Autogenous deformations of cement pastes: Part II. W/C effects, micro-macro correlations, and threshold values

A broad experimental study has been performed, from the end of mixing up to 2 years, on a set of plain cement pastes prepared with the same type I ordinary portland cement (OPC) and various water-to-cement ratios (W/C), and cured at various constant temperatures. Several parameters have been measured on the hydrating materials, such as chemical shrinkage, volumetric and one-dimensional autogenous deformations, degree of hydration of the cement, Ca(OH)₂ content and Vicat setting times. Drying shrinkage has also been measured on the mature materials. In this part II of the paper, the effects of W/C within the range 0.25-0.60 have in particular been analysed in relation to the microstructural characteristics of the materials. This micro-macro analysis has highlighted a W/C threshold value (located around 0.40) both at the macro-level (on autogenous, but also on drying deformations and durability-related properties) and at the micro-level (characteristics of the hydration products, MIP porosity and pore size distribution, etc.).In addition, volumetric and one-dimensional autogenous shrinkage deformations have been compared in the case of W/C=0.25 and T=20 °C. Finally, a critical twofold (chemical and structural) effect of calcium hydroxide has been found. When significant structural effects, generated by the formation and the growth of large-size Ca(OH)₂ crystals, take place, swelling can become prominent, as observed for one-dimensional autogenous deformations in the case of medium and high W/C, and deviations are recorded on linear relationships.     

Autogenous deformations of cement pastes: Part I. Temperature effects at early age and micro-macro correlations

A micro-macro experimental study has been performed, from the end of mixing up to 2 years, on a set of plain cement pastes prepared with the same type I ordinary Portland cement (OPC) and various water-to-cement ratios (W/C), and cured at various constant temperatures. In this part I of the paper, volumetric autogenous shrinkage has been analysed in relation to various parameters characterizing the hydration process: chemical shrinkage, degree of hydration of the cement, Ca(OH)₂ content and Vicat setting times, within the early-age period (≤24 h). The effects of the curing temperature (ranging from 10 up to 50 °C) have in particular been investigated. Its effects recorded on both the rate and the magnitude of volumetric autogenous shrinkage vs. time have pointed out the irrelevance of the usual maturity concept to describe such phenomenon within the whole early-age period. An improved maturity concept has hence been proposed. It is based on separating the early-age period in different phases and on using chemical shrinkage data for the calculation of the apparent activation energy applied to the prediction of autogenous deformations occurring after the setting period. Furthermore, micro-macro relationships have been pointed out, illustrating in particular the determining role of Ca(OH)₂.     

Influence of steam curing on the compressive strength of concrete containing supplementary cementing materials

Heat treatment is widely used to accelerate the strength-gaining rate of concrete. In general, the ultimate strengths of the heated-treated concrete are lower than those of the standard cured specimens. When ultrafine fly ash (UFA) is included in concrete, the pozzolanic reaction is accelerated through the heat treatment. Sometimes, various chemical activators were used to activate the reactivity of fly ash. In the current study, UFA and slag were used as a replacement for cement, steam curing and chemical activators were used to accelerate hydration of cement and fly ash, and then compared with moist curing. This paper presents the influence of steam curing on the compressive strength of concrete containing UFA with or without slag. The experimental results indicated that the concrete containing UFA has low early strength after 13-h steam curing and that the difference between the 28-day compressive strength of concrete through 13-h steam curing and that of moist-cured concrete is large, but the concrete with UFA and CaSO₄ or Ca(OH)₂ has a high early strength, thus, the reactivity of fly ash must be accelerated. Concrete containing UFA and ground slag was prepared, whose compressive strengths were improved.     

Influence of the availability of calcium on the hydration of tricalcium aluminate (C3A) in seawater-mixed C3A–gypsum system

This work examined the effects of seawater (SW) on the hydration of tricalcium aluminate (C₃A) in C₃A–gypsum and C₃A–gypsum–Ca(OH)₂ systems through the characterization of hydration heat release, the evolution of aqueous phase composition and hydration products with the hydration time. It was found that SW increased the dissolution driving force of C₃A and solubility of gypsum, which accelerated the early hydration of C₃A and the formation of ettringite (AFt), leading to a higher hydration degree of C₃A at an early age compared with the deionized (DI) water–mixed pastes. After gypsum depletion to form AFt, and in the absence of Ca(OH)₂, the formation of chloroaluminate hydrates was slower due to the insufficient Ca resulted in an accumulation of Al in solution. This would delay the subsequent transformation of AFt to monosulfate (SO₄–AFm) and the formation of hydrogarnet (C₃AH₆), which would further reduce the hydration degree of the C₃A at the later ages. However, in the presence of Ca(OH)₂, the hydration degree of C₃A–gypsum–Ca(OH)₂ at later ages was increased, which was similar to that of the corresponding DI pastes. This can be inferred that the amount of Ca available in SW-mixed cement concrete can affect the hydration degree of C₃A in cement.     

Pozzolanic properties of reject fly ash in blended cement pastes

Low-grade fly ash (reject fly ash, r-FA), a significant portion of the pulverized fuel ash (PFA) produced from coal-fired power plants and rejected from the ash classifying process, has remained unused due to its high carbon content and large particle size. But it may be used in certain areas, such as in solidification and stabilization processes of hazardous waste and materials for road base or subbase construction, which require relatively lower strength and reactivity. It is therefore necessary to extend research on the properties of r-FA and explore its possible applications. This paper presents experimental results of a study on the mechanical and hydration properties of cementitious materials prepared by blending r-FA with ordinary Portland cement (OPC). Parallel mixes were also prepared with the good ash [i.e., classified fine fly ash (f-FA)] for comparison. Selective chemical activators were added to the mix to study the effects of the activators on the properties of the blend system. The results show that r-FA generally has a lower rate of hydration than f-FA particularly at the early stage of hydration. Adding Ca(OH)₂ alone almost had no effect on accelerating the hydration of r-FA. But adding a small quantity of Na₂SO₄ or K₂SO₄ together with Ca(OH)₂ significantly accelerated the hydration reaction. The results of the compressive strength measurement correlated nicely with the degree of hydration results. It was also found that water-to-binder ratio (w/b) was an important factor in affecting the strength development and the hydration degree of r-FA pastes.     

Electrical conductivity and rheological properties of ordinary Portland cement-silica fume and calcium hydroxide-silica fume pastes

Two groups of solids mixtures were prepared: (i) the first group includes four mixes having different ordinary Portland cement/silica fume (OPC/SF) weight ratios and (ii) the second group consists of four blends having different Ca(OH)₂/SF molar ratios. Electrical conductivity measurements were carried out on the pastes of the first group mixes using two initial water-to-solid (W/S) ratios of 0.55 and 0.70 by weight; while the W/S ratios used for the second group mixes were 1.00 and 1.20 by weight. The measurements were done at 25 and 45 °C for each paste during setting and hardening processes after gauging with deionized water. Rheological properties were studied at room temperature for all mixes using various W/S ratios. The results obtained indicate clearly the effect of SF and W/S ratio on the rheological properties and electrical conductivity of all pastes under investigation. The relation between the electrical conductivity and rheological properties for different mixes were discussed based on the chemical nature and physical state of the hydration products formed at early ages of hydration.     

Structural and hydration properties of amorphous tricalcium silicate

Mechanical milling was carried out to synthesize amorphous tricalcium silicate (Ca₃SiO₅) sample, where Ca₃SiO₅ is the most principal component of Portland cement. The partial phase transformation from the crystalline to the amorphous state was observed by X-ray and neutron diffractions. Moreover, it was found that the structural distortion on the Ca-O correlation exists in the milled Ca₃SiO₅. The hydration of the milled Ca₃SiO₅ with D₂O proceeds as follows: the formation of hydration products such as Ca(OD)₂ rapidly occurs in the early hydration stage, and then proceeds slowly after about 15 h. The induction time for the hydration of the milled Ca₃SiO₅ is approximately one half shorter than that for the hydration of the unmilled one. This result means that the mechanical milling brings about the chemical activity of Ca₃SiO₅ for hydration, and may be particularly useful for increasing the reactivity in the early hydration stage.     

Effect of calcite on lead-rich cementitious solid waste forms

The effect of calcite on lead-rich solidified waste forms generated using Portland cement has been investigated. Samples of cementitious wastes in the absence and presence of Pb and in the absence and presence of calcite were examined separately at 2, 7, 14 and 28 days of hydration by X-ray diffraction and SEM/EDS and for compressive strength. The presence of lead was observed to produce lead carbonate sulfate hydroxide (Pb₄SO₄(CO₃)₂(OH)₂), lead carbonate hydroxide hydrate (3PbCO₃·2Pb(OH)₂·H₂O) and two other unidentified lead salts in cavity areas, and was observed to significantly retard the hydration of cement. Calcite addition to the Pb wastes was found to induce the rapid crystallization of calcium hydroxide coincident with the onset of C-S-H gel germination. The rapid dissolution of lead precipitates was observed with the subsequent development of very insoluble gel products of the form C-Pb-S-H. These products are formed by chemical incorporation of re-dissolved Pb species into silicate structures.     

Early age hydration of rice hull ash cement examined by transmission soft X-ray microscopy

Early age hydration of barium-doped β-Ca₂SiO₄ cement, produced from rice hull ash (RHA), is examined by transmission soft X-ray microscopy. Use of low-energy cements produced from by-product materials, such as the cement considered here, may be economically and environmentally advantageous. However, the hydration kinetics and morphology and composition of the products of RHA-based β-Ca₂SiO₄ cements have not been investigated. Observation of the early age cement hydration shows evidence of cement dissolution and hydration product formation, including the formation of Hadley grains. The rates of the reaction and amount product formed appear to be related to the hydrothermal processing temperature and the chemical composition of the cement. That is, more rapid hydration is observed for barium-doped RHA cements produced at higher temperatures and for cements produced with higher barium contents, within the ranges examined.     

An X-Ray Diffraction,Fourier-Transform Infrared Spectroscopy,and Scanning Electron Microscopy/Energy-Dispersive Spectroscopic Investigation of the Effect of Sodium Lignosulfonate Superplasticizer on the Hydration of Portland Cement Type V

This study investigated the effects of a common superplasticizer, ligno-sulfonate, on the hydration of Portland cement Type V. Samples of plain cement and superplasticizer-treated cement have been examined by x-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy/energy-dispersive spectroscopy. Lignosulfonate has been observed to retard the hydration of cement through specific surface chemical reactions which involve Ca²⁺ ions in pore solution. The admixture has been found to retard the formation of Ca(OH)₂ and stabilize ettringite [Ca₆(Al₂2O₆)(SO₄)₃.32H₂O]. The inhibition of the rate of conversion of ettringite to monosulfate [Ca₄Al₂(OH)₁₂.SO₄.6H₂O] is attributed to charge-controlled reactions caused by large quantities of Ca²⁺ ions from initial hydration reactions. Leaching of the admixture doped cement by water-removed lignosulfonate and caused complete hydration of cement. A charge-controlled-reaction model involving the Ca²⁺ ions is proposed to explain the role of the admixtures during hydration of cement.     

Differential thermal method of estimating calcium hydroxide in calcium silicate and cement pastes

DTA is applied to estimate Ca(OH)₂ in cementitious phases by determining the peak areas caused by the decomposition of Ca(OH)₂ to CaO+H₂O. In the hydration of C₃S generally, the chemical method yields slightly higher values. DTA is also used as a monitoring technique in preparing a practically Ca(OH)₂-free product from hydrated portland cement or hydrated C₃S. Hydrated portland cement or C₃S has now been exposed to an unsaturated Ca(OH)₂ solution and extraction continued until the sample indicate no endothermal peak for Ca(OH)₂. The thermal method permits determination of the rate of formation of Ca(OH)₂ in portland cement hydrated in the presence of 0, 1, 2 and $\text{3}\text{1}\text{2}$ per cent CaCl₂.     

Drying and autogenous shrinkage of pastes and mortars with activated slag cement

Activated slag cement (ASC) shows significantly higher shrinkage than ordinary Portland cement agglomerates. Cracking generated by shrinkage is one of the most critical drawbacks for broader applications of this promising alternative binder. This article investigates the relationship between ASC hydration, unrestrained drying and autogenous shrinkage of mortar specimens. The chemical and microstructure evolution due to hydration were determined on pastes by thermogravimetric analysis, conduction calorimetry and mercury porosimetry. Samples were prepared with ground blast furnace slag (BFS) activated with sodium silicate (silica modulus of 1.7) with 2.5, 3.5 and 4.5% of Na₂O, by slag mass. The amount of activator is the primary influence on drying and autogenous shrinkage, and early hydration makes a considerable contribution to the total result, which increases with the amount of silica. Drying shrinkage occurred in two stages, the first caused by extensive water loss when the samples were exposed to the environment, and the second was associated with the hydration process and less water loss. Due to the refinement of ASC porous system, autogenous shrinkage is responsible for a significant amount of the total shrinkage.     

Studies on blended cement with a large amount of fly ash

The influence of the contents of the clinker, activators and fly ash on the properties of blended cement with high fly ash content was studied. Experimental data from X-ray diffraction and pore size distribution indicated that the main hydration product of the fly ash blended cement was C-S-H gel, ettringite and a small amount of Ca(OH)₂. The volume porosity of the pores with diameter bigger than 0.1 μm was lower than that of the micro pores and gel pores with diameter lower than 0.05 μm. The amount of chemical combined water has increased with the curing age duration, while the content of Ca(OH)₂ has reduced after 7 days.     

Influence of solution chemistry on the hydration of polished clinker surfaces—a study of different types of polycarboxylic acid-based admixtures

The influence of two different types of polycarboxylic acid-based superplasticizers (homo- [HP] and copolymer [CP]) on the early hydration of Portland cement clinker is investigated. Polished clinker surfaces are hydrated in aqueous phase extracted from cement paste with and without superplasticizers as well as in saturated Ca(OH)₂/CaSO₄ solution and in deionized water. Scanning electron microscopy (SEM) is used to characterize the early hydration products after 30 min and after 24 h of hydration. Contrary to the hydration reaction with deionized water, in the presence of ionic solutions, the texture and morphology of the clinker surface are significantly altered, and amorphous and crystalline hydration products, mainly ettringite, are formed. The amount and type of ettringite formed are strongly related to solution chemistry and type of superplasticizer. In the presence of superplasticizers, the formation of hydration products and ettringite is reduced and the hydration of the surface is suppressed. Furthermore, the rate of hydration in mortar is studied in the presence of the two admixtures by measuring the temperature evolution as a function of time. The concentration-dependent delay in temperature evolution correlates well with the microscopic observations of different amounts of ettringite formation.     

Effects of flue gas desulphurization sludge on the pozzolanic reaction of reject-fly-ash-blended cement pastes

Reject fly ash (rFA), a coarse portion of the pulverized fuel ash (PFA) produced from coal-fired power plants and rejected from the ash classifying process, has remained unused due to its high carbon content and large particle size (>45 μm). However, the reject ash may have potential uses in chemical stabilization/solidification (S/S) processes that require relatively low strength and low chemical reactivity. Flue gas desulphurization (FGD) sludge is a by-product of the air pollution control process in coal-fired power plants. Its chemical composition is mainly gypsum. As there is no effective usage of both of these materials, it was of interest to conduct research on the possible activation of rFA using FGD. This paper presents experimental results of the effect of FGD on the pozzolanic reaction of rFA-blended cement pastes with or without Ca(OH)₂ and chemical activators. The results show that FGD take effect as an activator only at late curing ages. Adding Ca(OH)₂ activates the hydration of rFA. Chemical activator, such as alkali sulphate, is more effective in enhancing the strength development and degree of hydration of rFA than CaCl₂ in the rFA-Ca(OH)₂-cement system. But CaCl₂ is more effective in the rFA-Ca(OH)₂-FGD-cement system. The chemical activators speed up the reaction of the rFA through the formation new hydration products and elevating the pH value.     

Investigation of the carbonation front shape on cementitious materials: Effects of the chemical kinetics

Carbonation depth-profiles have been determined by thermogravimetric analysis and by gammadensitometry after accelerated carbonation tests on ordinary Portland cement (OPC) pastes and concretes. These methods support the idea that carbonation does not exhibit a sharp reaction front. From analytical modelling, this feature is explained by the fact that the kinetics of the chemical reactions become the rate-controlling processes, rather than the diffusion of CO₂. Furthermore, conclusions are drawn as to the mechanism by which carbonation of Ca(OH)₂ and C-S-H takes place. Carbonation gives rise to almost complete disappearance of C-S-H gel, while Ca(OH)₂ remains in appreciable amount. This may be associated with the CaCO₃ precipitation, forming a dense coating around partially reacted Ca(OH)₂ crystals. The way in which CO₂ is fixed in carbonated samples is studied. The results indicate that CO₂ is chemically bound as CaCO₃, which precipitates in various forms, namely: stable, metastable, and amorphous. It seems that the thermal stability of the produced CaCO₃ is lower when the carbonation level is high. It is also shown that the poorly crystallized and thermally unstable forms of CaCO₃ are preferentially associated with C-S-H carbonation.     

Working mechanism of calcium nitrate as an accelerator for Portland cement hydration

Precast concrete, cold weather concreting, and the emerging technique of concrete additive manufacturing are applications in which the acceleration of cement hydration plays a critical role. To allow precise control of early cement hydration in these applications, a thorough understanding of the working mechanisms of cement hydration accelerators is required. This study contributes to the understanding of the mechanism by which calcium nitrate (Ca(NO₃)₂) influences early cement hydration. The influence of Ca(NO₃)₂ on the hydration of an ordinary Portland cement has been followed by isothermal calorimetry, in situ X-ray diffraction (XRD), quantitative XRD, compressive strength testing, and the analysis of the pore solution composition. Further, the initial pore solution, the initial phase composition, and the phase composition in the fully hydrated cement have been estimated by thermodynamic calculations to corroborate the experimentally obtained results. The results indicate that Ca(NO₃)₂, especially at the highest analyzed dosage of 5 wt.%, enhances the formation of ettringite and a nitrate-containing AFm phase. Furthermore, Ca(NO₃)₂ accelerates alite hydration. Besides the increased Ca concentration in solution, it has been found that a reduction of the Al concentration in the initial pore solution by Ca(NO₃)₂ possibly contributes to the accelerating effect of Ca(NO₃)₂ on alite hydration.     

Autogenous shrinkage of concrete: a balance between autogenous swelling and self-desiccation

According to physical analyses, the driving force of autogenous shrinkage of concrete is the change in the capillary pressure induced by self-desiccation in its cement matrix. Self-desiccation is caused by the balance between the absolute volume reduction (chemical shrinkage) and the building up of the capillary network. The aim of this study was to quantify the influence of the cement characteristics on the chain of mechanisms leading from hydration to autogenous deformations. Four parameters were selected: (i) for clinker, the amount of C₃A and free lime and the SO₃/K₂O ratio; (ii) for cement, the fineness. To master the experimental area, 16 cements were prepared at the laboratory from pure raw materials. An important number of characterizing techniques were used in the experimental study. Their choice was based on the important parameters drawn from the physical analysis: setting time, suspension-solid transition, hydration kinetics through isothermal calorimetry and nonevaporable water, chemical shrinkage, evolution of relative humidity, capillary porosity and autogenous shrinkage. Using different techniques allowed to determine the precise mechanism of action of each parameter. Results showed that these mechanisms are generally different, even if their macroscopic consequences may be identical. This point will probably be useful for modeling and determining the industrial keys reducing the autogenous shrinkage. The physical mechanisms involved in autogenous deformations were further understood. In particular, this study shows that initial autogenous shrinkage should be considered as a balance between the self-desiccation and an initial swelling phase. The influence of the four parameters considered on this last phenomenon were also characterized.     

A critical comparison of 3D experiments and simulations of tricalcium silicate hydration

Advances in nano‐computed X‐ray tomography (nCT), nano X‐ray fluorescence spectrometry (nXRF), and high‐performance computing have enabled the first direct comparison between observations of three‐dimensional nanoscale microstructure evolution during cement hydration and computer simulations of the same microstructure, using HydratiCA. nCT observations of a collection of triclinic tricalcium silicate () particles reacting in a calcium hydroxide solution are reported and compared to simulations that duplicate, as nearly as possible, the thermal and chemical conditions of those experiments. Particular points of comparison are the time dependence of the solid phase volume fractions, spatial distributions, and morphologies. Comparisons made at 7 hours of reaction indicate that the simulated and observed volumes of consumed by hydration agree to within the measurement uncertainty. The location of simulated hydration product is qualitatively consistent with the observations, but the outer envelope of hydration product observed by nCT encloses more than twice the volume of hydration product in the simulations at the same time. Simultaneous nXRF measurements of the same observation volume imply calcium and silicon concentrations within the observed hydration product envelope that are consistent with Ca(OH)₂ embedded in a sparse network of calcium silicate hydrate (C–S–H) that contains about 70% occluded porosity in addition to the amount usually accounted as gel porosity. An anomalously large volume of Ca(OH)₂ near the particles is observed both in the experiments and in the simulations, and can be explained as originating from the hydration of additional particles outside the field of view. Possible origins of the unusually large amount of observed occluded porosity are discussed.     

Studies on improving moisture resistance of sulfur-infiltrated mortars

High-strength products can be made by sulfur infiltration of dried portland cement mortars or concretes. However, on exposure to moisture, strength deterioration takes place due to complex chemical reactions involving Ca(OH)₂ and sulfur.This paper presents the results of two series of tests. In the first series an attempt was made to inhibit the reaction between sulfur and Ca(OH)₂ by coating the Ca(OH)₂ with a film of insoluble calcium salts. In the second series, cements which did not contain Ca(OH)₂ in hydration products were investigated for sulfur infiltration. Both the approaches gave encourging results in improving moisture resistance of sulfur-infiltrated mortars.