non-isothermal conversion. Analytical solutions for the particle overshoot temperature due to heat of reaction are derived from the governing differential equation pertaining to conservation of energy, considering the limiting cases of small and large Thiele moduli. The solutions are used to assess the effect of interaction between chemical reaction rate and particle overshoot temperature on particle conversion. The analytical solutions are shown to compare favourably with numerical simulation results

Prograde spin-up during gravitational collapse

Asteroids, planets, stars in some open clusters, as well as molecular clouds appear to possess a preferential spin-orbit alignment, pointing to shared processes that tie their rotation at birth to larger parent structures. We present a new mechanism that describes how collections of particles or 'clouds' gain a prograde rotational component when they collapse or contract while subject to an external, central force. The effect is geometric in origin, as relative shear on curved orbits moves their shared center-of-mass slightly inward and toward the external potential during a collapse, exchanging orbital angular momentum into aligned (prograde) rotation. We perform illustrative analytical and N-body calculations to show that this process of prograde spin-up proceeds quadratically in time ($\delta L_\mathrm{rot} \propto t^2$) until the collapse nears completion. The total rotational gain increases with the size of the cloud prior to its collapse: $\delta L_\mathrm{rot}/L_\mathrm{H} \propto (R_\mathrm{cl}/R_\mathrm{H})^5$, and typically with distance to the source of the potential ($L_\mathrm{H}\propto r_0)$. For clouds that form at the interface of shear and self-gravity ($R_\mathrm{cl} \sim R_\mathrm{H}$), prograde spin-up means that even setups with large initial retrograde rotation collapse to form prograde-spinning objects. Being a geometric effect, prograde spin-up persists around any central potential that triggers shear, even those where the shear is strongly retrograde. We highlight an application to the Solar System, where prograde spin-up can explain the frequency of binary objects in the Kuiper belt with prograde rotation.Comment: Accepted for publication in A&A. Co-first authors. Comments and questions welcom

Hydration kinetics study of class G oil-well cement and olivine nano-silica mixtures at 20–60 °C

In this study the heat evolution of standard density slurries (1.89 g/cm3) of Class G oil-well cement and olivine nano-silica additions (0.5–2.0 % bwoc), cured under different temperatures (20–60 °C) and atmospheric pressure, were examined by isothermal calorimetry. Under isothermal and isobaric conditions, the dependency of cement hydration kinetics on curing temperature is related to the activation energy of the cementing slurry. The estimated apparent activation energy of the different slurries with olivine nano-silica varies from 38 to 44 KJ/mol using a dynamic method, at the temperature range of 20–60 °C. It is demonstrated that the addition of olivine nano-silica increases the rate and the heat of hydration of oil-well slurries. These effects are temperature dependent. Finally, comparable hydration degrees were obtained between slurries containing 0.5 % bwoc of olivine nano-silica and 10 % bwoc of oil-well grade micro-silica (mS)

Effect of olivine nano-silica additions on cement based system

In this study, the influence of olivine nano-silica (OnS) additions in cement based systems has been addressed. The obtained results demonstrate that the addition of OnS (1.5–3.8 % bwoc) increases the viscosity, yield point and hydration degree of the cementitious systems, mainly due to the increase of the total specific surface area of the mix. This holds also for the case when a fixed amount of SP is applied. Based on the performed analysis, it is concluded that the OnS acts as an accelerating and pozzolanic agent in concrete

Water layer thickness of silica fines and their effect on the workability of cement pastes

Concrete is used in infrastructure and in buildings. It is composed of granular materials of different sizes and the grading of the composed solid mix covers a wide range. The overall grading of the mix, containing particles from 300 nm to 32 mm, determines the mix properties of the concrete. The properties in fresh state (flow properties and workability) are for instance governed by the particle size distribution (PSD) and the resulting particle packing (PP). One way to further improve the packing is to increase the solid size range, e.g. by including particles with sizes below 300 nm. Possible materials, which are currently available, are limestone and silica fines like silica fume (mS) and nano-silica (nS). This paper addresses the characterization of six different silica fines with respect to their application in cement paste. Given that the fines provide by far the highest percentage of specific surface area in a mix, their packing behavior and water demand is of vital interest for the design of concrete. In the present work, different mixes are compared and analyzed using the mini spread-flow test method. In this way, a deformation coefficient derived by the spread-flow test is confirmed to correlate with the product of computed specific surface area (SSA) based on measured PSD and intrinsic density of the individual silica fines. Similarly, correlations with equal accuracy are found with a computed SSA using the BET method. With the flow experiments of different mixes it is possible to derive an individual deformation coefficient of the silica particles. It is demonstrated that the computed and the BET surface area values have a constant ratio (0.76 to 0.70). Finally, the value of a constant water layer thickness around the powder particles (24.8 nm) is computed for all silica fines at the onset of flowing. This implies the possibility to predict the flow behavior of paste only based on the knowledge of their SSA, either determined by computation or by BET measurements

Realist Evaluation : an overview

This report summarises the discussions and presentations of the Expert Seminar ‘Realist Evaluation’ with Gill Westhorp, which took place in Wageningen on March 29, 2011. The Expert Seminar was organised by the Wageningen UR Centre for Development Innovation in collaboration with Learning by Design and Context, international cooperation