Ultrasonic measurements during early-stage hydration of ordinary Portland cement

The variations of ultrasonic pulse velocity, peak amplitude of transmitted pulse and heat evolution rate were measured during the hydration process of ordinary Portland cement with water/cement ratio (W/C ratio) ranging from 0.4 to 0.6 at environmental temperatures between 20 and 40° C. The change of heat evolution rate reflected the kinetics of hydration and was sensitively affected by the W/C ratio and environmental temperature. The velocity and peak amplitude were compared with the heat evolution rate to investigate the correlation of elastic properties and structural development with the kinetics of hydration. The dependence of the velocity and the peak amplitude on W/C ratio and environmental temperature are discussed in terms of the concentration of solid phase in the cement paste varying with W/C ratio and the extent of hydration.     

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.     

The pozzolanic reaction of metakaolinite and its effects on Portland cement hydration

Metakaolinite was prepared from kaolinite by thermal treatment. Subsequent reaction of the metakaolinite with calcium hydroxide and water resulted in a cementitious material whose reaction kinetics were followed at a molecular level at both ambient and elevated temperatures using trimethylsilylation. Metakaolinite as a mineral admixture to ordinary Portland cement was also investigated.     

The effect of drying method on ordinary Portland cement surfaces during the early stages of hydration

During the early stages of ordinary Portland cement (OPC) hydration, ettringite, calcium hydroxide and C–S–H form on the OPC surface while nanoscale pits are formed in the exposed surface areas. In order to study these features using high resolution electron microscopy, samples must be appropriately dried to remove unbound water. The work reported here compares the effects of freeze-drying to drying by solvent exchange using four different solvents on the surface features of OPC after four hours of hydration in order to determine whether the solvent exchange process produces nanoscale changes in the surface features that can be detected using high resolution scanning electron microscopy (SEM). A quantitative image analysis of secondary electron SEM images was performed and the results analyzed using small sample statistics. They suggest that all of the solvent exchange methods damage the OPC surface and produce surface structures not seen in the freeze-dried samples. Caution is therefore warranted in the use of solvent exchange in the preparation of samples for the study of the dissolution of OPC surfaces.     

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.

     

Aspects of Portland cement hydration studied using atomic force microscopy

Aspects of the mechanisms of hydration and microstructural evolution in Portland cement are still not fully understood. Atomic force microscopy (AFM) is in many ways a powerful tool for investigating changes in surface structure that accompany the hydration of Portland cement, especially because surfaces can be imaged under aqueous solutions at normal temperature, pressure and high magnification. We have investigated changes in the surface characteristics of sections of Portland cement clinker immersed initially in saturated calcium hydroxide solution which was then replaced by water, and in sucrose solution. In the case of the former, the observations are consistent with the early formation of a protective membrane and the subsequent growth of calcium silicate hydrate (CSH) structures by an osmotic process. The dissolution of the clinker in sucrose solution has also been directly observed. It is concluded that the use of AFM will help to resolve many questions relating to cement hydration.Sadly deceased.Editors note: I am very saddened by the sudden loss of Derek Birchall and acknowledge his outstanding contribution to the subject and this journal over many years.     

Effect of barium hydrosilicates on the early hydration rate of Portland cement

We have studied the effect of a barium hydrosilicate-based modifier on the phase composition of Portland cement hydration products. The results demonstrate that the addition of a modifier containing barium hydrosilicates, silicic acid, and calcium carbonate makes it possible to reduce the nucleation rate of crystals of new phases during the induction period of the hydration process, increase alite (3СaO · SiO₂) hydration, and reduce the rate of aluminate phase (3CaO · Al₂O₃) hydration. The use of such a modifier increases the degree of cement hydration and the amount of calcium hydrosilicates and reduces the amount of forming portlandite and ettringite, thereby improving the mechanical properties and durability of the set cement.     

Study on hydration of Portland cement with fly ash using electrical measurement

The electrical resistivity method was used for measuring the electrical resistivity of cement paste incorporating with fly ash. The study found that the bulk electrical resistivity ρ(t) was a function of the solution electrical resistivity ρ0(t) and porosity Φ. A two-component model was proposed, in which ρ(t) was dominated by ρ0(t) at the early period of hydration and dominated by Φ at a later period. The porosity formation curve was derived from the 2-point method. A logarithmic equation ρ(t)=K^*In(t/t₀) was proposed to express the electrical resistivity development with time after hardening, where K represents the rate of hydration, and the pastes with fly ash had a lower K. The setting times and compressive strengths were tested and the results were compared with the electrical properties.     

碳酸化钢渣和矿渣作硅酸盐水泥混合材水化性能研究

通过对钢渣碳酸化前后的硅酸盐相提取及水化放热性能和将碳酸化钢渣和矿渣作为混合材的硅酸盐水泥的胶砂强度和水化产物种类的测定,以及对它们微观形貌的观察,研究了碳酸化钢渣对胶凝体系水化性能的影响.结果表明,碳酸化使钢渣中硅酸盐相的含量由47.06％下降至14.38％;碳酸化促进了钢渣的早期水化,抑制其后期水化;在配比相同的条件下,碳酸化钢渣-矿渣-硅酸盐熟料体系试样的3、28d抗压强度较未碳酸化钢渣-矿渣-硅酸盐熟料体系试样的高;碳酸化生成的CaCO3促进了熟料的水化;碳酸化钢渣促进了胶凝体系中AFt的生成,且生成水合碳铝酸钙.     

Dielectric and electrical properties of ordinary Portland cement and slag cement in the early hydration period

The dielectric constant and electrical conductivity of ordinary Portland cement (OPC) with water-cement ratios (w/c) of 0.30, 0.35 and 0.40 were measured for the first 30 h hydration, using a microwave technique in the frequency range 8.2–12.4 GHz. It was found that both the dielectric constant and electrical conductivity of the cement paste are sensitive to the water-cement ratio, the higher the w/c value, the greater the dielectric constant and electrical conductivity, and the longer the hydration time. We also found that the higher the frequency the greater the electrical conductivity but the smaller the dielectric constant. The dielectric constant and electrical conductivity of high- and low-slag cement with water-solid ratio (w/s) of 0.40 were measured in the first 30 h after mixing. The changes in dielectric constant and electrical conductivity of low-slag cement with time are similar to that of OPC, but the high-slag cement shows very different dielectric and electrical properties compared with OPC and low-slag cement. The relationship between the dielectric and electrical properties of cement paste and cement hydration was also discussed.     

Mitigating the effects of system resolution on computer simulation of Portland cement hydration

CEMHYD3D is an advanced, three-dimensional computer model for simulating the hydration processes of cement, in which the microstructure of the hydrating cement paste is represented by digitized particles in a cubic domain. However, the system resolution (which is determined by the voxel size) has a prominent influence on the simulation results and, thus, is difficult to choose a priori. In this paper, it is shown that the effects of system resolution on the simulation results are mainly due to the lack of considerations of the diffusion-controlled reactions in the model. A new concept “hydration layer” is proposed for mitigating the effects of system resolution on the model predictions. By performing simulations with different system resolutions, the robustness of the improved model is demonstrated. Comparisons of model predictions with experimental measurements further demonstrate that the use of hydration layer can successfully mitigate the bias brought by the system resolution.     

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.     

A conduction calorimetric study of early hydration of ordinary Portland cement/high alumina cement pastes

Conduction calorimetry was applied to an investigation of the early hydration of ordinary Portland cement (OPC)/high alumina cement (HAC) pastes. Three different rate of heat-evolution profiles were observed, depending on the HAC/OPC ratio. Relevant processes affecting heat development include ettringite formation, HAC and OPC hydration. Results from SEM examination and X-ray diffraction studies are also presented. An acceleration of OPC hydration was observed in pastes containing less than 12.5% HAC. A similar acceleration effect on HAC hydration was also obtained with the addition of OPC. A large amount of ettringite was formed and OPC hydration delayed in the pastes containing 15%–30% HAC. The latter could be one of the factors attributed to poor strength development in these HAC/OPC systems. Early hydration mechanisms of OPC/HAC systems are also discussed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Investigating entrained air voids and Portland cement hydration with low-temperature scanning electron microscopy

This paper describes the application of low temperature scanning electron microscopy to the materials science of Portland cement. The details of low-temperature scanning electron microscopy are described, along with a number of specimen preparation techniques. There are three main research topics presented in this paper: (1) ice morphology in entrained air voids, (2) development of air voids during early hydration and (3) progression of hydration in Portland cement. The first research focus examines ice in air voids at freezing temperatures, and various cement paste ages. The second research focus tracks the development of the air voids during the first hour of hydration. In the third research focus, the progression of hydration with and without accelerating and retarding admixtures is described. Each of these research programs demonstrates how low-temperature scanning electron microscopy can be an effective tool in Portland cement research.     

Observation of chemical hydration in inorganic mineral fibres

An experimental study has been carried out and it was observed that inorganic mineral fibres of the slag wool and rock wool types exhibit a physical disintegration and morphological modifications on the fibre surface when these fibrous products have been exposed to water vapour or immersed in a liquid water medium under heating at temperatures up to 100° C for a period of several months. The physical disintegration transformed the fine and thread-like fibres into small segments and granules. Scanning electron microscopic examinations showed that a rough and eroded fibre surface was formed for those fibres which have been immersed in water and heated at temperatures below 60° C for about 3 months and that a reticular network structure or honeycomb morphology was developed on the Jibre surface for those fibres which have been immersed in water and heated at temperatures above 85° C and up to 100° C for about 4 weeks. Measurement of the pH value of the water solution and thermogravimetric analysis of the disintegrated fibres suggest that these modifications of the fibrous products are caused by two possible mechanisms: chemical leaching and chemical hydration. Thermal conductivity measurements showed that the disintegrated fibres have thermal insulating values about 15% lower than that of the normal and thread-like fibres.     

The impact of zirconium oxide radiopacifier on the early hydration behaviour of white Portland cement

Zirconium oxide has been identified as a candidate radiopacifying agent for use in Portland cement-based biomaterials. During this study, the impact of 20 wt.% zirconium oxide on the hydration and setting reactions of white Portland cement (WPC) was monitored by powder X-ray diffraction (XRD), ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR), transmission electron microscopy (TEM) and Vicat apparatus.The presence of 20 wt.% zirconium oxide particles in the size-range of 0.2 to 5 μm was found to reduce the initial and final setting times of WPC from 172 to 147 min and 213 to 191 min, respectively. Zirconium oxide did not formally participate in the chemical reactions of the hydrating cement; however, the surface of the zirconium oxide particles presented heterogeneous nucleation sites for the precipitation and growth of the early C-S-H gel products which accelerated the initial setting reactions. The presence of zirconium oxide was found to have little impact on the development of the calcium (sulpho)aluminate hydrate phases.     

Early chemical shrinkage due to dissolution and hydration of cement

New investigations into very early volume change of aqueous cement dispersions allow us to understand better what is meant by the general term ‘chemical shrinkage’. In fact, early chemical shrinkage should be subdivided into dissolution shrinkage and hydration shrinkage, the latter taking place before as well as after the induction period. Shrinkage is evaluated by picnometry and by linear measurement carried out in three directions by means of an experimental set-up specially built to measure endogenous shrinkage right from the time after mixing. As expected, the volume of dissolved cement is a function of w/c-ratio, whereas the volume of early hydrated cement is independent of w/c-ratio. This relation, is linear and the slope as well as the intersection of the straight line with the ordinate vary depending on the cement composition. Therefore, these values may be considered to be indicating characteristic cement properties. In this way it becomes possible to classify cements and to predict the extent of early chemical shrinkage of mortar and concrete by just knowing their w/c-ratio.