The hydration of Portland and aluminous cements with added polymer dispersions

We have studied the hydration of ordinary Portland cement and Secar 71 aluminous cement with added polyvinyl alcohol, polyvinyl acetate, acrylic, poly (vinylidene chloride-acrylic), poly (styrene-acrylic) polymers and poly (styrene-butadiene) rubber latex. We have used the techniques of conduction and differential scanning calorimetry, infrared spectroscopy and X-ray diffraction in our studies.The nature of the interactions between the polymers and the two cements is significantly different. For Portland cement all polymers were found to retard the hydration rate to some extent, with the acrylics producing the maximum affect, whereas in the case of Secar 71, polymer additions had minimal effects on the hydration of or heat evolution from the cement. Our results suggest that many polymers interact with Portland cements in a chemical way although for aluminous cements the observations are less clear except in the case of polyvinyl alcohol-acetate.     

Electrical characterisation as a function of frequency: application to aluminous cement during early hydration

Electrical measurement as a function of frequency is a non-destructive technique which has been applied on numerous ceramic based systems such as single crystals, heterogeneous materials or concentrated suspensions of ceramic powders. It is a method for looking at the dynamic response of the system under test. Several approaches have been developed by different authors to give the associated relaxation times based on a single or on a distribution of times. In order to support the interpretation of the dynamic signal, one has to vary the physical or chemical parameters such as temperature, time, concentration and so on. Application of high frequency measurements (1 MHz–1.8 GHz) on aluminous cement pastes (W/C=0.4) is described. The variations of the capacitance with respect to setting time (from 0 to a few hours after mixing water and cement) and temperature (20 and 30 °C) are related to hydration chronology and to the quantity of formed hydrates. The high frequency response of C₃AH₆, which has been prepared by a chemical route, is also discussed.     

Cement content in concretes made with aluminous cement

An equation for calculating the cement content in concretes made with aluminous cement is proposed, based on determining the Fe₂O₃ content of the concrete, assuming that the proportions of this oxide, as provided by the manufacturing company, throughout the production process are very stable. The values are corrected according to the Fe₂O₃ content of the aggregates. The Fe content is determined by x-ray fluorescence analysis (XRF).

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Resume On propose une équation pour calculer la quantité de ciment dans les bétons de ciment alumineux fondu. La méthode calcule la teneur en Fe₂O₃ du béton, attendu que les concentrations de cet oxide sont très constants pour le ciment alumineux. Ou décompte le taux de Fe₂O₃ des granulats. Pour l'analyse du Fe on a appliqué la fluorescence x. *** DIRECT SUPPORT *** A00G7067 00003

     

Controlling cement hydration with nanoparticles

Nanoparticles have recently become a focus in the development of new accelerators for cement hydration. We have produced nanoparticles of different materials like Al₂O₃, C–S–H-phase, and quartz by different top-down and bottom-up production methods. The effect of these particles on the cement hydration was followed by heat flow calorimetry to evaluate their acceleration potential as a function of the particles material, particles size and their percentage in the cement paste. In this work we will demonstrate which particles have the best potential as accelerators for cement hydration and therefore are most suitable for further investigations. Interestingly, nanoparticles which have a retarding effect on cement hydration were also found.     

Ultrasonic characterization of model mixtures of hydrated aluminous cement

In an aluminous cement, which mainly consists of CA, the stable phases arising during hydration are C₃AH₆ and AH₃. This communication presents a chemical route for the synthesis of C₃AH₆ which is based on hydration of C₃A. Mixtures of CA, C₃AH₆ and AH₃ are characterised by ultrasonic testing. The ultrasonic velocity obtained on these mixtures is lower than what is observed in hydrated aluminous cement of similar chemical composition. Interfaces are thought to play a significant role in the ultrasonic response of aluminous cements.     

Properties of polyvinyl alcohol cement pastes

Properties of cement pastes containing varying amounts of each of polyvinyl alcohol (PVAL), mixtures of polyvinyl alcohol and phenol formaldehyde (PF) and mixtures of poly vinyl alcohol and borax were studied in this paper. Though the strength parameters of the PVAL-cement pastes are comparable to virgin cement paste their resistance to acid is far superior. Soxhlet extraction with water, done to determine leachability of the polymer from the polymer cement paste, revealed that the percentage of polyvinyl alcohol leached out was less when borax or PF resin was added to the PVAL cement paste. The compressive strength of the poly vinyl alcohol–phenol formaldehyde cement paste was found to be inferior to the other two cases but the retention of compressive strength after immersing in each of acid, base and kerosene was much better. In general, polyvinyl alcohol when added to cement pastes improves the chemical resistance properties in terms of retention of compressive strength after exposure to chemicals.     

Identification of hydration stage of calcium sulfoaluminate cement at an early age with helium pycnometry

This work is to study the mechanism of the hydration of calcium sulfoaluminate (CSA) cement at an early age by identifying the hydration stage with helium pycnometry, heat evolution test, X-ray diffraction, thermo-gravimetric/differential scanning calorimetry and scanning electron microscope. The results show that the hydration process of CSA cement has four stages: rapid dissolution stage, dissolution-crystallization stage, crystal growth stage and stable stage. In the rapid dissolution stage, exothermic wetting and fast hydration result in the shrinkage occurring from 0 to 15 min. A significant volume decrease happens in the dissolution-crystallization stage. In the crystal growth stage, ettringite prefers to crystalize at the neighboring places of pores at the nano scale due to the combined action of crystallization pressure and size effect. The volume increase considerably slows down throughout the stable stage indicating the deceleration of CSA cement hydration. In addition, the influence of the proportions of clinker and gypsum in the CSA cement on its hydration is studied as well.     

The role of phosphonates in the hydration of Portland cement

Six phosphonates, comprising the three acids aminotri(methylenephosphonic acid) (ATMP), 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and diethylenetriaminepenta-(methylenephosphonic acid) (DTPMP) and their corresponding sodium salts Na₅ATMP, Na₄HEDP and Na₆DTPMP, were added to Portland cement in dosages ranging from 0.03 to 0.09%, at a constant water/cement ratio of 0.35 and their conduction calorimetric behaviour was investigated up to 72 h. The induction period, the time to attain the maximum heat effect and the integral heat developed at different times were determined. All phosphonates increased the induction period, from about 3 h to greater than 72 h, with respect to the reference cement with an induction period of 2 h. The acid phosphonates were more effective retarders than their corresponding salts. At a concentration of 0.05% the induction period extended from 10.1 to 21.1 h with the acids and only from 4.1 to 16.2 h with the salts. DTPMP was the most effective retarder among all the phosphonates, a concentration of 0.05% producing an induction period of 21.1 h and an exothermic inflection at 42.4 h compared with values of 2.2 and 7.9 h, respectively, for the reference. The corresponding salt (Na₆DTPMP) was the most efficient of all the salt retarders. At a concentration of 0.05%, the induction period was extended to 16.2 h and the exothermal inflection to 31.4 h. In most instances the degree of extension of the induction period increased with the dosage of retarder. Phosphonates appear to be much more efficient retarders than many other retarders used in concrete practice.

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Resume On a ajouté au ciment Portland six phosphonates, comprenant les trois acides aminotri (acid méthylènephosphonique) (ATMP), 1-acide hydroxyéthylidène-1,1-acide diphosphonique (HEDP) et diéthylènetriaminepenta(acide méthylènophosphonique) (DTPMP) et les sels de sodium correspondants Na₅ATMP, Na₄HEDP et Na₆DTPMP, à des dosages allant de 0,03 à 0,09%, avec un rapport eau/ciment constant de 0,35. On a étudié leur comportement sous conduction calorimétrique pendant une durée de 72 h. On a déterminé la période d’induction, le temps nécessaire pour atteindre l`effet de chaleur maximal et la chaleur intégrale produite à certains moments. Tous les phosphonates ont augmenté la période d’induction de 3 h à plus de 72 h, par rapport au ciment de référence qui présente une période d’induction de 2 h. Les phosphonates sont des retardateurs plus efficaces que les sels qui leur correspondent. Pour une concentration de 0,05%, la période d’induction est passée de 10 à 21,1 h avec les acides, et seulement de 4,1 à 16,2 h avec les sels. Le DTPMP a été le retardateur le plus efficace parmi les phosphonates: une concentration de 0,05% a déterminé une période d’induction de 21,1 h et une manifestation exothermique à 42,4 h, par rapport à des valeurs respectivement de 2,2 et 7,9 h pour le ciment de référence. Le sel correspondant (Na₆DTPMP) a été le plus efficace de tous les sels retardateurs. Pour une concentration de 0,05, la période d’induction a atteint 16,2 h et la manifestation exothermique 31,4 h. Dans la plupart des cas, le degré d’allongement de la période d’induction était en relation avec le dosage de retardateurs. Les phosphonates semblent être bien plus efficaces que beaucoup d’autres retardateurs utilisés dans la pratique du béton.

     

Hollow-shell formation – an important mode in the hydration of Portland cement

The characteristics and quantities of hollow shells (Hadley grains) in the microstructure of Portland cement paste have been studied. It is shown that at relatively early hydration stages, the hydration largely occurs in a manner resulting in hollow shells. At later ages, the “hollow-shell hydration” mode becomes much less prominent as “inner products” are increasingly formed. The intrinsic porosity associated with hollow shells can be significant at later ages, between about 1% and 9% depending on the mix composition and curing regime. The presence of silica fume increases the amount of hollow-shell porosity considerably, while the hollow-shell porosity appears to decrease with decreasing water/binder ratio. This revised version was published online in November 2006 with corrections to the Cover Date.     

The nature of the hydration products in hardened cement pastes

An understanding of the performance of portland cement-based materials requires knowledge at the microstructural level. Developments in the instrumentation of several techniques have led to improved understanding of the composition, morphology, and spatial distribution of the various products of cement hydration. In particular, our understanding of the nature of the nearly amorphous calcium silicate hydrate (C–S–H) phases – which are the principal binding phases in all portland cement-based systems – has been advanced by developments in solid-state NMR spectroscopy and analytical TEM. This paper presents an overview of the nature of the hydration products formed in hardened portland cement-based systems. It starts with the most straightforward cementitious calcium silicate systems, C₃S and β-C₂S, and then considers ordinary portland cement and blends of portland cement with silica fume, ground granulated iron blast-furnace slag, and finally alkali hydroxide-activated slag cements.     

Three-dimensional computer modeling of slag cement hydration

A newly developed version of a three-dimensional computer model for simulating the hydration and microstructure development of slag cement pastes is presented in this study. It is based on a 3-D computer model for Portland cement hydration (CEMHYD3D) which was originally developed at NIST, taken over in the authors’ group and further developed. Features like the digitized 3-D microstructure, the cellular automata (CA) algorithm for simulating the random walking, phase transformation for simulating the chemical reactions, are retained. But, the 3-D microstructure was reconstructed allowing for slag particles as binder in the system. Algorithms and rules are developed to account for the interaction between Portland cement hydration and slag reaction in the paste, of which the mechanisms were revealed in the studies by Chen and Brouwers [(2007) J Mater Sci 42(2):428; (2007) J Mater Sci 42(2):444] Methods for considering the various factors on the reactivity of slag in hydrating slag cement pastes are proposed, mainly for the oxide composition of slag and the alkalinity in the pore solution composition. A comparison between the model predictions and the experimental results in literature shows that the presented computer model can successfully predict the hydration process and the microstructure development of hydrating slag cement paste.     

Properties and interaction of cement with polymer in the hardened cement pastes added absorbent polymer

Ordinary Portland cement mixed with various amounts of absorbent polymer in the form of sodium acrylate ((–CH–)_nCOONa) have been studied. As the content of absorbent polymer increased, heat evolution of samples decreased up to 1.15 wt % of absorbent polymer addition and conversely increased over 1.75 wt %. Flexural strength of cement paste with absorbent polymer was improved more than 20%. As the content of absorbent polymer increased, the porosity values decreased and mean pore diameter shifted to small pore diameter region. Flexural strength of ordinary Portland cement paste had a linear correlation with non-evaporable water content but, that of cement paste with absorbent polymer deviated from a linear correlation with non-evaporable water content. The chemical difference between cement pastes with and without absorbent polymer was found by the inductively coupled plasma-atomic emission spectroscopy and the infrared spectroscopy. For the infrared spectra of absorbent polymer, bands at 1416 and 1560 cm^{– 1} were assigned to C–O single bond and C=O double bond respectively, namely, unidentate complex. As the curing time increased, the absorption bands near 1416 cm^{– 1} shifted to longer wave number and the absorption bands near 1560 cm^{– 1} to shorter wave number and finally bidentate complex was formed. Absorbent polymer released sodium ions to pore solution under the basic condition of pH 12.5–13.5 and became polyacrylic acid. Then some of these polyacrylic acid were crosslinked with others by calcium ions leached from cement grains. Calcium ion was regarded as a central charge connecting the negative parts in carbon-oxygen polarization of absorbent polymer' functional groups.     

Simulation of silica fume blended cement hydration

A model is proposed in this paper to simulate silica fume (SF) blended cement hydration based on the kinetics, stoichiometry and physical chemistry of cement hydration and pozzolanic reaction. The pozzolanic reaction degree, volume fraction of hydration products, capillary porosity and gel porosity can be obtained from model simulation. By using proper amount of silica fume replacement, the microstructure of silica fume blended cement paste is improved since the volume fraction of C-S-H gel is increased, Ca(OH)₂ content and capillary porosity are decreased due to pozzolanic reaction compared with ordinary Portland cement (OPC) paste. The effects of silica fume particle size, glass phase content and the percentage of silica fume replacement on pozzolanic reaction degree, volume fraction of hydration products, and capillary porosity are simulated. The simulation results show that finer silica fume particles with higher glass phase content (GP) are of higher reactivity. There is an optimum silica fume replacement; extra silica fume only acts as inert filler because there is no enough Ca(OH)₂ from cement hydration to react with it pozzolanically.

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Résumé Le modèle proposé dans cet article simule l'hydratation de ciment mélangé à de la fumée de silice et est basé sur la cinétique, la stoichiométrie et la physico-chimie de l'hydratation du ciment et la réaction pozzolanique. Le degré de réaction pozzolanique, la fraction volumique des produits de l'hydratation, la porosité capillaire et la porosité de gel peuvent être obtenues par un modèle de simulation. En utilisant la bonne quantité de remplacement de fumée de silice, la microstructure de la fumée de silice mélangée à la pate de ciment est améliorée étant donné que la fraction volumique du gel C-S-H augmente, que le taux de Ca(OH)₂ et la porosité capillaire décroissent en raison de la réaction pozzolanique lorsque l'on compare avec une pate de ciment à base de Portland ordinaire (OPC). Les effets de la taille des particules de fumée de silice, le contenu de la phase de verre et le pourcentage de remplacement de fumée de silice sur le degré de réaction pozzolanique, la fraction volumique des produits d'hydratation et la porosité capillaire sont simulés. Les résultats de cette simulation montrent que de plus fines particules de fumée de silice avec une plus grande quantité de phase de verre (GP) sont de forte réactivité. Il existe un remplacement de fumée de silice optimal; de la fumée de silice en plus ne sert que de filler inerte car il n'y a pas assez de Ca(OH)₂ à partir de l'hydratation du ciment pour provoquer une réaction pozzolanique

     

On the initial stages of cement hydration

After the initial mixing of cement, an induction period occurs during which its consistency remains constant. Thickening occurs at the end of this period when the consistency is observed to increase very rapidly. In this paper we propose a reaction-diffusion model for the hydration of tricalcium silicate, a principal constituent of cement, which is believed to be responsible for the initial development of its strength. Our model is based on the assumption that the hydration of cement can be described as a dissolution -precipitation reaction. The mathematical solutions enable us to determine some of the factors that control the length of the induction period and make predictions of the ionic concentrations which are in agreement with experimental data.     

The influence of nano-silica on the hydration of ordinary Portland cement

Nucleation seeding is a new approach to control the kinetics of cement hydration. It is known that nano-silica has an accelerating effect on cement hydration. It is assumed that the surface of these particles act as a nucleation site for C–S–H-seeds which then accelerate the cement hydration. In this case the acceleration should depend on the particles surface area. To verify this, nano-silica particles of different sizes and specific surface areas were synthesised. The acceleration of cement hydration clearly correlates with the total surface size of the added particles, which was varied by either using smaller particles or higher concentration of particles in the cement lime. Additional in situ-XRD experiments show that the consumption of C₃S and the formation of portlandite are accelerated by the addition of nano-silica. In both cases the surface size is the major factor for the hydration kinetics.     

Influence of finely ground limestone on cement hydration

Some work has been carried out on the effect of calcium carbonate on cement paste, but there is no general agreement on the relative effects of different amounts of calcium carbonate on cement paste properties. The objective of the present work is to assess the effect of various amounts of calcium carbonate on the hydration of tricalcium silicate in order to explain the physico-chemical changes occurring during Portland cement hydration. It is shown that calcium carbonate has an accelerating effect on C₃S and cement hydration and leads to the precipitation of some calcium carbosilicate hydrate.     

Alongterm study of MgO hydration in cement pastes

The effect of various additives on the longterm hydration of Magnesium Oxide, (MgO) in cement pastes cured in water at 18±2°C up to 12 years was studied by X-ray diffraction (XRD), seanning electron microscopy (SEM) and EPMA. It was found that cement with high MgO content, is stabilized even after a longterm period of hydration by active pozzolans.     

Monitoring the early hydration mechanisms of hydraulic cement

Cement is one of the most widely used construction materials with one billion tonnes used annually. From an engineering point of view, it is essential that cement sets and hardens in a correct manner, indeed, modification of the setting and hardening characteristics of cement by the use of admixtures is becoming widespread in the construction industry. The reaction between cement clinker and water is a complicated chemical process which results in a rigid matrix capable of sustaining load. The increase in strength of the cement matrix is the consequence of hydration and crystal formation within the paste. Understanding the mechanisms of hydration and how they can be modified could result in new cement blends and admixtures tailor-made to suit any particular set of design criteria. In this paper it is shown that the temporal change in electrical response can be used to monitor the progress of hydration, and give an insight into mechanisms of hydration. Data are presented for several cement paste consistencies over the frequency range 20 Hz to 300 kHz.     

Clinkering and hydration of belite-alite-ye´elimite cement

Some belite-ye´elimite-ferrite (BYF) cements present low mechanical strengths mainly due to the slow reactivity of belite. A solution to this problem may be the activation of BYF clinkers by preparing them with a coexistence of alite and ye'elimite, which are known as belite-alite-ye´elimite (BAY) cements.The objective of this work was the preparation of BAY mortars that show higher mechanical strengths than BYF mortars. In order to attain this, the clinkering conditions to prepare BAY-clinker (2 kg) with the following mineralogical composition 60.6 (2) wt% of belite, 14.3 (2) wt% of alite and 10.4 (1) wt% of ye'elimite were optimized (900°C/30 min-1300°C/15 min). The hydration mechanism of cement pastes (with 12 wt% of anhydrite and water-to-cement ratios of 0.4 and 0.5) was studied through laboratory X-ray powder diffraction and thermo-analyses. Finally, BAY mortars with higher compressive strengths than BYF-mortars were obtained (viz. 24.8 and 17.1 MPa for BAY and BYF mortars at 7 days of hydration, respectively).