Contribution of 1H combined rotation and multipulse spectroscopy nuclear magnetic resonance to the study of tricalcium silicate hydration

Hydration products of tricalcium silicate (C₃S) are the calcium silicate hydrates (C–S–H) and portlandite. Silica fume, added to anhydrous cement in industrial formulations, reacts with portlandite and leads to C–S–H slightly different from the previous one (this reaction is called the pozzolanic reaction). C₃S hydration at 120 °C with and without silica fume has been studied by two ¹H nuclear magnetic resonance techniques: combined rotation and multi-pulse spectroscopy (CRAMPS) and magic angle spinning (MAS). The static spectra are broadened by the proton–proton dipolar interaction. The ¹H MAS technique does not allow us to remove the effects of this interaction completely and cannot be in this case a quantitative method. Therefore the CRAMPS technique, which can remove the broadening of the interaction because of the use of a multipulse sequence associated with the sample rotation, was used. It is shown that the CRAMPS NMR spectra allow us to describe the action of silica fume on the hydrates and to reveal the competition between portlandite formation around the C₃S grains and portlandite dissolution near the silica grains until the complete dissolution of portlandite. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence des fumées de silice sur l’évolution de l’hydratation des pâtes de chaux ou de Ciment Portland

The silica fume (SF) is used in civil engineering, in particular for the manufacture of high performance concrete.In order to better understand the gain in strength of the concretes containing SF, the microstructural aspect has been examined.Mixtures of SF–Lime pastes present a hydraulic setting which is due to the formation of a C–S–H phase (calcium silicate hydrate). The latter is semi-crystallized. It is characterized by the lines of X-ray diffraction, hk0 such as: 3,06 Å (220), 2,80 Å (400) and 1,83 Å (040).The mix design SF–Lime paste is thus a simplified approach of that of the mixtures SF–OPC in which the main reaction is the fixation, by the SF, of lime coming from the hydration of C₃S in the form of C–S–H.Tests have been carried out on two varieties of SF resulting from the same furnace with the presence of lime or Portland cement. The results show that the presence of certain impurities, by their actions on the solubility of silica plays a significant role on the evolution of the hydration of the principal components of Portland cements and the kinetics of lime fixation by the SF.Among the impurities contained in the SF, carbon delays considerably the hydration of the principal components of Portland cement (C₃S and C₃A) as well as the pozzolanic reactivity of the silica fume without removing it.     

The hydration of tricalcium silicate in the presence of colloidal silica

Reactions of colloidal silica fumes with calcium hydroxide or hydrating tricalcium silicate (C₃S) have been studied using calorimetry, chemical analyses, and scanning electron microscopy. Silica fume reacts immediately with calcium hydroxide forming a colloidal calcium silicate hydrate (C-S-H) similar to that formed by the hydration of C₃S. When excess silica is present it reacts with C-S-H already formed to produce a new, highly polymerized C-S-H, having a very low C/S ratio (1.0). Silica fume accelerates the hydration of C₃S, reduces the amount of calcium hydroxide formed by reacting with it, and slightly lowers the C-S-H ratio of the C-S-H formed by hydration. When large amounts of silica fume are present the formation of calcium hydroxide may be entirely suppressed and a highly polymerized C-S-H is formed. Silica fume is considered a good model for reactive pozzolans used in concrete.     

Hydration of tricalcium silicate (C3S) at high temperature and high pressure

Hydration products of tricalcium silicate (C₃S) are the calcium silicate hydrates (C-S-H) and Portlandite. Silica fume, added to anhydrous cement in industrial formulations reacts with Portlandite and leads to C-S-H different from the previous one. C₃S hydration with and without silica fume has been studied under high pressure (1000 bar) and high temperature (160°C) by numerous techniques (²⁹Si and ¹H NMR, XRD, Thermal analysis, SEM) for different hydration times. In these conditions, high temperature more stable crystalline phases are formed and their kinetics of formation is dependent on pressure. Besides, electrical conductivity measurement on hydrating cement under pressure have been carried out in order to evidence the great dependence of hydration kinetics with pressure. This study proposes a practical phase diagram which allows on a thermodynamical base to understand the change of equilibrium temperature with pressure. The kinetics of reaction has been studied and mechanisms of reaction proposed to explain the results.     

Lattice defects and the effect of Melment on the hydration of alite

The hydration of alite has been studied in the presence of different concentrations of the superplasticizer Melment by using various experimental techniques. Heat of hydration and non-evaporable water content determinations show that Melment retards the hydration; pH measurements indicate that in presence of Melment, Ca²⁺ ion dissolution is reduced. Zeta potential measurements give definite proof that Melment molecules are adsorbed at alite surfaces and the retardation of hydration may be due to adsorption. Thermoelectric power measurements prove that the material is an n-type semiconductor, whereas electrical conductivity measurements of solid pellets of C₃S (C = CaO, S = SiO₂) show that the material is an intrinsic semiconductor above 746 K and an extrinsic semiconductor below 746 K. Extrinsic semiconductivity may be due to the presence of defects or impurities in the crystal lattice. The results also show that oxide ion vacancies are created in the crystal lattice and the reactivity of C₃S is related to the defects. Probably Melment molecules are adsorbed at the sites of defects and oxide ion vacancies and hence retard the hydration.     

Influence of slag substitution on some properties of sand-lime aerated concrete

The effect of substitution of sand with granulated slag on some properties of sand-lime aerated concrete was investigated. The compressive strength, the hydration kinetics and the nature of the hydration products were studied for samples autoclaved under 8 atm for different periods from 0.5 to 24 h. The results indicate that the substitution of slag leads to a marked increase of the compressive strength compared with that of the unsubstituted samples. The extent of hydration, as measured by the chemically combined water, free lime and free silica contents, is enhanced due to the slag substitution. The X-ray diffraction results show that the hydration products are mainly tobermorite-like phases, C₄AH₁₃,C₃AH₆ and CAH₁₀ in the slag-containing samples. Cement notations are used in the text, e.g. C=CaO, S=SiO₂, A=Al₂O₃ and H=H₂O.     

Step-by-step observation of the hydration of C3S by magic-angle spinning 29Si nuclear magnetic resonance: the masking effect of D2O

High-resolution solid-state ²⁹Si nuclear magnetic resonance (NMR) characterization of hydrated C₃S provides a means of selectively observing the hydrogen-containing phases. In particular, by applying the cross-polarization (CP) technique, we demonstrated that a contrast can be obtained, not only for hydrated against anhydrous phases, but also for hydrogen-hydrated against deuterium-hydrated phases. Selectively deuterated samples were thus prepared by a H₂O/D₂O alternated hydration procedure. The progress of the polycondensation of orthosilicates as derived by H₂O was followed, demonstrating that a separate microphase of anhydrous C₃S prevailingly reacted at any step. An induction period, showing a low reactivity and mainly formation of Q^o hydrated was observed during the first hydration steps of selectively deuterated samples. The efficiency of the masking effect of D₂O was also indirectly proven by comparison with calorimetric measurements.     

An in-situ Raman spectroscopy study of the hydration of tricalcium silicate

Raman spectroscopy has been used to study the hydration kinetics of tricalcium silicate (C₃S). The progress of the reaction was followed by two different methods: the first one is based on measurement of the intensity of characteristic C₃S Raman bands compared with that of an internal Standard Raman peak (TiO₂); the second one consists in measuring the relative intensity of Raman peaks associated with C₃S and calcium hydroxide. These two concordant data sets show that a change in the hydration mechanism occurs at about 13 hours, and enable the kinetic parameters associated with the two hydration processes to be derived.     

In situ solid-state NMR studies of Ca3SiO5: hydration at room temperature and at elevated temperatures using 29Si enrichment

²⁹Si isotopic enrichment was used for acquisition of multiple ²⁹Si magic-angle spinning (MAS) and cross-polarization magic-angle spinning (CPMAS) nuclear magnetic resonance (NMR) spectra, in situ in an NMR probe, from a single sample of hydrating Ca₃SiO₅ (C₃S). Data with excellent signal-to-noise ratios were obtained at 20, 50 and 75 °C, with minimal use of spectrometer time, and without the need for the quenching of multiple samples. Spectral line widths and polymer-chain lengths derived from the spectra had no detectable differences from experiments in which the quenching was carried out with propan-2-ol. Furthermore, the effects of the MAS technique on the hydration reaction appeared to be minimal. At 20 °C, the bulk hydrate initially produced was dimeric; at later stages of the reaction, polymerization occurred. Arrhenius energies of 35 and 100 kJ mol^–1, respectively, were calculated for these two reactions. The cross-polarization (CP) spectra acquired throughout the hydration showed that at 20 °C, 2% of the hydrated monomeric Q _o ^(H) species persisted from after the induction period through to the late stages of the hydration reaction; this indicates that this species is unlikely to result from surface hydroxylation of C₃S; an upfield shift of this species occurred with increasing hydration, indicating a possible change of environment for the silicate species. The amount of Q _o ^(H) produced was found to increase at higher temperatures. Potential mechanisms for polymerization were assessed and a model in which dimeric-silicate units are linked together by insertion of monomers (dimer pentamer octomer) was found to give the best fit to the observed data; these results support a dreierketten model for the structure of the hydrate.     

Tricalcium aluminate hydration: Microstructural observations by in-situ electron microscopy

Environmental scanning electron microscopy (ESEM) was used to study the microstructural changes that take place during the hydration of tricalcium aluminate (C₃A) in the absence and presence of gypsum (CS¯H₂; where A = Al₂O₃, C = CaO, H = H₂O, S¯ = SO₃). The ESEM proves to be a valuable tool in the observation of cement hydration and no specialised equipment other than the ESEM is required. The hydration process can be observed at any time without the need to halt the hydration process prior to specimen preparation. Subsequently, artefacts associated with specimen preparation, such as water loss and desiccation, are now avoided. In the absence of sulphate, amorphous gel, poorly crystalline hexagonal calcium aluminate hydrate (? C₄AH₁₉) and cubic calcium aluminate hydrate (C₃AH₆) are observed on the surface of C₃A grains. When small amounts of sulphate (2% gypsum) are present the same phases are observed. If larger amounts of sulphate (25% gypsum) are added to the system amorphous gel products, crystalline ettringite (C₆AS¯₃H₃₂) and monosulphate (C₄AS¯H₁₂) are observed. The crystalline products grow both within the amorphous gel and, where space allows, in interstices suggesting a through solution mechanism of transport.     

The influence of an Austrian fly ash on the reaction processes in the clinker phases of Portland cements

The aim of the paper is to establish the influence of fly ash on the hydration process of the cement and fly ash mixtures. Particular attention was paid to the influence on the main clinker phases, C₃S, C₂S and C₃A being investigated by X-ray methods at various points during the reaction period. The reaction partners used were two normal Austrian Portland cements plus an Austrian brown-coal ash. In addition to the pure cements, a mixture with 30% fly ash and 70% of the respective cements was also investigated. For purposes of comparison, it was also necessary to analyse in each case a cement mixture with an inert substance in the same ratio.     

Preparation and in vitro bioactivity of zinc incorporating tricalium silicate

Zinc incorporating tricalcium silicate (Zn-C₃S) at various concentrations (0.80-8.00 wt% Zn) were prepared and investigated as compared with the pure C₃S. The incorporation of Zn promotes the formation of C₃S phase during the calcining process, and inhibits the phase transformation and decomposition of C₃S during the quenching process. The excessive incorporation of Zn (≥ 4.80 wt%) results in the disappearance of the main heat libration during the hydration of Zn-C₃S. The incorporation of Zn decreases the pH value of SBF solution after soaking, and increases the ionic dissolution concentrations. The deposition of apatite on Zn-C₃S paste surface is inhibited during the first 12 hours of soaking. However, the apatite layer could deposit on the surface of Zn-C₃S paste after soaking for 3 days. Our results indicate that the incorporation of Zn significantly improves the formation and stable of C₃S, and alters the bioactive behaviors and ion releases to the SBF solution.     

Analysis of cubic and orthorhombic C<Subscript>3</Subscript>A hydration in presence of gypsum and lime

Field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) have been used to study the microstructural changes and phase development that take place during the hydration of cubic (pure) and orthorhombic (Na-doped) tricalcium aluminate (C₃A) and gypsum in the absence and presence of lime. The results demonstrate that important differences occur in the hydration of each C₃A polymorph and gypsum when no lime is added; orthorhombic C₃A reacts faster with gypsum than the cubic phase, forming longer ettringite needles; however, the presence of lime slows down the formation of ettringite in the orthorhombic sample. Additional rheometric tests showed the possible effects on the setting time in these cementitious mixes.     

矿物外加剂对丁苯聚合物/水泥复合胶凝材料凝结硬化过程的影响及机制

为了比较沸石、纳米二氧化硅和稻壳灰这3种矿物外加剂对丁苯聚合物/水泥复合胶凝材料凝结硬化过程作用的差异,分别采用这3种矿物外加剂为调凝材料,并从凝结时间、早期强度、水化进程以及水化产物等角度比较3种矿物外加剂对丁苯聚合物/水泥复合胶凝材料的影响。结果表明,3种矿物外加剂都能促进复合胶凝材料的凝结硬化,大幅缩短凝结时间,提高早期强度。但3种矿物外加剂的调凝效果互不相同,调凝机理也有差异:沸石对AFt的生成有较大的促进作用,它不仅能促进C3A的水化,自身也能与Ca(OH)_2反应生成AFt和CSH凝胶;而纳米二氧化硅和稻壳灰对C3S水化的促进作用较强,自身也会与Ca(OH)_2反应生成CSH凝胶。     

Spectroscopic studies and evanescent optical fibre wave sensing of Cu<Superscript>2+</Superscript> based on activated mesostructured silica matrix

This paper report on the synthesis and test of eriochrome cyanine R (ECR) doped mesostructured silica films as Cu²⁺ sensing without prior removal of the surfactant. ECR activated mesostructured silica (via sol-gel technique) coated optical fibre core were used to Cu²⁺ sensing by evanescent wave method. The sol was prepared via acidic hydrolysis-condensation of tetraethoxysilane (TEOS) in the presence of cetyltrimethylammonium bromide (CTAB:C₁₆H₃₃NCH₃Br) as the structure-directing agent. Spectrophotometric investigations using doped sol-gel thin films dip-coated on glass slides were reported. Heat treatment temperature effect (optimum temperature 150°C), absorption dependence on the pH of the aqueous phase (pH = 8 seems to be the optimal absorption pH), sensitivity, reversibility and response range (0.05 mmol/l) were reported.     

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.     

Microwave assisted preparation of Fe doped β-dicalcium silicate by sol-gel method

Fe³⁺ doped and undoped β-dicalcium silicates (β-Ca₂SiO₄ or β-C₂S) were prepared by sol-gel method. The gels formed were heated in a microwave oven and subsequently in a muffle furnace. The particle sizes of doped and undoped samples were found to be about 150 nm and 600 nm respectively. The materials were characterized by scanning electron microscopic, powder X-ray diffraction, BET specific area measurements, infrared spectroscopic and Mössbauer spectroscopic techniques. IR spectroscopic studies showed the possibility of distorted tetrahedral symmetry in Fe³⁺ doped β-C₂S. Calorimetric studies have shown that iron doped β-C₂S is highly reactive. From Mössbauer spectroscopic studies it is found that electronic environment of Fe³⁺ in β-C₂S is changed due to hydration. Degree of hydration calculated from non-equilibrium water content measurements have shown that Fe³⁺ doped β-C₂S hydrates at a much higher rate. Scanning electron microscopic studies have revealed that the hydration products in both the cases are almost identical. Formation and hydration of β-C₂S in the presence of Fe³⁺ have been understood.     

纳米SiO2与粉煤灰协同改性水泥基材料性能研究

通过调整纳米SiO_2与粉煤灰的比例,研究了两者协同作用对水泥基材料性能的影响。结果表明,纳米SiO_2(NS)和粉煤灰协同作用效果优于NS单一掺加,3%(质量分数,下同)纳米SiO_2和不大于30%的粉煤灰同时掺加可以补偿粉煤灰引起的早期强度降低,且砂浆28d抗压强度不降低。随着NS掺量增加水泥基材料的干燥收缩增大,粉煤灰可以改善纳米SiO_2对干燥收缩的不利影响。随着NS掺量的增加,试件的抗冻性和抗氯离子渗透性能均得到提升,掺加3%NS与30%粉煤灰使水泥基材料达到最佳耐久性能。NS可以缩短水泥水化诱导期,加速水泥水化进程,且使胶凝体系总放热量增加。在水泥粉煤灰体系中掺入NS后,非蒸发水含量在早期明显增多,但在后期增长缓慢。     

In-situ reaction of the very early hydration of C3A-gypsum-sucrose system by Micro-Raman spectroscopy

This paper studied in situ, by Micro-Raman spectroscopy, the very early hydration of C₃A in the presence and absence of sulphates and with sucrose as an additive. For C₃A hydration in the absence of gypsum, when carbonation is not avoided, carbonate-AFm phases are formed, but in the presence of gypsum, hydroxi-AFm are the main phases. Ettringite is the AFm stable phase developed initially at 70 min of hydration with gypsum and no monosulphate is formed. In the presence of sucrose, this salt, instead of sulphate, is adsorbed over the surface of the C₃A, avoiding its reaction with sulphates until sucrose desorption. Three hours are necessary to lead to ettringite formation. A nucleation poisoning/adsorption surface mechanism is proposed for added sucrose systems.     

Chemical activation of calcium aluminate cement composites cured at elevated temperature

The influence of sodium sulfate, as an activator, on the hydration of calcium aluminate cement (CAC)–fly ash (FA)–silica fume (SF) composites was investigated. Different mixes of CAC with 20% pozzolans (20% FA, 20% SF and 10% FA + 10% SF) were prepared and hydrated at 38 °C for up to 28 days. The hydration products were investigated by XRD, DSC and SEM. The results showed that sodium sulfate accelerated the hydration reactions of calcium aluminate cement as well as the reactions of FA and SF with CAH₁₀ and C₂AH₈ to form the strätlingite (C₂ASH₈). The later reactions prevent the strength loss by preventing the conversion of CAH₁₀ and C₂AH₈ to the cubic C₃AH₆ phase. The acceleration effect of Na₂SO₄ on the reactivity of fly ash was more pronounced than on the reactivity of silica fume with respect to reaction with CAH₁₀ and C₂AH₈ phases.