Jeffrey J. Thomas

GCP (Saint-Gobain)

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 sho...

Mean random vitrinite reflectance (Ro) is the most widely accepted method to determine thermal maturity of coal and other sedimentary rocks. However, oil-immersion Ro of polished rock or kerogen samples is commonly lower than Ro values measured in samples from adjacent vitrinite-rich coals that have undergone the same level of thermal stress. So-ca...

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 contr...

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...

Shrinkage can be critical for the strength and durability of drying cement pastes. Shrinkage becomes particularly severe at very low relative humidity, < 20%, 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...

The nanopore network in cement paste controls the water-sorption and shrinkage hysteresis. We show that the water-induced stresses in pores larger or smaller than ~1 nm can be treated separately and additively. Below ~1nm, molecular models and experiments indicate that water evaporates only below ~20% RH, causing clay-like hysteresis. Above ~1 nm,...

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 model...

An unpublished paper describing a method of preparing samples of homogeneous, single phase C-S-H by rapidly and efficiently decalcifying hydrated C3S paste. Using this method, C-S-H with Ca/Si molar ratios in the range of about 0.8 to 1.7 can be formed. The structure of the C-S-H formed by this method is believed to be more similar to that of C–S–H...

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 lab...

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...

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 o...

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...

An apparatus has been designed to study the volumetric changes under controlled curing conditions. The apparatus is capable of measuring the chemical shrinkage as well as the macroscopic volumetric changes of sealed and saturated specimens under controlled pressure and temperature. Specifically, when measuring separately the macroscopic deformation...

Hydration of portland cement is the cornerstone of the process responsible for microstructure development in concrete and ultimately controls the kinetics of all materials properties that make concrete such a useful product for society (properties such as setting, strength, permeability, and durability). A comprehensive understanding of, and model...

A new calorimetry‐based method for calculating the activation energy of cement hydration is described. The method requires a scanning calorimeter that can change the sample temperature relatively quickly and has a stable baseline, but the technique is straightforward to perform. Activation energy values can be calculated at multiple hydration times...

With the search for hydrocarbons moving to more extreme environments, includingdeepwater, one of the challenges associated with cementing is ensuring thelong-term integrity and mechanical properties of the cement at hightemperatures (HT). To avoid strength retrogression at temperatures above about 110°C, silicais added to the cement. This makes the...

The permeability and elastic modulus of mature cement paste cured at temperatures between 8 degrees C and 60 degrees C were measured using a previously described beam bending method. The permeability increases by two orders of magnitude over this range, with most of the increase occurring when the curing temperature increases from 40 degrees C to 6...

Alkali-activated slag (AAS) paste was analyzed using small-angle neutron scattering (SANS). The scattering response indicates that the microstructure consists of a uniform matrix of hydration product with a high surface area studded with unhydrated cores of slag particles. In contrast with portland cement paste, no surface fractal scattering regime...

A new high-performance material known as frangible concrete was developed with the goal of eliminating the potential for lethal-sized fragments of broken concrete when a structure is impacted by a dynamic load, such as the blast force from a terrorist vehicle bomb. Based on a strategy of developing concrete with moderate, quasi-static, load-bearing...

The current state of knowledge of cement hydration mechanisms is reviewed, including the origin of the period of slow reaction in alite and cement, the nature of the acceleration period, the role of calcium sulfate in modifying the reaction rate of tricalcium aluminate, the interactions of silicates and aluminates, and the kinetics of the decelerat...

Efforts to model and simulate the highly complex cementhydration process over the past 40 years are reviewed, covering different modeling approaches such as single particle models, mathematical nucleation and growth models, and vector and lattice-based approaches to simulating microstructuredevelopment. Particular attention is given to promising de...

Addition of pure calcium silicate hydrate (C–S–H) to alkali-activated slag (AAS) paste resulted in an earlier and larger hydration rate peak measured with isothermal calorimetry and a much higher compressive strength after 1d of curing. This is attributed to a nucleation seeding effect, as was previously established for Portland cement and tricalci...

A recently published constitutive model for drying of a partially saturated porous material is extended to take into account finite air (gas) pressure as well as finite external load, variables that are absent during simple drying under atmospheric conditions. We further use the result to derive a general form of effective stress in a partially sat...

The use of microwave heating for the processing of reaction-bonded silicon nitride offers at least two advantages over conventional heating methods. First, the reaction can be made to proceed preferentially in the interior of the reacting compact by maximizing the temperature gradient resulting from microwave heating. This helps maintain the flow o...

Silicon compacts nitrided utilizing the temperature gradient inherent to microwave heating were more fully converted to silicon nitride than was possible with similar compacts nitrided isothermally. Although nitrogen depletion prevented the reaction rate in the center from exceeding that at the surface, the temperature gradient partially counteract...

Relationships between composition, mass density, and atomic packing density for CaO−SiO2−H2O (C−S−H), the main hydration product of cement, and its mineral analogues tobermorite and jennite, are examined. A graphical approach, similar to a phase diagram, is used to display the variation in density as a function of water content. In order to provide...

A laser based ultrasonic technique is explored for predicting the mechanical integrity and durability of cementitious materials. It is well known that these properties depend on the mechanical properties, and structural properties including porosity and permeability. The technique used involves the measurement of the phase velocity of fast and slow...

Microstructure development and the kinetics of hydration of pure tricalcium silicate (C3S) and CaCl2-accelerated C3S pastes were investigated by performing isothermal calorimetry and in situ small-angle neutron scattering (SANS) measurements on parallel specimens during the first few days of hydration, as well as on 28-day old specimens hydrated un...

The fundamental chemical hydration process of portland cement and its main mineral component, tricalcium silicate, was studied by investigating the effects of various additives. A relatively small amount (1-4 wt %) of well-dispersed calcium silicate hydrate (C-S-H), a pure form of the main hydration product, significantly increases both the early h...

A new constitutive model for drying of an elastic porous material is presented. The model is derived by decomposing the boundary value problem of drying into parts for which an explicit solution is readily available. The mathematical form differs from models developed using either the concept of equivalent pore pressure or average pore pressure. In...

The effects of drying to various relative humidity (RH) levels on the internal structure of hydrated cement paste were investigated using small-angle neutron scattering (SANS). Specimens of young and mature portland cement paste were analyzed in the initial saturated state, in the dried state, and then again after resaturation, allowing reversible...

Cement-based materials have complex multi-component, multiscale structures that first form through chemical reaction and then continue to change with time. As with most classes of materials, the porosity of cement paste strongly influences its properties, including strength, shrinkage, creep, permeability and diffusion. Pores in cement paste range...

Cement-based materials have complex multi-component, multiscale structures that first form through chemical reaction and then continue to change with time. As with most classes of materials, the porosity of cement paste strongly influ- ences its properties, including strength, shrinkage, creep, permeability and diffusion. Pores in cement paste rang...

The hydration kinetics of tricalcium silicate (C3S), the main constituent of portland cement, were analyzed with a mathematical “boundary nucleation” model in which nucleation of the hydration product occurs only on internal boundaries corresponding to the C3S particle surfaces. This model more closely approximates the C3S hydration process than do...

A reply to discussion of the paper written by Hamlin M. Jennings, Jeffrey J. Thomas, Julia S. Gevrenov, Georgios Constantinides, and Franz-Josef Ulm on 'a multi-technique investigation of the nanoporosity of cement paste' done by Beaudoin and Alizadeh is presented. The authors are correct in noting that the water sorption length change isotherms pu...

Although Portland cement concrete is the world's most widely used manufactured material, basic questions persist regarding its internal structure and water content, and their effect on concrete behaviour. Here, for the first time without recourse to drying methods, we measure the composition and solid density of the principal binding reaction produ...

The nanometer-scale structure of cement paste, which is dominated by the colloidal-scale porosity within the C-S-H gel phase, has a controlling effect on concrete properties but is difficult to study due to its delicate structure and lack of long-range order. Here we present results from three experimental techniques that are particularly suited to...

The role of small-angle X-ray and neutron scattering (SAXS and SANS) in the characterization of cement is briefly reviewed. The unique information obtainable from SANS analysis of C–S–H gel in hydrating cement is compared with that obtainable by other neutron methods. Implications for the nature of C–S–H gel, as detected by SANS, are considered in...

Decalcification of cement paste in concrete is associated with several modes of chemical degradation including leaching, carbonation and sulfate attack. The primary aim of the current study was to investigate the effects of decalcification under saturated conditions on the dimensional stability of cement paste. Thin (0.8 mm) specimens of tricalcium...

The properties, structure, and behavior of cement paste, including surface area, drying shrinkage, creep, and permeability are discussed with the assumption that the C-S-H gel is an aggregation of precipitated, colloidal-sized particles that undergoes chemical aging. A basic thesis of this paper is that C-S-H particles bond together over time, incr...

The chemical equilibrium between the C-S-H gel phase and the aqueous phase in the CaO-SiO2-H2O system was modeled using the free-energy-based Thermo-Calc commercial software system. Unlike programs that are based on solubility products, the minimization of free energy allows the composition and solubility of the C-S-H phase to vary, as in real ceme...

Thin coupons of white portland cement (WPC) and tricalcium silicate paste were decalcified by leaching in concentrated ammonium nitrate solutions, resulting in calcium-to-silicon molar ratios (C/S) ranging from 3.0 (control) down to 0.3. The microstructure and surface area were measured using both small-angle neutron scattering (SANS) and nitrogen...

In the paper referred to in the title of this commentary, Odler provides a comprehensive and useful review of pertinent data and interpretations of BET gas sorption measurements of cement paste. The focus of that paper was the well-known differences in the surface area and pore volume obtained using water vapor (BET H2O) or nitrogen (BET N2) as the...

The poorly crystalline calcium silicate hydrate (C-S-H) phases that form near room temperature, which include the technically important C-S-H gel phase formed during the hydration of Portland cement, have a broad similarity to the crystalline minerals tobermorite and jennite, but are characterized by extensive atomic imperfections and structural va...

Thin coupons of white portland cement (WPC) and tricalcium silicate paste were decalcified by leaching in concentrated ammonium nitrate solutions, resulting in calcium-to-silicon molar ratios (C/S) ranging from 3.0 (control) down to 0.3. The microstructure and surface area were measured using both small-angle neutron scattering (SANS) and nitrogen...

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement and white Portland cement pastes were measured during the first 28 d of curing at temperatures ranging from 5–50 °C. Saturation indices with respect to solid phases known to form in cement paste were calculated from a thermodynamic analysis of the eleme...

The often-argued and seldom-resolved issue of relating the bulk shrinkage of concrete to changes in the size of pores is analyzed using a simple model. It is shown that the pores in a composite containing both shrinking and nonshrinking solid phases can themselves either shrink or expand when the matrix shrinks, depending on the amount of restraint...

Inelastic neutron spectroscopy (INS) was used to determine the content of Ca-OH bonds in a series of mature tricalcium silicate (3CaO‚SiO 2) and white portland cement (WPC) pastes that were decalcified to calcium/silicon molar ratios (Ca/Si) as low as 1. These measurements indicate that the band in the INS data at 41 meV, which originates from latt...

The introduction of the environmental scanning electron microscope (ESEM) greatly Improved the functionality of scanning electron microscope (SEM) technology by allowing objects to be imaged in water-vapor under low vacuum, and without the need for coating to facilitate electrical conductivity. The low vacuum environment allows Imaging of delicate...

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement (OPC) and white Portland cement (WPC) pastes were measured during the first 28 days of hydration at room temperature. Saturation indices (SI) with respect to various solid phases known to occur in cement pastes were calculated from a thermodynamic analy...

The effect of a short heat treatment on hydrated cement paste has been investigated by measuring the weight and length changes of specimens as they undergo various combinations of heating, drying, and resaturation. Heating a cement paste to 60°C coarsens the capillary pore system, decreases the volume of mesopores, and increases the degree of polym...

Quasi-elastic neutron scattering was used to monitor the temperature-dependant hydration of tricalcium silicate and Portland cement. Results show that for some samples the degree of hydration is in fact higher at lower curing temperatures. To investigate this effect further, we performed a series of experiments in which samples are initially hydrat...

Quasi-elastic neutron scattering (QENS) was used to monitor the state of water in portland cement and tricalcium silicate pastes during the first 2 days of hydration at three different temperatures. By applying a double-Lorentzian rather than a single-Lorentzian fitting function, the QENS signal from water at a given hydration time was divided into...

A revealing fact concerning the sophistication of modern concrete durability research is that, for the most part, structures still are categorized as sufficiently intact, irreparably damaged, or usually somewhere subjectively in between. Standard compression tests may indicate loss of serviceability in the advanced stages of deterioration when macr...

The hydration of tricalcium silicate powder with H[sub 2]O and D[sub 2]O was followed at three different temperatures using isothermal calorimetry. For both types of paste, the early kinetics could be accurately modeled, from the time of mixing to a point on the downslope of the main hydration peak, with a four-parameter nucleation and growth model...

The hydration of tricalcium silicate powder with H2O and D2O was followed at three different temperatures using isothermal calorimetry. For both types of paste, the early kinetics could be accurately modeled, from the time of mixing to a point on the downslope of the main hydration peak, with a four-parameter nucleation and growth model. Pastes mad...

Hydrated cement paste has a high specific surface area due almost entirely to the calcium-silicate-hydrate reaction product. The surface area of cement paste is closely related to many crucial properties, including strength and permeability, and is therefore a useful predictive measurement. It is also a useful parameter for studying the nature of t...

The chemistry of the aqueous phase of ordinary portland cement paste at early ages (<2 h) has been analyzed in terms of the concentrations of the elemental components in the pore fluid. The concentrations of calcium, sulfur, aluminum, and silicon are rationalized by plotting the data on “phase diagrams.” To simplify the analysis, the portland cemen...

Small-angle neutron scattering was used to measure the effect of water-to-cement ratio (w/c) and cement batch variation on the surface area of ordinary Portland cement (OPC) paste after hydration for 28 days at room temperature, and to measure the development of surface area over the first 3 days of hydration at 30°C. The 28-day surface area was fo...

Accurate measurements of the surface area of cement based materials using small-angle neutron scattering (SANS) require determination of the neutron scattering contrast between the C-S-H gel and H2O. Because the C-S-H gel has a poorly understood structure and a variable composition, calculated values of the C-S-H:H2O neutron scattering contrast bas...

The chemical equilibrium between the C-S-H gel phase and the aqueous phase in the CaO-SiO 2-H 2O system was modeled using the free-energy-based Thermo-Calc commercial software system. Unlike programs that are based on solubility products, the minimization of free energy allows the composition and solubility of the C-S-H phase to vary, as in real ce...

Quasielastic neutron scattering (QNS) measurements of the hydration kinetics of tricalcium silicate (C3S) have been made with 60 μeV energy resolution at a momentum transfer q = 1 Å−1. Monitoring the fraction of neutrons elastically scattered from C3S paste specimens follows the progress of the C3S hydration reactions. Three different water/cement...

The progress of the hydration reactions of tricalcium silicate (C3S) has been followed using quasielastic neutron scattering (QNS) at an energy resolution of 60 mueV and a momentum transfer of q=1 Å-1. The degree of reaction in the hydrating cement paste is inferred from the fraction of water that is chemically bound to the cement reaction products...

Rod-shaped silicon compacts were nitrided in a microwave applicator using a minimum of insulation in order to maximize the temperature gradients to effect an inside-out reaction. These specimens exhibited very nonuniform conversion to Si3N4, and fully nitrided areas had poor microstructures consisting of alternating regions of high and low density....

Gas sorption surface area measurements have been important for developing microstructural models for the structure of calcium-silicate-hydrate (C-S-H), the major hydration product of portland cement. However, surface area measurements using the nitrogen BET method have historically had high variability and have not always been reproducible. In this...

The progressive decrease in the nitrogen BET surface area of dried hydrated portland cement paste during storage in plastic vials was found to be caused by carbonation, a reaction between the C-S-H phase and carbon dioxide in the air. A strong correlation between the surface area and the amount of carbonation as measured by thermogravimetric analys...

A finite-difference model was used to simulate the temperature and composition distributions produced inside a specimen heated with microwave energy during a process involving a change in composition. The dielectric properties of the specimen change with composition, resulting in nonuniform microwave power absorption and steady-state temperature gr...

Thesis (Ph. D., Materials science and engineering)--Northwestern University, 1994.

The size and density of reaction-bonded silicon nitride (RBSN) specimens are limited by the reduction in pore size and pore volume associated with the nitridation reaction. In particular, under conventional heating, pores at the surface of dense compacts close before the center has reacted fully. Microwave heating offers a unique advantage over con...

Although the preparation of single-phase C–S–H specimens is usually from the reaction of CaO and SiO 2 or from the double decomposition of a Ca-salt and an alkali silicate in aqueous solution, recent evidence in the literature shows that these 'synthetic' preparations (especially double decomposition) produce nanostructures at high Ca/Si ratios qui...

Blended cements are mixtures of artificial or natural pozzolans with ordinary portland cement which increases strengths and reduces fabrication costs. We have been studying the hydration rates of cements mixed with D_2O and H_2O using neutron methods. With model cement compounds mixed with heavy water, the hydration process can be monitored by the...

Implications for a colloid model for C-S-H on creep and shrinkage are discussed. A basic thesis of this paper is that C-S-H undergoes chemical aging, causing it to become stiffer, stronger, and denser over time. This process is accelerated by increased temperature and also by drying, which introduces large local strains that may provide a microstru...

In the advanced stages of concrete deterioration, macroscopic features such as focused crack networks and surface spalls may indicate damage to a structural element. In such cases, compressive strength measurements provide a quantitative criterion for evaluating the engineering serviceability of the construction. Although microstructures may record...