C3S Paste Research Articles

Adsorption and Desorption Characteristics of Alkali Ions in Hydrated C3S-nano SiO2 Pastes

C3S pastes containing 0%, 5%, 10%, and 15% nano-SiO2 mixed with de-ionized water and alkali solutions were prepared. When C3S was completely hydrated, the pastes were ground into powders with a particle size less than 80 μm. Adsorption and desorption characteristics of alkali ions adsorbed by C3S-nano SiO2 pastes mixed with de-ionized water immersed in alkali solutions and those in C3S-nano SiO2 pastes mixed with alkali solutions, were investigated. Meawhile, the adsorption mechanisms of alkali ions were discussed. Results showed that the contents of alkali ions adsorbed by C3S-nano SiO2 pastes mixed with de-ionized water increased with increasing substitution levels of nano-SiO2 and/or the initial alkali concentrations. In C3S-nano SiO2 pastes mixed with de-ionized water, each paste was characterized by having a fixed alkali-adsorption capacity that was essentially independent of alkali concentration. No obvious difference between the adsorption capacity of a given paste for K+ and Na+ was observed. Adsorption of alkali ions in the pastes is considered to be caused by surface force which is related to the BET specific surface area of the paste, and charge compensation of C-S-H gel, mainly by electrostatic interactions. In C3S-nano SiO2 pastes mixed with alkali solutions, alkali ions may enter the structure of C-S-H gel to replace a part of Ca2+ in the interlayer. This assumption is supported by the structural characterization of C-S-H gel using 29Si MAS NMR.

Insights into the influencing factors on the micro‐mechanical properties of calcium‐silicate‐hydrate gel

AbstractSo far, many researchers have attempted to unveil the micro‐mechanical features of calcium‐silicate‐hydrate (C‐S‐H) gel that contributes the most to the engineering performances of cement‐based materials. Due to the highly heterogeneous nature of C‐S‐H gel present in cement‐based materials, the influencing factors of the micro‐mechanical properties of C‐S‐H gel are still not clear. This study aims to provide an insight into the factors that govern the micro‐mechanical properties of C‐S‐H gel by investigating the morphology, composition, and micro‐mechanical properties of C‐S‐H gels present in a relatively simple system, ie, a C3S paste. Through this study, it is revealed that the reinforcement by the crystal phase and the size of C3S clinker influence the micro‐mechanical properties of inner product C‐S‐H gel.

Early Age Carbonation Heat and Products of Tricalcium Silicate Paste Subject to Carbon Dioxide Curing.

This paper presents a study on the carbonation reaction heat and products of tricalcium silicate (C3S) paste exposed to carbon dioxide (CO2) for rapid curing. Reaction heat was measured using a retrofitted micro-calorimeter. The highest heat flow of a C3S paste subject to carbonation curing was 200 times higher than that by hydration, and the cumulative heat released by carbonation was three times higher. The compressive strength of a C3S paste carbonated for 2 h and 24 h was 27.5 MPa and 62.9 MPa, respectively. The 24-h carbonation strength had exceeded the hydration strength at 28 days. The CO2 uptake of a C3S paste carbonated for 2 h and 24 h was 17% and 26%, respectively. The X-ray diffraction (XRD), transmission electron microscope coupled with energy dispersive spectrometer (TEM-EDS), and 29Si magic angle spinning–nuclear magnetic resonance (29Si MAS-NMR) results showed that the products of a carbonated C3S paste were amorphous silica (SiO2) and calcite crystal. There was no trace of calcium silicate hydrate (C–S–H) or other polymorphs of calcium carbonate (CaCO3) detected.

Role of Polycarboxylate-ether superplasticizers on cement hydration kinetics and microstructural development

Polycarboxylate-ether (PCE) superplasticizers are a fundamental constituent of modern cementbased materials due to their impact on the rheology of the fresh mix and mechanical performance of the hardened material. The effect of PCEs on cement hydration kinetics has been known since their introduction in the early 1980s. However, detailed knowledge of the role played by PCE macromolecules on the basic mechanisms of cement hydration (dissolution, diffusion, precipitation) is still lacking. A better understanding of how such mechanisms are influenced by the addition of PCE is no doubt beneficial to the design of novel superplasticizing admixtures. Here, I report on some recent findings about the role of PCE superplasticizers on cement hydration kinetics and microstructural development. The interaction between PCE and C3S pastes was investigated by an ad-hoc kinetic model based on a combination of generalized forms of the Avrami and BNG (Boundary Nucleation and Growth) models. The model is used to fit the rate of C-S-H precipitation measured by in-situ X-ray powder diffraction combined with mass balance calculations. The results show that a switch from heterogeneous to homogeneous C-S-H nucleation occurs in the presence of PCEs and that the C-S-H growth rate decreases proportionally to the amount of PCE used. The predicted switch to homogeneous nucleation is in agreement with experimental results obtained by XRD-enhanced micro-tomography imaging, showing that, in the presence of PCE, C-S-H preferentially forms in the pore space rather than at the surface of clinker particles.

Discussion of the paper “A new view on the kinetics of tricalcium silicate hydration,” by L. Nicoleau and A. Nonat, Cem. Concr. Res. 86 (2016) 1–11

In contradiction to the proposition of Nicoleau and Nonat, it is shown that the rate of hydration of anhydrous tricalcium silicate (C3S) cannot be controlled by the rate of dissolution of the anhydrous phase in typical C3S pastes except at extremely early ages. The strongest evidence for this is the observation that the hydration of C3S in pastes is strongly accelerated by the addition of afwillite seeds, to the extent that it hydrates almost completely by 7days. It is suggested that the hydration rate at later ages is primarily controlled by the porosity and permeability of the hydration products.

Pair distribution function analysis of nanostructural deformation of calcium silicate hydrate under compressive stress

AbstractDespite enormous interest in calcium silicate hydrate (C–S–H), its detailed atomic structure and intrinsic deformation under an external load are lacking. This study demonstrates the nanostructural deformation process of C–S–H in tricalcium silicate (C3S) paste as a function of applied stress by interpreting atomic pair distribution function (PDF) based on in situ X‐ray scattering. Three different strains in C3S paste under compression were compared using a strain gauge, Bragg peak shift, and the real space PDF. PDF refinement revealed that the C–S–H phase mostly contributed to PDF from 0 to 20 Å whereas crystalline phases dominated that beyond 20 Å. The short‐range atomic strains exhibited two regions for C–S–H: I) plastic deformation (0‐10 MPa) and II) linear elastic deformation (>10 MPa), whereas the long‐range deformation beyond 20 Å was similar to that of Ca(OH)2. Below 10 MPa, the short‐range strain was caused by the densification of C–S–H induced by the removal of interlayer or gel‐pore water. The strain is likely to be recovered when the removed water returns to C–S–H.

Effects of Incorporating High-Volume Fly Ash into Tricalcium Silicate on the Degree of Silicate Polymerization and Aluminum Substitution for Silicon in Calcium Silicate Hydrate.

This study assesses the quantitative effects of incorporating high-volume fly ash (HVFA) into tricalcium silicate (C3S) paste on the hydration, degree of silicate polymerization, and Al substitution for Si in calcium silicate hydrate (C–S–H). Thermogravimetric analysis and isothermal conduction calorimetry showed that, although the induction period of C3S hydration was significantly extended, the degree of hydration of C3S after the deceleration period increased due to HVFA incorporation. Synchrotron-sourced soft X-ray spectromicroscopy further showed that most of the C3S in the C3S-HVFA paste was fully hydrated after 28 days of hydration, while that in the pure C3S paste was not. The chemical shifts of the Si K edge peaks in the near-edge X-ray fine structure of C–S–H in the C3S-HVFA paste directly indicate that Al substitutes for Si in C–S–H and that the additional silicate provided by the HVFA induces an enhanced degree of silicate polymerization. This new spectromicroscopic approach, supplemented with 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy and transmission electron microscopy, turned out to be a powerful characterization tool for studying a local atomic binding structure of C–S–H in C3S-HVFA system and presented results consistent with previous literature.

The effects of seeding C3S pastes with afwillite

The addition of 1–4% s/s (dry solids by mass of C3S) of afwillite (C3S2H3) seeds to C3S pastes made with two different commercial polyacrylate-based superplasticizers (SP) allows the pastes to be cast at low water/C3S mass ratios (w/c) and overcomes the hydration retardation produced by the SPs. SP-free C3S pastes seeded with afwillite at an initial w/c of 0.50 gave about 30% lower 28-day compressive strengths than the unseeded controls, due to higher porosities. However, at w/c=0.35, with the addition of 0.4% s/s SP, the afwillite-seeded pastes gave similar or higher strengths than the unseeded controls at all ages tested. Hydration rate data obtained by chemical shrinkage measurements suggest that this is because the degrees of hydration of the C3S in the low w/c afwillite-seeded pastes made with added SP reach higher values than in the unseeded controls, compensating for the difference in density of the hydrates.

Effects of functionality and stereochemistry of small organic molecules on the hydration of tricalcium silicate

The retardation and the maximum rate of tricalcium silicate (C3S) hydration induced by organic molecules were rarely measured as a function of their molar concentration and prevent us to identify the effect of specific functional groups carried by the molecules. In this study, the maximum rate of C3S hydration which is proportional to the maximum heat flow and the retardation led by different concentrations of organic molecules was determined using the monitored calorimetric curves of the C3S pastes. The effect of the charge, functionality and stereochemistry was studied by using a set of sugar alcohols (d-glucitol, d-galactitol…), sugar acid anions (d-gluconate, d-galactarate…), carboxylate (adipate) and amines with functionalized sulfonate, carboxylate and phosphonate groups (EDTA, EDTMP…).This study reveals that the maximum rate of C3S hydration is enhanced in the presence of the organic molecules and that the retardation is increased with their concentration. The nature of the ending functional group of the molecules (hydroxyl, hydroxy-carboxylate, carboxylate, sulfonate or phosphonate), their main chain group (hydrocarbon or polyol chain) and the stereochemistry of their hydroxyl groups are shown to play a role.

Effect of Carbon‐Based Materials on the Early Hydration of Tricalcium Silicate

Two types of carbon‐based materials, i.e., mesoporous carbon and HNO3‐oxidized carbon nanotubes, with nearly the same specific surface area and abundant in surface oxygen‐containing functional groups were selected in order to examine their effect on the hydration of tricalcium silicate (C3S), the main portland cement component, in early stages. Different methods, including XPS and TG‐MS analyses, electrokinetic potential measurements, as well as determination of adsorption capacity for calcium ions from aqueous solutions, were used to investigate the physicochemical surface properties of the selected carbon‐based materials. It was found that the carbon‐based materials with high specific surface area and rich in oxygen‐containing functional groups on their surfaces have a catalytic effect on early C3S hydration. It was observed that the modification of C3S paste with the selected materials added in high concentrations (1 wt% and higher) led to an increase in the rate and degree of C3S hydration in the early stages. The mechanism of early C3S hydration accelerated by carbon‐based materials rich in surface functional groups was clarified by the example of the mesoporous carbon. It was found that the oxygen‐containing functional groups present on the carbon surface have both an influence on the content of calcium ions in the aqueous phase of the C3S paste and an indirect positive effect in relation to the specific surface of C3S.

Atomic and nano-scale characterization of a 50-year-old hydrated C3S paste

This paper investigates the atomic and nano-scale structures of a 50-year-old hydrated alite paste. Imaged by TEM, the outer product C–S–H fibers are composed of particles that are 1.5–2nm thick and several tens of nanometers long. 29Si NMR shows 47.9% Q1 and 52.1% Q2, with a mean SiO4 tetrahedron chain length (MCL) of 4.18, indicating a limited degree of polymerization after 50years' hydration. A Scanning Transmission X-ray Microscopy (STXM) study was conducted on this late-age paste and a 1.5year old hydrated C3S solution. Near Edge X-ray Absorption Fine Structure (NEXAFS) at Ca L3,2-edge indicates that Ca2+ in C–S–H is in an irregular symmetric coordination, which agrees more with the atomic structure of tobermorite than that of jennite. At Si K-edge, multi-scattering phenomenon is sensitive to the degree of polymerization, which has the potential to unveil the structure of the SiO44− tetrahedron chain.

Investigation of the swelling behavior of cationic exchange resins saturated with Na+ ions in a C3S paste

Ion exchange resins (IERs) are widely used by the nuclear industry to decontaminate radioactive effluents. Spent products are usually encapsulated in cementitious materials. However, the solidified waste form can exhibit strong expansion, possibly leading to cracking, if the appropriate binder is not used. In this work, the interactions between cationic resins in the Na+ form and tricalcium silicate are investigated during the early stages of hydration in order to gain a better understanding of the expansion process. It is shown that the IERs exhibit a transient swelling of small magnitude due to the decrease in the osmotic pressure of the external solution. This expansion, which occurs just after setting, is sufficient to damage the material which is poorly consolidated for several reasons: low degree of hydration, precipitation of poorly cohesive sodium-bearing C–S–H, and very heterogeneous microstructure with zones of high porosity.

FTIR study of the effect of temperature and nanosilica on the nano structure of C–S–H gel formed by hydrating tricalcium silicate

Fourier transform infrared spectroscopy (FTIR) was used to explore the effect of temperature (25°, 40° and 65°C) and the presence of amorphous nanosilica (nSA) on the nanostructure of the C–S–H gel generated during tricalcium silicate (C3S) hydration.Rising temperatures were shown to modify the nanostructure of C–S–H gel. The bands at around 1076cm−1, 909cm−1 and 540cm−1 that appeared on the FTIR spectrum for C3S paste with rising temperatures were attributed to the formation a jennite-like structure. Moreover, irrespective of temperature, as the reaction progressed, the initial tobermorite-like C–S–H gel changed to a jennite-like structure. With the addition of nSA, the band generated at 960–970cm−1 appeared from the first day of hydration, an indication that nanosilica accelerates C3S hydration. The presence of nSA induced no narrowing of the bands at 1076cm−1 or 909cm−1 at any curing time, a sign that these gels had a smaller proportion of jennite-like species than the gels in the nSA-free pastes. Finally, compositional date of C–S–H was provides.

In vivo investigation of biological responses to tricalcium silicate pastes in muscle tissue

The biocompatibility of tricalcium silicate (Ca3SiO5; C3S) paste was studied by intramuscular implantation. C3S paste can induce the deposition of apatite layer on its surface in the muscle tissue. The widths of apatite layer between C3S paste and muscle tissue are increased with the increasing of implantation time. Pure C3S and fluorine-doped C3S (F-C3S) pastes induce less inflammatory reactions to the muscle tissue than PMMA paste. Pure C3S and F-C3S pastes are embedded with connective tissue after 12 weeks of implantation as well as PMMA paste. However, more living fibroblasts and fewer macrophages are observed in the connective tissue around F-C3S paste. F-C3S paste is better biocompatibility to the muscle tissue than pure C3S paste, because F-C3S paste has a better deposition ability of apatite layer. Those results confirm that F-C3S may be more biologically suitable for bone cement.

The di- and tricalcium silicate dissolutions

In this study, a specially designed reactor connected to an ICP spectrometer enabled the careful determination of the dissolution rates of C3S, C2S and CaO, respectively, over a broad range of concentration of calcium and silicates under conditions devoid of C–S–H. The kinetic laws, bridging the dissolution rates and the undersaturations, were obtained after extrapolation of rate zero allowing the estimation of the true experimental solubility products of C3S (Ksp=9.6·10−23), C2S (Ksp=4.3·10−18) and CaO (Ksp=9.17·10−6). The latter are then compared to the solubilities calculated from the enthalpies of formation. We propose that the observed deviations result from the protonation of the unsaturated oxygen atoms present at the surface of these minerals. Hydration rates measured in cement pastes or in C3S pastes are in excellent agreement with the kinetic law found in this study for C3S under conditions undersaturated with respect to C–S–H.

Microindentation creep of secondary hydrated cement phases and C–S–H

Microindentation creep measurements were obtained on compacted specimens of several secondary hydrated cement phases in equilibrium with water vapor at 11%RH. Values of creep modulus, indentation modulus and indentation hardness for calcium hydroxide, ettringite, gypsum and calcium carbonate are reported. The porosity dependence of these parameters was established and the significance of porosity on the time-dependent deformation of these materials was discussed. In addition the microindentation creep behavior of pure C–S–H and C3S paste hydrated 32 years was determined. The discussion focuses on the relative importance of the contribution of the secondary phases in hydrated cement-based materials to creep with respect to the more ‘active’ C–S–H phase.

Effects of molecular structure of polycarboxylate-type superplasticizer on the hydration properties of C3S

Effects of polycarboxylate-type superplasticizer (PC) molecular structure on the hydration heat of tricalcium silicate (C3S) paste and polymerization degree of hydration products (C-S-H gel) were researched by using TAM AIR isothermal microcalorimetry (TA) and 29Si nuclear magnetic resonance (NMR). Methoxy polyethylene glycol-methacrylates-based polycarboxylate superplasticizers with different side chain lengths and main chain lengths were employed. PC molecules with shorter main chain or longer side chains caused stronger retardation of C3S early hydration and lesser increase of C3S 3 d hydration degree. NMR measurement indicated that the incorporation of PC increased the hydration degree of C3S paste and the polymerization degree of silicon-oxygen tetrahedron of C-S-H gel. The tendency for C3S 7 d hydration degree to improve was more pronounced while PC molecules with longer main chain or shorter side chain were added. Whereas, PC molecules with longer main chains or longer side chains increased the 7 d polymerization degree of C-S-H gel.

Comparative study of accelerated decalcification process among C3S, grey and white cement pastes

This study compared the resistance of Triclinic-C 3 S, grey (OPC) and white (WOPC) Portland cement paste to decalcification induced by accelerated leaching in concentrated ammonium nitrate solutions. Paste microstructure was studied with scanning and backscattering electron microscopy (SEM and BSEM) and nitrogen BET surface area techniques. Ca 2+ leached content was quantified by ICP, XRD and FTIR techniques were used to study phase mineralogy. The conclusions drawn from the findings were that calcium leaching-induced decay in the cementitious materials studied (C 3 S, OPC and WOPC), accelerated by immersion in ammonium nitrate, affected the main calcium phases in the samples (CH, C–S–H gel and ettringite), i.e., both the anhydrous and the hydrated phases. The present study showed that the Ca/Si ratio of C–S–H gels declines on a gradient from the sample core outward. Specimen surface area and nanoporosity rose in cementitious materials after Ca leaching-induced decay and subsequently declined as a result of the collapse of the structure of the hydrated cement , and in particular of the C–S–H gel. C 3 S paste was impacted more quickly and intensely by leaching than the WOPC and OPC pastes. Further to the findings of this study, the leaching resistance of these three materials, in descending order, is: OPC > WOPC > C 3 S.

Portlandite content and ionic transport properties of hydrated C3S pastes

This paper presents the results of a C3S paste characterization study. The objective was to determine the parameters needed to model the process of degradation. The experimental study focused on determining the portlandite content and the ionic diffusion coefficients of C3S paste. The molar C/S ratio of C–S–H in hydrated C3S pastes was also investigated. The portlandite content was determined with an experimental method based on an electron microprobe analysis.This method leads to a portlandite mass content of 24.4±2.3%. The diffusion coefficient of each ionic species was determined by inverse analysis of diffusion test data performed on hydrated C3S samples using a multiionic transport model.

Bonding Force Investigation between C-S-H Clusters in the C<sub>3</sub>S Pastes and AFM Tip

With the experimental objects of the C3S pastes curing at 60 days with water-solid ratio of 0.5, the micro mechanical characteristic between C-S-H clusters and tip has been investigated in the low scale. The results show that the experimental method is feasible for bonding force between C-S-H clusters and tip using atomic force microscopy (AFM) in the real atmospheric conditions. Though bonding force values are discrete in view of the anisotropy character of the cement-based material, the normal distribution can be fitted for the variation of bonding force between C-S-H blusters in the C3S pastes and AFM tip with a mean of 6.2nN. Even if bonding force values is somewhat higher due to the roughness and the measuring conditions in the paper, the experimental method using AFM could still be effective for the lateral correlation for bonding force of the different systems. The research on bonding force between C-S-H clusters would play the important role in establishing the new microscopic structural model.