Thermodynamic modeling of hydrated white Portland cement–metakaolin–limestone blends utilizing hydration kinetics from 29Si MAS NMR spectroscopy

Hydration kinetics for the principal phases of Portland cement blends have been incorporated in thermodynamic modeling (GEMS package), utilizing degrees of hydration from ²⁹Si MAS NMR. An empirical relationship for the reaction of these phases is established which includes three variable parameters that all can be estimated from the degrees of hydration. This approach is compared with thermodynamic equilibrium modeling (full hydration) for white Portland cement–metakaolin (0–30 wt.%) blends and for ternary blends of white Portland cement (65 wt.%)–metakaolin–limestone. The predicted phase assemblages have been compared with the phases identified by XRD, ²⁷Al and ²⁹Si MAS NMR which reveals that the incorporation of hydration kinetics improves the agreement between modeling and experiments. The results show also that the formation of strätlingite depends critically on the quantity of charge-balancing anions in the AFm phases, especially carbonate and sulfate anions, and on the degree of hydration for metakaolin.     

Effect of temperature on the hydration of Portland cement blended with siliceous fly ash

The effect of temperature on the hydration of Portland cement pastes blended with 50 wt.% of siliceous fly ash is investigated within a temperature range of 7 to 80 °C.The elevation of temperature accelerates both the hydration of OPC and fly ash. Due to the enhanced pozzolanic reaction of the fly ash, the change of the composition of the C–S–H and the pore solution towards lower Ca and higher Al and Si concentrations is shifted towards earlier hydration times. Above 50 °C, the reaction of fly ash also contributes to the formation of siliceous hydrogarnet. At 80 °C, ettringite and AFm are destabilised and the released sulphate is partially incorporated into the C–S–H. The observed changes of the phase assemblage in dependence of the temperature are confirmed by thermodynamic modelling.The increasingly heterogeneous microstructure at elevated temperatures shows an increased density of the C–S–H and a higher coarse porosity.     

Characterization of behavior and cracking of a cement paste confined between spherical aggregate particles

In this work, the cement–grain interactions in a pair of 8 mm spherical grains of quartz or calcite, linked by a Portland cement, are analyzed experimentally at a local scale (cement–grain interface) during mechanical testing. The volume of the cement paste, the water/cement ratio and the gap between the grains are constant. Examined samples are conditioned in an atmosphere of constant temperature (21 °C) and high relative humidity (~ 100%) to minimize drying shrinkage. At various stages of hydration, the samples are subjected to tensile or compressive tests, monitored by a high-speed camera. Stiffness and force at rupture are estimated and correlated to the crack initiation and propagation followed through image analysis. The results concern mechanical properties and kinetics of cracking in function of hydration time, for both types of loading and both types of grain. Such local characteristics will be used to propose local interaction laws and to support numerical modeling of the concrete at macroscopic scale.     

The effect of gyrolite additive on the hydration properties of Portland cement

The influence of gyrolite additive on the hydration properties of ordinary Portland cement was examined. It was found that the additive of synthetic gyrolite accelerates the early stage of hydration of OPC. This compound binds alkaline ions and serves as a nucleation site for the formation of hydration products (stage I). Later on, the crystal lattice of gyrolite becomes unstable and turns into C–S–H, with higher basicity (C/S ~ 0.8). This recrystallization process is associated with the consumption of energy (the heat of reaction) and with a decrease in the rate of heat evolution of the second exothermic reaction (stage II). The experimental data and theoretical hypothesis were also confirmed by thermodynamic and the apparent kinetic parameters of the reaction rate of C₃S hydration calculations. The changes occur in the early stage of hydration of OPC samples and do not have a significant effect on the properties of cement stone.     

Hydration kinetics modeling of the effect of curing temperature and pressure on the heat evolution of oil well cement

The heat evolution of Class G and Class H oil well cements cured under different temperatures (25 °C to 60 °C) and pressures (2 MPa to 45 MPa) was examined by isothermal calorimetry. Curing pressure was found to have a similar effect on cement hydration kinetics as curing temperature. Under isothermal and isobaric conditions, the dependency of cement hydration kinetics on curing temperature and pressure can be modeled by a scale factor which is related to the activation energy and the activation volume of the cement. The estimated apparent activation energy of the different cements at 2 MPa varies from 38.7 kJ/mol to 41.4 kJ/mol for the temperature range of 25 °C to 40 °C, which decreases slightly with increasing curing temperature and pressure. The estimated apparent activation volume of the cements at 25 °C varies from − 23.1 cm³/mol to − 25.9 cm³/mol for the pressure range studied here, which also decreases slightly in magnitude with increasing curing temperature.     

Change in reaction kinetics of a Portland cement caused by a superplasticizer — Calculation of heat flow curves from XRD data

The hydration process of a commercial Portland cement was followed by means of heat flow calorimetry. The measured heat flow was compared with calculated heat flow curves based on XRD data. Examined in particular was the influence of one selected superplasticizer on the hydration of the Portland cement. It was shown that the superplasticizer in question retards both the aluminate reaction and the silicate reaction. It is certainly conceivable that there are more than only one explanation for the interaction between the superplasticizer and the cement. A complexation of Ca^2 + ions from pore solution by the superplasticizer is as thinkable as the adsorption of the polymer on the nuclei or the anhydrous grain surfaces which in turn might lead to the prevention of the growth of the nuclei or the dissolution of the anhydrous grains.     

Hydration of Portland cement with high replacement by siliceous fly ash

The effects of two different low calcium fly ashes on the hydration of ordinary Portland cement (OPC) pastes containing 50 wt.% of fly ash were investigated over a hydration time of 550 days. The results were compared with a reference blend of OPC containing 50 wt.% of inert quartz powder allowing the distinction between "filler effect" and pozzolanic reaction.Until 2 days, no evidence of fly ash reaction was measured and its influence on the hydration is mainly related to the “filler effect”. From 7 days on, the effects of the pozzolanic reaction were observed by the consumption of portlandite, the change of the pore solution chemistry, the formation of a presumably water-rich inner hydration product and the change of the C–S–H composition towards higher Al/Si ratio compared to the C–S–H of neat OPC. Additional strength due to the pozzolanic reaction developed after 28 days of hydration.     

Phase changes during the early hydration of Portland cement with Ca-lignosulfonates

The influence of two Ca-lignosulfonates (LS), one softwood (LSs) and one hardwood (LSh) based, on the phase changes during the early hydration of ordinary Portland cements was investigated using isothermal calorimetry, in-situ XRD, and thermal analysis. In the presence of LS the hydration of C₃S and C₃A was retarded. LS was found to influence the solubility of the sulphate phase; in case of bassanite/gypsum the initial dissolution was accelerated. An acceleration of the initial ettringite formation was observed in the presence of LS. However, the second ettringite formation was retarded. The amounts of bound water (H) and calcium hydroxide (CH) formed were measured using TG/DTG on cement pastes hydrated for 90 min, 5, 12 and 24 h. At 90 min the amount of H was increased the higher the concentration of LS. The amount of CH formed between 5–24 h was decreased the higher the concentration of LS.     

Influence of silica fume on the microstructure of cement pastes: New insights from 1H NMR relaxometry

¹H NMR has been used to characterise white Portland cement paste incorporating 10 wt.% of silica fume. Samples were measured sealed throughout the hydration without sample drying. Paste compositions and C–S–H characteristics are calculated based on ¹H NMR signal intensities and relaxation analysis. The results are compared with a similar study of plain white cement paste. While the presence of silica fume has little influence on C–S–H densities, the chemical composition is impacted. After 28 days of sealed hydration, the Ca/(Si + Al) ratio of the C–S–H is 1.33 and the H₂O/(Si + Al) ratio is 1.10 when 10% of silica fume is added to the white cement. A densification of the C–S–H with time is observed. There are no major changes in capillary, C–S–H gel and interlayer pore sizes for the paste containing silica fume compared to the plain white cement paste. However, the gel/interlayer water ratio increases in the silica fume blend.     

The effect of temperature on the rate of sulfate attack of Portland cement blended mortars in Na2SO4 solution

Sulfate attack on Portland cement and Portland blended cement concretes is a well-researched field. However, the effect of varying temperature on the rate of sulfate attack requires further attention. This laboratory experiment studied temperatures of 23 °C, 10 °C, 5 °C, and 1 °C. Both Portland and Portland limestone cements were studied in combination with several supplementary cementing materials. The mortar bars were submerged in 5% Na2SO4 (33,800 ppm SO₄^2 −) solution for 15–30 months. At higher temperatures the supplementary cementing materials, particularly the fly ashes, greatly improved the resistance to external sulfate attack. At lower temperatures the metakaolin improved the resistance to sulfate attack; the fly ashes had little to no effect on the low-temperature sulfate resistance. The alterations to sulfate resistance are attributed to: dilution of Portland cement in the presence of supplementary cementing materials; additional nucleation sites provided by finely ground SCMs; and the pozzolanic and hydraulic reactions of the SCMs.     

Time-dependent behaviour of hardened cement paste under isotropic loading

The experimental results of isotropic compression tests performed at 20 °C and 90 °C on a class G hardened cement paste hydrated at 90 °C (Ghabezloo et al., 2008, Cem. Conc. Res. 38, 1424–1437) have been revisited considering time-dependent response. Within the frame of a viscoplastic model, the non-linear responses of the volumetric strains as observed in drained and undrained tests and of the pore pressure in undrained tests are analysed. The calibration of model parameters based on experimental data allows to study the effect of the test temperature on the viscous response of hardened cement paste showing that the creep is more pronounced for a higher test temperature. The effect of the hydration temperature on the time dependent behaviour is also studied by evaluating the model parameters for a cement paste hydrated at 60 °C. The time-dependent deformations are more pronounced for hydration at a higher temperature.     

Hydration kinetics modeling of Portland cement considering the effects of curing temperature and applied pressure

A hydration kinetics model for Portland cement is formulated based on thermodynamics of multiphase porous media. The mechanism of cement hydration is discussed based on literature review. The model is then developed considering the effects of chemical composition and fineness of cement, water-cement ratio, curing temperature and applied pressure. The ultimate degree of hydration of Portland cement is also analyzed and a corresponding formula is established. The model is calibrated against the experimental data for eight different Portland cements. Simple relations between the model parameters and cement composition are obtained and used to predict hydration kinetics. The model is used to reproduce experimental results on hydration kinetics, adiabatic temperature rise, and chemical shrinkage of different cement pastes. The comparisons between the model reproductions and the different experimental results demonstrate the applicability of the proposed model, especially for cement hydration at elevated temperature and high pressure.     

Experimental studies and thermodynamic modeling of the carbonation of Portland cement,metakaolin and limestone mortars

The carbonation of Portland cement, metakaolin and limestone mortars has been investigated after hydration for 91 days and exposure to 1% (v/v) CO₂ at 20 °C/57% RH for 280 days. The carbonation depths have been measured by phenolphthalein whereas mercury intrusion porosimetry (MIP), TGA and thermodynamic modeling have been used to study pore structure, CO₂ binding capacity and phase assemblages. The Portland cement has the highest resistance to carbonation due to its highest CO₂ binding capacity. The limestone blend has higher CO₂ binding capacity than the metakaolin blends, whereas the better carbonation resistance of the metakaolin blends is related to their finer pore structure and lower total porosity, since the finer pores favor capillary condensation. MIP shows a coarsening of the pore threshold upon carbonation for all mortars. Overall, the CO₂ binding capacity, porosity and capillary condensation are found to be the decisive parameters governing the carbonation rate.     

Chemistry of cement hydration in polymer-modified pastes containing lead compounds

The role of polymeric additives on the hydration process of cement pastes admixed with a lead compound (Pb₃O₄) was investigated. Three series of pastes were prepared: the reference series, mixing water with Ordinary Portland Cement (OPC), and two series in which whether a styrene–butadiene rubber latex or a superplasticiser based on acrylic-modified polymer was added to the pastes. For each series, 5 and 10 wt% of Pb were mixed with the pastes. Phase analysis and microstructural characterisation were carried out by means of X-ray powder diffraction and SEM–EDX. Thermogravimetric analysis was performed to monitor the hydration degree of the three pastes; indeed, quantitative determination of portlandite and calcite was performed.Dynamic leach tests were performed on solidified monoliths to evaluate the effective immobilisation of Pb₃O₄. After 384 h leaching, excellent results were obtained by pastes mixed with superplasticiser that showed a cumulative release of Pb equal to 0.62 mg/l for samples containing 5 wt% of Pb, and equal to 0.84 mg/l for samples bearing 10 wt% of Pb.     

How to increase the hydration degree of CA — The influence of CA particle fineness

It is shown that the hydration degree of CA is directly dependent on the fineness of CA-particles. Finer particles lead to an increased degree of hydration and also an increased hydration rate.The reaction of a sample with mainly coarse particles of CA (d₅₀ = 50 μm) is characterized by a low hydration rate and only 34 rel.-% of CA dissolved after 22 h. Whereas in a very fine CA-sample (d₅₀ = 4 μm) hydration starts delayed but then shows the highest hydration rate and a dissolution of 62 rel.-% CA. The behaviour is explained by the coverage of CA-particles with a dense hydrate layer of C₂AH_x and AH_x. This reacted CA-rim is supposed to have the same thickness for different sized CA-particles. Optimization of Gauss distribution curves, which were applied to simulate a more realistic particle size distribution, leads to a reacted rim thickness of 1.3 μm until reaction is stopped.     

13C chemical shift anisotropies for carbonate ions in cement minerals and the use of 13C, 27Al and 29Si MAS NMR in studies of Portland cement including limestone additions

¹³C isotropic chemical shifts and chemical shift anisotropy parameters have been determined for a number of inorganic carbonates relevant in cement chemistry from slow-speed ¹³C MAS or ¹³C{¹H} CP/MAS NMR spectra (9.4 T or 14.1 T) for ¹³C in natural abundance. The variation in the ¹³C chemical shift parameters is relatively small, raising some doubts that different carbonate species in Portland cement-based materials may not be sufficiently resolved in ¹³C MAS NMR spectra. However, it is shown that by combining ¹³C MAS and ¹³C{¹H} CP/MAS NMR carbonate anions in anhydrous and hydrated phases can be distinguished, thereby providing valuable information about the reactivity of limestone in cement blends. This is illustrated for three cement pastes prepared from an ordinary Portland cement, including 0, 16, and 25 wt.% limestone, and following the hydration for up to one year. For these blends ²⁹Si MAS NMR reveals that the limestone filler accelerates the hydration for alite and also results in a smaller fraction of tetrahedrally coordinated Al incorporated in the C-S-H phase. The latter result is more clearly observed in ²⁷Al MAS NMR spectra of the cement–limestone blends and suggests that dissolved aluminate species in the cement–limestone blends readily react with carbonate ions from the limestone filler, forming calcium monocarboaluminate hydrate.     

Local study of the corrosion kinetics of hardened Portland cement under acid attack

Whereas the chemical reactions occurring during the acid corrosion of cementitious materials are now well known, their reaction rates have still not been measured. We propose here a methodology using digital holographic interferometry to access to these pure surface reaction rates. This method makes possible the differentiation between the dissolution and precipitation steps. We use it to measure the reaction rate constant of the dissolution of hardened Portland cement in aqueous solutions of nitric, sulfuric and hydrochloric acids at pH 2. This quantity is seen to be unexpectedly similar for the three acids, with a value of the order of 1 mg/m²/s. We have measured the evolution of this reaction rate constant with the pH in nitric acid. We have also measured the real pH of the solution at the material surface, which is always alkaline (pH  > 11), even for attacks by solution with pH as low as 1.     

Effect of mixing on the early hydration of alite and OPC systems

The kinetics of hydration of cementitious materials is sensitive to the mixing procedure. High shear mixing conditions lead to an increase in the kinetics of hydration at early age compared to low shearing conditions such as hand mixing. In this study the effect of mixing speed and procedure was studied on alite and Portland cement in the presence or not of aggregates. The kinetics of hydration was monitored using isothermal calorimetry at 20 °C. The early reactivity was enhanced both with an increase in the speed of mixing and the shearing conditions. The principal features are a shortening of the induction period; a higher rate of hydrate precipitation during the acceleration period as well as an increase in the height of the main heat evolution peak. Analysis of the results in terms of dissolution theory, coupled with quantitative simulation with the μic modelling platform indicate different effects of mixing prior to and after the end of the induction period. Before the end of the induction period mixing has an impact on the rate of dissolution in the fast dissolution regime and high undersaturation, which appears to be (at least partially) controlled by the rate of transport of ions away from the alite surface. After the end of the induction period the main effect of mixing appears to be the production of more C-S-H nuclei, due to the possible detachment of the primary C-S-H (metastable) by mechanical action. This higher nucleation density leads to a denser microstructure for systems mixed at high intensities.     

Hydration reactions and physicochemical properties in a novel tricalcium-dicalcium silicate-based cement containing hydroxyapatite nanoparticles and calcite: A comparative study

The mineral trioxide aggregate (MTA) is Portland type cement whose main application in dentistry is retrograde filling. The purpose of this study was to analyze hydration reactions and physicochemical properties of a new tricalcium-dicalcium silicate-based cement containing nanocrystalline hydroxyapatite (nHAp) and calcite. The new formulation was compared with Biodentine™ and MTA-Angelus™ as control samples.Hydration reactions were monitored by Raman spectroscopy, X-ray diffraction, radiopacity, pH, setting time, and compressive strength. The compressive strength reaches its higher value at 7 days following the sequence: Biodentine™ (104.8 MPa) > Cement + 5% nHAp (59 MPa) > MTAAngelus™ (27.3 MPa), in agreement with the pH values measured at 24 h: Biodentine™, Cements + nHAp or + calcite (10.6–11.6) > MTA-Angelus™ (9.7). Mean setting times was around 30 min and no significative differences (p = 0.0001) were observed. In the Biodentine™ control samples, Ca₃SiO₅ diminishes until disappear at 28 days of hydration. On their turn, calcium silicate hydrate (CSH) increases continuously in the range of time analyzed. The present results suggest that the physicochemical properties were improved for the new cement with nanosized hydroxyapatite nanoparticles and relevant information on chemical properties is of valuable importance for testing predictive models for Biodentine™ and MTA-Angelus™.     

In situ synchrotron X-ray powder diffraction study of the early age hydration of cements blended with zeolitite and quartzite fines and water-reducing agent

A comparison was made between the early-age hydration of cements blended with micronized zeolitite and quartzite powders. The Portland cement replacement in the mixes was 30%, and the effect of introducing a superplasticiser to lower the required water to solid ratio was assessed. The cement pastes were hydrated at 40 °C and monitored in situ by time-resolved synchrotron X-ray powder diffraction combined with Rietveld quantitative phase analysis.The quantitative evolution of phase weight fractions showed that the addition of the zeolite tuff accelerated the hydration rate of the main C₃S cement component. Blending with the quartzite powder of similar fineness did not affect the C₃S hydration rate. Reduction of the water to solid ratio by introduction of the superplasticiser had a retarding effect on the hydration of the zeolitite-blended cement over the early hydration period up to 3 days.The AFt or ettringite reaction products, formed promptly after the addition of water to the mixtures, underwent a crystal structural modification over the induction period up to 4 to 6 hours of reaction. The continuous contraction of the c-cell parameter and expansion of the a-cell parameter towards the ideal values for AFt or ettringite reflects the structural adaptation of the AFt to the changing availability of sulphate over the course of the first hours of hydration. The observed structural changes were less pronounced in the zeolitite blended cement. This is related to the dilution of the overall sulphate content in the blended cement and highlights the need to control and optimise sulphate additions in blended cements.