Comparison of proton field-cycling relaxometry and molecular dynamics simulations for proton-water surface dynamics in cement-based materials

Diffusion coefficients of water in hydrated cement pastes and mortars obtained from proton field cycling NMR spin lattice relaxation over three orders of magnitude in magnetic field strength are in good agreement with values from molecular dynamics simulations of water on the surface of tobermorite. The level of agreement from these two independent approaches provides mutual support for their validity.     

NMR Relaxation Study of Adsorbed Water in Cement and C3S Pastes

The deuteron and proton spin-lattice and spin-spin relaxation times T ₁ and T ₂ of adsorbed water in commercial portland cement and tricalcium silicate pastes were studied as functions of the hardening time at room temperature. The time dependence of the water self-diffusion coefficient of tricalcium silicate pastes was also followed. The proton and the deuteron T ₁ and T ₂ decrease markedly as hydration increases and the pastes harden due to the increase in the active surface and the number of adsorptive sites, thus providing convenient tools for studying the nature of the hydration process.     

Nuclear magnetic relaxation dispersion investigations of water retention mechanism by cellulose ethers in mortars

We show how nuclear magnetic spin–lattice relaxation dispersion of proton-water (NMRD) can be used to elucidate the effect of cellulose ethers on water retention and hydration delay of freshly-mixed white cement pastes. NMRD is useful to determine the surface diffusion coefficient of water, the specific area and the hydration kinetics of the cement-based material. In spite of modifications of the solution's viscosity, we show that the cellulosic derivatives do not modify the surface diffusion coefficient of water. Thus, the mobility of water present inside the medium is not affected by the presence of polymer. However, these admixtures modify significantly the surface fraction of mobile water molecules transiently present at solid surfaces. This quantity measured, for the first time, for all admixed cement pastes is thus relevant to explain the water retention mechanism.     

Hardened portland blast-furnace slag cement pastes: I. Effects of additives on surface area and on pore structure

Portland blast-furnace slag cement pastes were prepared with various water/cement ratios. Specific surface areas and pore structures of the hardened pastes were investigated by nitrogen adsorption. The “accessibility” of the nitrogen molecules to the pore structure is discussed in terms of degree of hydration and total porosities of the pastes. Effect of presence of CaCl₂, a typical steel reinforcement corrosive agent, was also studied, and results indicated that it alters the area and pore structure extensively, to a more “open structure,” thus facilitating its own accessibility. Lime and gypsum addition was also studied in presence and in absence of CaCl₂, and the effect of the Blaine surface area of the unhydrated cement is particularly emphasized in this investigation.     

Hardened slag–cement pastes of various porosities. III. Surface area in relation to pore structure

The total surface areas, total pore volumes, and the distribution of pore surface and pore volume in pores of different sizes were determined on the hardened slag-cement pastes. Adsorption isotherms of four different adsorbates, namely water, methanol, nitrogen and cyclohexane, were used for surface area and pore structure determinations. The results obtained indicated that water can penetrate into the total pore system, namely both the micropores and the wide pores; whereas methanol can measure wide pores and some of the micropores; moreover, nitrogen and cyclohexane molecules tend to measure only wider groups of pores. The indication gained from this result is that the molecular sieve property plays the most significant role in surface area and pore structure determinations of the hardened cement pastes. The polarity of the molecules seems to be important at low relative vapour pressures, and at high pressures it causes expansion of the pastes.     

NMR diffusion and relaxation studies during cement hydration—A non-destructive approach for clarification of the mechanism of internal post curing of cementitious materials

Internal post-curing of hardening cement pastes by addition of alginate spheres, which contain 98% of water, is studied by non-destructive ¹H NMR measurements of transverse relaxation time and self-diffusion. The onset and amount of water transition from the alginate gel used as additive with temporary delayed release of water to cement pastes was observed continuously during the dormant and accelerated period of cement hydration. During hydration, the water transition from the alginate into the cement matrix as well as the development of pore size is monitored quantitatively by studying the time dependence of characteristic peaks in the transverse relaxation time distribution. Comparison between samples without and with internal post curing shows that the addition of alginate gel does not influence the pore size in the micropore region. NMR diffusion studies demonstrate that the physically bound pore water has sufficient mobility to ensure homogeneous distribution of water from the alginate source into the surrounding cement matrix during the dormant and accelerated period.     

Effect of some superplasticizers on the mechanical and physicochemical properties of blended cement pastes

Blended cement pastes made of Portland cement and fine sand (known in Egypt as El-Karnak cement) were made using a water–cement ratio of 0.25 by weight. Three pastes containing admixture (water-soluble condensates) were also prepared using a water–cement ratio of 0.25 and condensate (superplasticizer) content of 0.25% by the weight of cement; the superplasticizers used are Na-phenol sulfonate formaldehyde, Na-polystyrene sulfonate, and Na-ß-naphthol sulfonate formaldehyde condensates. All pastes were cured for various time intervals within the range of 0.02–90 days. Compressive strength tests, hydration kinetics, X-ray diffraction analysis, thermal analysis, and surface properties were studied and related as much as possible to the pore structure of the hardened pastes. © 1995 John Wiley & Sons, Inc.     

Influence of superplasticizers on the evolution of ultrasonic P-wave velocity through cement pastes at early age

The paper discusses a possibility of using an ultrasonic wave transmission method to study the influence of superplasticizers on the formation of structure of cement pastes at early ages. When compared to mixtures without additives, lower P-wave velocity was found through superplasticized cement pastes, indicating that superplasticizers prevent formation of a solid network frame. Comparing to sulfonate naphthalene-formaldehyde superplasticizers, polycarboxylate ether (PCE) admixtures retarded the solid network frame development more intensively, resulting in a plateau on a P-wave velocity curve during the setting period. The length of the plateau is proportional to the dosage of the PCE and inversely proportional to the specific surface area of the hydration products developed, proving that the specific surface area of a solid phase affects the performance of the PCEs. Validation of ultrasonic results was determined on the basis of the temperature evolution of the material in time.     

Investigation of blended cement hydration by isothermal calorimetry and thermal analysis

Hydration of portland cement pastes containing three types of mineral additive; fly ash, ground-granulated slag, and silica fume was investigated using differential thermal analysis, thermogravimetric analysis (DTA/TGA) and isothermal calorimetry. It was shown that the chemically bound water obtained using DTA/TGA was proportional to heat of hydration and could be used as a measure of hydration. The weight loss due to Ca(OH)₂ decomposition of hydration products by DTA/TGA could be used to quantify the pozzolan reaction. A new method based on the composition of a hydrating cement was proposed and used to determine the degree of hydration of blended cements and the degree of pozzolan reaction. The results obtained suggested that the reactions of blended cements were slower than portland cement, and that silica fume reacted earlier than fly ash and slag.     

Early-age acoustic emission measurements in hydrating cement paste: Evidence for cavitation during solidification due to self-desiccation

In this study, the acoustic emission activity of cement pastes was investigated during the first day of hydration. Deaired, fresh cement pastes were cast in sealed sample holders designed to minimize friction and restraint. The majority of acoustic emission events occurred in lower water to cement ratio pastes, while cement pastes with higher water to cement ratios showed significantly less acoustic activity. These acoustic events occurred around the time of setting. A layer of water on the surface of the cement pastes substantially reduced acoustic emission activity at the time of setting. According to these experimental results, the acoustic emission measured around setting time was attributed to cavitation events occurring in the pores of the cement paste due to self-desiccation. This paper shows how acoustic emission might be used to indicate the time when the fluid–solid transition occurs in a cement paste, often referred to as time-zero. Knowledge of time-zero is fundamental for determining when mechanical properties develop and in calculations of residual stresses.     

Modeling of diffusive mass transport in micropores in cement based materials

In order to predict long-term leaching behavior of cement constituents for safety assessments of radioactive waste disposal, we modeled diffusive mass transport in micropores in cement based materials. Based on available knowledge on the pore structure, we developed a transport porosity model that enables us to estimate effective porosity available for diffusion (transport porosity) in cement based materials. We microscopically examined the pore structure of hardened cement pastes to partially verify the model. Effective diffusivities of tritiated water in hardened cement pastes were also obtained experimentally, and were shown to be proportional to the estimated transport porosity.     

Effects of Microsilica (Silica Fume) on Pore-Solution Chemistry of Cement Pastes

Pore solutions expressed from hydrated Portland cement pastes containing up to 30% microsilica (silica fume) as replacement for cement were analyzed. The presence of microsilica enhances early concentrations of alkalies and hydroxyl ions, but after 1 day the effect is reversed, and the concentrations of these ions are progressively reduced to very low values. Alkali reduction seems more pronounced for pastes of higher water content, and low levels of microsilica replacement (5 or 10%) exert substantially greater than proportional effects.     

Model for the Developing Microstructure in Portland Cement Pastes

A method is proposed for quantitatively predicting the volume of the major phases in hydrated cement pastes as a function of (1) the composition of the cement, (2) the degree of reaction, and (3) the initial water:cement ratio. This procedure is then used to develop a quantitative model for the surface area and volume of porosity that is accessible to nitrogen in C-S-H. Published values for surface areas and volume of pores are compared with the predictions made by the model. An implication of the model is that there are two types of C-S-H, or perhaps regions within the C-S-H: one that nitrogen can penetrate and one that it cannot.     

Alkali binding in hydrated Portland cement paste

The alkali-binding capacity of C-S-H in hydrated Portland cement pastes is addressed in this study. The amount of bound alkalis in C-S-H is computed based on the alkali partition theories firstly proposed by Taylor (1987) and later further developed by Brouwers and Van Eijk (2003). Experimental data reported in literatures concerning thirteen different recipes are analyzed and used as references. A three-dimensional computer-based cement hydration model (CEMHYD3D) is used to simulate the hydration of Portland cement pastes. These model predictions are used as inputs for deriving the alkali-binding capacity of the hydration product C-S-H in hydrated Portland cement pastes. It is found that the relation of Na⁺ between the moles bound in C-S-H and its concentration in the pore solution is linear, while the binding of K⁺ in C-S-H complies with the Freundlich isotherm. New models are proposed for determining the alkali-binding capacities of C-S-H in hydrated Portland cement paste. An updated method for predicting the alkali concentrations in the pore solution of hydrated Portland cement pastes is developed. It is also used to investigate the effects of various factors (such as the water to cement ratio, clinker composition and alkali types) on the alkali concentrations.     

A water‐soluble acrylate/sulfonate copolymer. I. Its synthesis and dispersing ability on cement

A new water‐soluble methacrylate/2‐acrylamido‐2‐methylpropane sulfonate copolymer (PMAMP) was synthesized and evaluated as a dispersion agent for cement particles. PMAMP was prepared from methacrylic acid and 2‐acrylamido‐2‐methylpropane sulfonic acid (AMP). The structure of the prepared polymer was verified by its NMR and IR spectra. The dispersing properties of PMAMP were evaluated by a minislump test on cement pastes. The test results indicated that this copolymer could disperse the cement particles and improve the minislump of cement pastes. Compared with a commercial superplasticizer (sulfonated naphthalene formaldehyde condensates), PMAMP performed better in enhancing the fluidity of the cement pastes. The polymer with about 40–50% AMP and a weight‐average molecular weight of about 5 × 10⁴ was most effective in dispersing cement particles and promoting the fluidity of cement pastes. Nevertheless, PMAMP with a higher AMP content or a higher molecular weight appeared to cause less slump loss. This was related to the interaction of this admixture with the cement particles and its adsorption behavior onto the cement particles. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2490–2496, 2006     

Hydration Kinetics and Phase Stability of Dicalcium Silicate Synthesized by the Pechini Process

Reactive dicalcium silicate (Ca₂SiO₄) has been synthesized by the Pechini process, and hydration kinetics studied. With increasing calcination temperature, the amorphous product first crystallizes to α'_L-phase and subsequently to the ß- and γ-phases. The specific surface area, ranging from 40 to 1 m²/g, strongly depends on the calcination temperature of 700°-1200°C for 1 h. Samples with a high surface area have a high water demand; a water/cement ratio >2.0 is required to produce formable pastes in some instances. Hydration kinetics are determined by XRD, ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR), and differential scanning calorimetry/thermogravimetry (DSG/TG). The hydration rate depends only on the surface area, not on the polymorph. Complete hydration occurs in as early as 7 d. Very little calcium hydroxide (Ca(OH)₂) is formed in the most reactive specimens (calcined at 700° and 800°C), which indicates the Ca/Si ratio in C-S-H gels is ∼2.0, but more Ca(OH)₂ forms from samples calcined at higher temperature. The silicate structure of the hydrated Ca₂SiO₄ pastes is investigated using ²⁹Si MAS NMR spectroscopy and trimethylsilylation analysis.     

The morphology of C–S–H: Lessons from 1H nuclear magnetic resonance relaxometry

¹H nuclear magnetic resonance has been applied to cement pastes, and in particular calcium silicate hydrate (C–S–H), for the characterisation of porosity and pore water interactions for over three decades. However, there is now renewed interest in the method, given that it has been shown to be non-invasive, non-destructive and fully quantitative. It is possible to make measurements of pore size distribution, specific surface area, C–S–H density and water fraction and water dynamics over 6 orders of magnitude from nano- to milli-seconds. This information comes in easily applied experiments that are increasingly well understood, on widely available equipment. This contribution describes the basic experiments for a cement audience new to the field and reviews three decades of work. It concludes with a summary of the current state of understanding of cement pore morphology from the perspective of ¹H NMR.     

Studies on water and nitrogen adsorption on hardened cement pastes. II-Thermodynamics of adsorption

Heats as well as entropies of water and nitrogen adsorption on hardened cement pastes of various pore structures were computed. Water vapor adsorption is accompanied by a decrease in integral entropy relative to the liquid state, while nitrogen adsorption is accompanied by an increase in integral entropy. The results could be related to the pore structures of the various pastes. For water adsorption, the molecules seem to be oriented in a definite array with more order than the liquid structure, and depending on the pore size, the normal liquid structure is resumed at some distance far from the surface. Nitrogen results could be described in terms of the inaccessibility and exclusion of the molecules from parts of the pore system.     

Evaluation of the pozzolanic activity of fluid catalytic cracking catalyst residue (FC3R). Thermogravimetric analysis studies on FC3R-Portland cement pastes

Spent fluid catalytic cracking catalyst (FC3R) from a petrol refinery has shown a great pozzolanic activity in lime pastes as have been demonstrated in previous studies. Based on these results, the pozzolanic activity of the FC3R in Portland cement pastes has been investigated. This evaluation has been carried out by means of thermogravimetry (TG) of cured FC3R-Portland cement pastes. The influence of water/binder ratio and the replacement percentage of FC3R on the pozzolanic reaction were investigated. Due to the chemical composition of FC3R that is similar to metakaolin (MK), and knowing that MK has a high pozzolanic activity, the latter was used as a material of comparison in the study of the water/binder ratio influence. The scope of this study is the determination of pozzolanic activity of FC3R when incorporated to Portland cement, and the evaluation on amount and nature of pozzolanic products. FC3R has shown a similar reactivity to MK, yielding similar pozzolanic products: CSH, CAH and CASH. The optimum replacing percentage in Portland cement pastes was in the 15-20% range.     

NMR studies of hydrating cement: A spin-spin relaxation study of the early hydration stage

The early stages of cement hydration were investigated by proton spin-spin relaxation time (T_c) measurements because these experiments can be performed much faster (in several seconds) than the characteristic time for a change in hydration evolution. In addition, comparing the free induction decays of nuclear magnetization and its spin echoes allows the separation of solid-like and liquid-like proton groups. The values of T₂ and the associated magnetization fractions were monitored for cement pastes with different water/cement ratios in the first 20 hours of hydration.

Results show that already at the time of the first measurement, i.e. six minutes after mixing dry white cement powder with water, a quasi steady state is reached with protons distributed in three groups: loosely bound water (in the gel coatings and interstital spaces between coated clinker grains with T₂ of several ms), tightly bound water (T₂ 100μs) and water/hydroxyl protons in crystalline structure (T₂ 10 μs). The values of T₂ and their associated magnetizations are fairly constant during the first several hours with the solid-like components only slowly growing at the expense of the liquid one. The fraction of the loosely bound water as well as the absolute value of its T₂ increases with increasing w/c ratio. These data indicate that although nothing vigorous is happening in the dormant period of cement hydration there is a continuous growth of the solid matrix.