The early age volume changes occurring during the geopolymerisation reaction are not sufficiently understood yet, due to shortage of experimental data and theoretical models. This work presents new results on chemical and autogenous deformation of sodium-activated geopolymers from metakaolin, focussing on the first 72 h of reaction. The results show that the geopolymers undergo early-age chemical expansion, not shrinkage. A model is proposed to explain the experimental result, leveraging recent advances from molecular simulations. The model predicts how the extent of chemical expansion is controlled by confined water in the molecular structure of the geopolymer. However, despite this underlying chemical expansion, geopolymer samples undergo autogenous shrinkage at the macroscale, which excludes self-desiccation as the origin of autogenous deformation. A better insight is gained by monitoring the kinetics of geopolymerisation using isothermal and differential calorimetry. Two kinetic regimes are identified, with apparent activation energies of approximately 90 kJ/mol and 70 kJ/mol. This suggests that two microscopic mechanisms concur to determine the early-age volume changes of geopolymer pastes.

Understanding the effects of high temperature (HT) and high pressure (HP) conditions on the microstructure of cement-based materials is critical to the construction and safe operation of deep oil and gas wells. Under such conditions, the persistence of calcium-silicate-hydrate (C-S-H) gel is compromised by ongoing crystallization that, if not controlled, may adversely affect the durability of the cement sheath. This work investigates the effect of silica content > 35% by-weight-of-cement (BWOC), silica partide size, and solid volume fraction (SVF) on the microstructure and phase composition of cement-silica blends cured hydrothermally at 200 degrees C and 20.7 MPa. The results of X-ray diffraction and electron microprobe analysis revealed significant impact of these three mix design parameters on the final phase assembly, and on the conversion rate of semi-crystalline C-S-H to gyrolite and 11 angstrom tobermorite. Incorporation of more fine siliceous material suppressed dissolution of coarse silica particles, resulting in a matrix with improved homogeneity and dominated by fine gel pores. Mixes with lower SVF showed greater formation of 11 angstrom tobermorite, a higher degree of crystallinity and/or greater crystallite size. Prolonged HTHP curing of all systems (up to three months in this study), irrespective of the initial SW, increased the fraction of capillary pores, indicating void coalesce caused by crystal growth. However, we find that this coarsening is less pronounced in systems with less pore space available for crystallization.

Mean random vitrinite reflectance (R-o) is the most widely accepted method to determine thermal maturity of coal and other sedimentary rocks. However, oil-immersion R-o of polished rock or kerogen samples is commonly lower than R-o values measured in samples from adjacent vitrinite-rich coals that have undergone the same level of thermal stress. So-called suppressed R-o values have also been observed in hydrous pyrolysis experiments designed to simulate petroleum formation. Various hypotheses to explain R-o suppression, such as sorption of products generated from liptinite during maturation, diagenetic formation of perhydrous vitrinite or overpressure, remain controversial. To experimentally test for suppression of vitrinite reflectance, artificial rock was prepared using silica and a calcined blend of limestone and clay with various proportions of thermally immature vitrinite-rich Wyodak-Anderson coal and liptinite-rich kerogen isolated from the oil-prone Parachute Creek Member of the Green River Formation. The samples were subjected to hydrous pyrolysis for 72 h. at isothermal temperatures of 300 degrees C, 330 degrees C, and 350 degrees C to simulate burial maturation. Compared to artificial rock that contains only coal, samples with different proportions of oil-prone kerogen show distinct suppression of calibrated R-o at 300 degrees C and 330 degrees C. The reflectance of solid bitumen generated during heating of the samples is lower than that of the associated vitrinite and does not interfere with the R-o measurements. These results provide the first experimental evidence that R-o suppression occurs in vitrinite mixed with liptinite-rich kerogen in a rock matrix. Although the precise chemical mechanism for R-o suppression by liptinite remains unclear, free radicals generated from solid bitumen and associated volatile products during maturation of liptinite may contribute to termination reactions that slow the aromatization and rearrangement of polyaromatic sheets in vitrinite, thus suppressing R-o. This mechanism does not preclude Ro suppression that might result from overpressure or differences in redox conditions during diagenesis. (C) 2018 Elsevier Ltd. All rights reserved.

The activation energy for hydration of β–C2S paste was measured as a function of hydration time using a calorimetric method and was found to depend on the surface area and reactivity of the powder as well as on the addition of sodium silicate. For neat paste made with standard β–C2S (similar to that found in portland cement), the activation energy is approximately 32kJ/mol and is constant with time. For neat paste made with reactive β–C2S (calcined at lower temperature and with high surface area), the activation energy is about 55kJ/mol and is also constant with time. This large difference in activation energy reflects a difference in the rate-controlling step for hydration. After investigating the effects of sodium silicate and synthetic calcium–silicate–hydrate on the kinetics, we hypothesize that the lower activation energy represents C2S dissolution, while the higher value represents nucleation and growth of hydration product.

Simulations of tricalcium silicate (C3S) hydration using a kinetic cellular automaton program, HydratiCA, indicate that the net rate depends both on C3S dissolution and on hydration product growth. Neither process can be considered the sole rate-controlling step because the solution remains significantly undersaturated with respect to C3S yet significantly supersaturated with respect to calcium silicate hydrate (C–S–H). The reaction rate peak is attributed to increasing coverage of C3S by C–S–H, which reduces both the dissolution rate and the supersaturation of C–S–H. This supersaturation dependence is included in a generalized boundary nucleation and growth model to describe the kinetics without requiring significant impingement of products on separate cement grains. The latter point explains the observation that paste hydration rates are insensitive to water/cement ratio. The simulations indicate that the product layer on C3S remains permeable; no transition to diffusion control is indicated, even long after the rate peak.

Cement paste has a complex distribution of pores and molecular-scale spaces. This distribution controls the hysteresis of water sorption isotherms and associated bulk dimensional changes (shrinkage). We focus on two locations of evaporable water within the fine structure of pastes, each having unique properties, and we present applied physics models that capture the hysteresis by dividing drying and rewetting into two related regimes based on relative humidity (RH). We show that a continuum model, incorporating a poreblocking mechanism for desorption and equilibrium thermodynamics for adsorption, explains well the sorption hysteresis for a paste that remains above approximately 20% RH. In addition, we show with molecular models and experiments that water in spaces of ≲1 nm width evaporates below approximately 20% RH but reenters throughout the entire RH range. This water is responsible for a drying shrinkage hysteresis similar to that of clays but opposite in direction to typical mesoporous glass. Combining the models of these two regimes allows the entire drying and rewetting hysteresis to be reproduced accurately and provides parameters to predict the corresponding dimensional changes. The resulting model can improve the engineering predictions of long-term drying shrinkage accounting also for the history dependence of strain induced by hysteresis. Alternative strategies for quantitative analyses of the

Shrinkage can be critical for the strength and durability of drying cement pastes. Shrinkage becomes particularly severe at very low relative humidity, < 2 0 %, which can be met in some activities involving extreme temperatures. Experiments and simulations suggest that small pores in the cement paste, with approximate thickness <= 1 nm, stay saturated unless the humidity drops below 20%. Here we suggest that this pore size can define two different categories of pores in the paste: pores thicker than 1 nm, where the Kelvin's equation and the corresponding capillary (Laplace) pressure apply, and pores thinner than 1 nm, which can be considered as part of the solid skeleton if the humidity stays above 20%. We show that a continuum model, incorporating a pore-blocking mechanism for desorption and equilibrium thermodynamics for adsorption, explains well the sorption hysteresis for a paste that remains above similar to 2 0 %. At lower humidities, we assume that (1) during adsorpion water re-enters the smallest pores throughout the entire RH range (supported by experiments and simulations) and (2) there exists a simple linear relationship between water and strain in the smallest pores. These minimal assumptions are sufficient to explain the low-humidity hysteresis of water content and strain, but the underlying mechanistic explanation is still an open question. Combining the low-humidity and high-humidity models allows capturing the entire drying and rewetting hysteresis, and provides parameters to predict the corresponding dimensional changes.

Small angle neutron scattering (SANS) is an attractive technique for characterizing the structure of shale at length scales less than 100 nm that are relevant to the storage and transport of hydrocarbons. The fine structure of hydrocarbon-bearing shale is in large part governed by the mass fraction and thermal maturity of the immobile organic phase known as kerogen. To correctly interpret and normalize SANS data requires knowing the neutron scattering length density (SLD) of the individual phases in the material; for shales this includes mineral, kerogen, and pore phases. While the SLD of many phases can be estimated with reasonable accuracy from compositional data, it is not clear that this is the case for kerogen, as its SLD is expected to vary considerably with type and thermal maturity. To characterize the importance of this variation, we measured the SLD of pure kerogen samples that were separated from the host shale by acid demineralization. By varying the kerogen type (I, II or III) and the associated thermal maturity, samples with a broad range of hydrogen to carbon atomic ratios, 0.5 < H/C < 1.4 were obtained. The SANS measurements consisted of using deuterated methanol exchange to vary the SLD of the pore space in contact with the kerogen, which allowed the SLD of the solid (kerogen) phase to be directly determined. The SLD of the kerogen was also calculated from the measured elemental composition and mass density. We find that the SLD of kerogen varies over a rather large range, from 1 to 4 x 10(14) m (2), increasing significantly with decreasing H/C, and that the measured and calculated values are in fairly good agreement. An implication of our results is that for shales containing immature kerogen with an SLD close to 1, the scattering between the kerogen and mineral phases may be significant, such that the shale cannot be treated as a two-phase material of solid and pore. Moreover, the surface area of extracted kerogen does not increase with thermal maturity. This result is in contrast to the contemporary literature which clearly indicates that catagenesis leaves nanopores in kerogen. Therefore we conclude that the acid demineralization procedure alters the structure of kerogen. (C) 2013 Elsevier Ltd. All rights reserved.

The hydration kinetics of tricalcium silicate (C3S) has been the subject of much study, yet the experimentally observed effects of the water‐to‐cement (w/c) ratio and particle size distribution have been difficult to explain with models. Here, we propose a simple hypothesis that provides an explanation of the lack of any significant effect of w/c on the kinetics and for the strong effect of the particle size distribution on the amount of early hydration associated with the main hydration peak. The hypothesis is that during the early hydration period the calcium–silicate–hydrate product forms only in a reaction zone close to the surface of the C3S particles. To test the hypothesis, a new microstructure‐based kinetics (MBK) model has been developed. The MBK model treats the C3S particle size distribution in a statistical way to save computation time and treats the early hydration as essentially a boundary nucleation and growth process. The MBK model is used to fit kinetic data from two published studies for C3S with different size distributions, one for alite (impure C3S) pastes and one for stirred C3S suspensions. The model is able to fit all the data sets with parameters that show no significant trend with particle size, providing support for the reaction zone hypothesis.

Abstract Expansion of oilwell cement after placement in the annulus is a promising route to improve wellbore sealing. Expansion can potentially provide benefits such as: closing a microannulus, reducing the tendency for cracking/debonding, and improving cement logging evaluation. To properly evaluate the performance of expanding cement formulations in a laboratory setting, the cement should be hydrated under confined conditions similar to a wellbore. We conducted measurements using a new confinement cell developed in our laboratory. The cell provides radial confinement and elevated temperature as in a wellbore. The availability of water to the cement can also be controlled to simulate tight or porous formations. In the axial direction the cement sample contacts a piston that either confines the sample while measuring the axial stress development, or permits linear expansion that is measured. Using blends of oilwell cement with magnesium oxide (MgO), a standard expanding additive, we measured the axial stress development and linear expansion under different curing conditions. To better understand and optimize cement expansion we also characterized the hydration kinetics of the MgO additive alone and in the presence of the cement, using isothermal calorimetry. Finally, we measured the acoustic response of cement formulations with and without a commercial expanding agent with a standard ultrasonic pulseecho technique. This testing confirmed the benefits of expansion for the logging response.

With ever more challenging (T,p) environments for cementing applications in oil and gas wells, there is a need to identify the fundamental mechanisms of fracture resistant oil well cements. We report results from a multi-technique investigation of behavior and properties of API class G cement and silica-enriched cement systems subjected to hydrothermal curing from 30°C to 200°C; including electron probe microanalysis, X-ray diffraction, thermogravimetry analysis, electron microscopy, neutron scattering (SANS), and fracture scratch testing. The results provide a new insight into the link between system chemistry, micro-texture and micro-fracture toughness. We suggest that the strong correlation found between chemically modulated specific surface and fracture resistance can explain the drop in fracture properties of neat oil-well cements at elevated temperatures; the fracture property enhancement in silica-rich cement systems, between 110° and 175°C; and the drop in fracture properties of such systems through prolonged curing over 1year at 200°C.

Abstruct The authors have proposed a new method, epoxy-coating on MIP (Mercury Intrusion Porosimetry) sample, to measure threshold pore radius of concrete to obtain an indicator of pore structure which has correlation with air and water permeability. In this paper, first, the validity of the above method was studied through observation on splitting surface of samples after MIP analysis and comparison with obtained threshold pore radius and permeability. Results showed that the proposed method is suitable to extract threshold pore radius, and it showed good correlation with water permeability and, if concrete is enough dried, air permeability. Good correlation was found even on samples prepared with overseas concrete material and core samples taken from existing structures overseas. The above results indicate that pore structure governs both air and water permeability of concrete and that threshold pore radius can be an indicator of the permeability of concrete.

This paper describes a novel reactive composite material comprised of hydrogenated nitrile butadiene rubber (HNBR) compounded with slag cement. The composite initially looks and behaves like rubber, but when exposed to water it simultaneously swells and stiffens due to hydration of the cement component. The material eventually reaches a stiffness that is intermediate between that of HNBR and hydrated cement, while maintaining a relatively large ductility that is more characteristic of rubber. This behavior, which is ideal for sealing applications, differentiates this material from conventional swellable materials that become less stiff upon swelling. The development of this new type of material was motivated by the requirements of oilfield zonal isolation, where alternatives to cement are needed for some challenging sealing applications. A mechanism for the swelling and stiffening of the reactive composite is proposed: water diffuses into the HNBR matrix and is converted to bound water through hydration reactions with the cement, causing the effective solid filler content of the composite to increase. A model is proposed that treats the composite as a cellular solid with a continuous filler phase (hydrated cement). This model is able to reproduce the observed increase in the elastic modulus with time during exposure to water.

The kinetics of hydration of magnesium oxide (MgO) powder to form magnesium hydroxide (Mg(OH)2) were measured using isothermal calorimetry at different temperatures, and the morphology of the powders before and after hydration were examined. The hydration kinetics of light‐burned MgO exhibit a hydration rate peak similar to that of portland cement hydration, whereas the hydration kinetics of hard‐burned MgO are comparatively slower at the same temperature, and exhibit a continuously declining hydration rate after the first several minutes of reaction. The hydration kinetics of both light‐burned and hard‐burned MgO can be fit using a boundary nucleation and growth model that has previously been applied to the hydration of portland cement and tricalcium silicate. Activation energy values for MgO hydration were determined from the fitted rate constants and were also measured directly using small temperature excursions according to a recently proposed method. For light‐burned MgO the resulting values are in good agreement and indicate a value of 77 kJ/mol. For the hard‐burned MgO the activation energy values vary considerably depending on temperature and how the activation energy is measured, but are always lower than the value obtained for the light‐burned MgO.

Cement paste has a complex mesoscale structure, and small changes in its pore network potentially cause large variation in measurements such as the water isotherm (also nitrogen). We deconvolute the water isotherm with the help of advanced computational techniques, hypotheses, and a re-examination of published data. The pore system is divided into four different categories, each containing water with its own physical properties. By viewing the highly interdependent roles of water in each of the pore categories as a system, new insights are gained regarding possible mechanisms that control drying, shrinkage, and creep, and experimental strategies for verification.