The hydration characteristics of MSWI fly ash slag present in C3S

This paper reports on an investigation of the hydration characteristics of C₃S and the mixing of C₃S with municipal solid waste incinerator (MSWI) fly ash slag. The results can be summarized as follows: TGA observations show lower amounts of CSH and Ca(OH)₂ in samples that incorporated MSWI slag into C₃S, possibly due to the partial replacement of the C₃S by slag with less activity. In general, the incorporation of slag into C₃S decreases the initial hydration reactions, but it increases the pozzolanic reactions at a later stage by consuming Ca(OH)₂. As the hydration precedes, the C₃S peaks decrease, up to 90 days, due to the activation of the slag by liberated Ca(OH)₂. As well, the amount of hydration products (Ca(OH)₂) from the pure C₃S pastes are more than for the C₃S-slag pastes. Moreover, the degree of hydration of the C₃S pastes increases with the curing time, the C₃S-slag paste also shows that lower hydration degree values at all ages.     

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.     

Effect of CuO on the formation of clinker minerals and the hydration properties

The purposes of this study are to explore the mechanisms of Cu element in clinker burning and hydration processes and to make effective use of waste containing copper in cement production. The effect of CuO on clinker mineral composition, C₃S polymorph and size, Cu element distribution and state, compressive strengths, hydration products, non-evaporable water quantity and hydration heat release rate was analyzed by XRD, SEM, DTA, isothermal heat-conduction calorimetry, etc. Results show that as the amount of CuO increases the formation and growth of C₃S grain are accelerated, R C₃S is gradually transformed into M₃ and the content of C₄AF increases; a small quantity of CuO increases the 3-day and 28-day strengths and the hydration degree of clinker, but excessive CuO has adverse effects. Those effects of CuO on clinker burning process are attributed to the formation of low-melting Cu₂O and the dissolution of CuO in C₄AF which decrease the formation temperature of liquid phase and increase its quantity. The effects on hydration process result from the combined action of the following factors: the induction period is prolonged; the hydration reactions in the initial and acceleration periods are accelerated.     

Surface studies of hydrated β-C2S

ESCA (electron spectroscopy for chemical analysis) studies of β-C₂S in the early stages of hydration have yielded valuable information on the chemical changes on the hydrating surface. These changes are less pronounced than, but very similar to, those noted for C₃S. The data indicate that, although β-C₂S hydrates more slowly than C₃S, the same hydration mechanism applies to both. The studies also suggest surface hydroxylation as a first step in the hydration process.     

Hydration of C3S-pozzolana paste estimated by trimethylsilylation

The structure, quantity and the degree of polycondensation of silicate ion in C₃S-pozzolana pastes were examined and discussed in relation to the strenght and the rate of hydration of C₃S and pozzolana. Gel permeation chromatography was adopted to prepare the standard pure sample for the calibration curves and to determine the distribution of molecular weight of silicates. A small amount of monomer was found in the hydrates of C₃S and C₃S-pozzolana. Dimer decreased after a definite period and polysilicate increased gradually with age. The quantity and the degree of polycondensation of polysilicate in hydrate depend roughly on the silica content of pozzolana and the reactivity of silicate in pozzolana. Double chain structure of polysilicate was supported from the infra-red absorption spectra and the basic structure of polysilicate in hydrate was supposed to be identical regardless of the kinds of pozzolana and curing age.     

The effects of Cr2O3, Cu(OH)2, ZnO and PbO on the compressive strength and the hydrates of the hardened C3A paste

The purpose of this paper is to investigate the effects of Cr₂O₃, Cu(OH)₂, ZnO or PbO on the hydration of C₃A and characterization of its hydrates. C₃A pastes adding the above compound were examined on the basis of the hydration products and their structure, compressive strength and rate of early hydration.     

Study of the early hydration of Ca3SiO5 by X-ray photoelectron spectrometry

C₃S pastes (W/S = 0.5), have been studied from 5 seconds to 4 hours by X-ray Photoelectron Spectrometry. XPS reveals surface transformations of C₃S grains from very early ages of hydration. The modifications have been evidenced by a change in the environment of Si atoms and a variation of the Ca/Si ratio. A Primary Hydrate (C/S << 3), a Secondary Hydrate ($\text{C/S}\text{?2}$) and a Tertiary Hydrate (CSH type I) have been identified. XPS is a sensitive and reproducible method for the study of the surface C₃S hydration.     

Effect of some liquefying agents on properties and hydration of portland cement and tricalcium silicate pastes

The effect of a melamine sulfonate resin, a naphthalene sulfonate resin and a sulfonated lignin on the rheological properties and the hydration of portland cement and tricalcium silicate pastes was studied. In addition to improving the flow properties of the pastes all three substances retarded the hydration of C₃S and altered the stoichiometric composition of the CSH-phase formed. The rate of ettringite formation was altered by the agents differently in two different cements studied.     

Hydration reactions in pastes C3S+C3A+CaSO4.2aq+water at 25°C. II

Variation of C₃A/C₃S ratio in pastes if C₃S+C₃A+CaSO₄.2aq+water influences the hydration reactions in a way compatible with retardation of C₃A hydration by amorphous Al (OH)₃, but not compatible with retardation by dissolved ions or by a “C₄AH₁₃” retarding layer.     

Effect of particle size distribution of metakaolin on hydration kinetics of tricalcium silicate

The focus of this study is to elucidate the role of particle size distribution (PSD) of metakaolin (MK) on hydration kinetics of tricalcium silicate (C₃S–T₁) pastes. Investigations were carried out utilizing both physical experiments and phase boundary nucleation and growth (pBNG) simulations. [C₃S + MK] pastes, prepared using 8%_mass or 30%_mass MK, were investigated. Three different PSDs of MK were used: fine MK, with particulate sizes <20 µm; intermediate MK, with particulate sizes between 20 and 32 µm; and coarse MK, with particulate sizes >32 µm. Results show that the correlation between specific surface area (SSA) of MK's particulates and the consequent alteration in hydration behavior of C₃S in first 72 hours is nonlinear and nonmonotonic. At low replacement of C₃S (ie, at 8% _mass), fine MK, and, to some extent, coarse MK act as fillers, and facilitate additional nucleation and growth of calcium silicate hydrate (C–S–H). When C₃S replacement increases to 30% _mass, the filler effects of both fine and coarse MK are reversed, leading to suppression of C–S–H nucleation and growth. Such reversal of filler effect is also observed in the case of intermediate MK; but unlike the other PSDs, the intermediate MK shows reversal at both low and high replacement levels. This is due to the ability of intermediate MK to dissolve rapidly—with faster kinetics compared to both coarse and fine MK—which results in faster release of aluminate [Al(OH)₄⁻] ions in the solution. The aluminate ions adsorb onto C₃S and MK particulates and suppress C₃S hydration by blocking C₃S dissolution sites and C–S–H nucleation sites on the substrates’ surfaces and suppressing the post-nucleation growth of C–S–H. Overall, the results suggest that grinding-based enhancement in SSA of MK particulates does not necessarily enhance early-age hydration of C₃S.     

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.     

The mechanism of the hydration in the system C3S-pozzolana

A mixture of five kinds of Japanese pozzolanas and synthesized pure C₃S were hydrated. The hydration mechanism in the system C₃S-pozzolana was investigated. The hydration of C₃S was accelerated by the addition of pozzolanas. The reasons for the acceleration increase of the precipitation sites of hydrates and the increase of the dissolution speed of C₃S caused by the depression of Ca²⁺ ionic concentration in the liquid phase was due to the addition of pozzolanas. The reaction between pozzolana and formed Ca(OH)₂ is pronounced after 1 to 3 days. Zonal hydrates existing between C₃S and pozzolana grains have Ca ionic concentration gradient from C₃S to pozzolana. It was often observed that in intact pozzolana grains which had no precipitated hydrates, there was clearance between pozzolanas and hydrates, and cast of trace. That tendency was pronounced in pozzolanas which had substantial alkalies. The mechanism of the hydration in the system C₃S-pozzolana was considered from those results.     

The Effect of Magnesium and Zinc Ions on the Hydration Kinetics of C3S

Impure tricalcium silicate (C₃S) in portland cement may contain various foreign ions. These ions can stabilize different polymorphs of C₃S at room temperature and may affect its reactivity. In this paper, the effects of magnesium and zinc on the polymorph type, hydration kinetics, and the hydrate morphology of C₃S were investigated. The pure C₃S has the T1 structure while magnesium and zinc stabilize polymorphs M3 and T2/T3, respectively. The two elements have distinct effects on the hydration kinetics. Zinc increases the maximum heat released. Magnesium increases the hydration peak width. The C–S–H morphology is modified, leading to longer needles in the presence of zinc and thicker needles in the presence of magnesium. Zinc is incorporated into C–S–H, while magnesium is only incorporated slightly, if at all, but rather seems to inhibit nucleation. Implementing experimentally measured parameters for C–S–H nucleation and growth in the μic hydration model captured well the observed changes in hydration kinetics. This supports C–S–H nucleation and growth to be rate controlling in the hydration of C₃S.     

C3A hydration

C₃A is hydrated with time and temperature as variable parameters. The solid hydration products were observed using the scanning electron microscope and determined by XRD. The heat development was followed by means of isothermal microcalorimetry.The first hydration product is a gel-like material with no detectable XRD lines. The hexagonal phases which follow have a better crystallinity when formed below than above room temperature. In the latter case distinct XRD lines are only obtained after some time. C₃AH₆ as single crystals or aggregates develops earlier at high than at low temperatures. The morphology of C₃AH₆ varies with hydration time and temperature.This sequence of reactions occurs slowly in suspensions if a small amount of C₃A is used. In pastes, and in suspensions if a larger amount of C₃A is used, C₃AH₆ is formed very quickly and no hexagonal hydrates were detected by XRD.     

Impact of Annealing on the Early Hydration of Tricalcium Silicate

It was recently proposed that the induction period observed during the hydration of tricalcium silicate could be explained by the build‐up of ions in solution. Due to the importance of defects in this mechanism, this work describes the effect of different annealing effects on the defect structure and hydration behavior of C₃S. The impact of annealing on the crystal structure was checked by X‐ray diffraction and the defect structure studied by transmission electron microscopy. The hydration kinetics were followed by isothermal calorimetry of pastes. Scanning electron microscopy was used to look at the microstructure formation. It was observed that grinding created a highly deformed layer on the surface of the grains, which disappeared after annealing. The defect structure was closely related to the length of the induction period observed in pastes by calorimetry. There was no observable effect on the morphology of C–S–H during hydration, but the number of calcium hydroxide nuclei was less in pastes from annealed material.     

Influence of polymer on cement hydration in SBR-modified cement pastes

The influence of styrene-butadiene rubber (SBR) latex on cement hydrates Ca(OH)₂, ettringite, C₄AH₁₃ and C-S-H gel and the degree of cement hydration is studied by means of several measure methods. The results of DSC and XRD show that the Ca(OH)₂ content in wet-cured SBR-modified cement pastes increases with polymer-cement ratio (P/C) and reaches a maximum when P/C is 5%, 10% and 10% for the pastes hydrated for 3 d, 7 d and 28 d, respectively. With wet cure, appropriate addition of SBR promotes the hydration of cement, while the effect of SBR on the content of Ca(OH)₂ and the degree of cement hydration is not remarkable in mixed-cured SBR-modified cement pastes. XRD results illustrate that SBR accelerates the reaction of calcium aluminate with gypsum, and thus enhances the formation and stability of the ettringite and inhibits the formation of C₄AH₁₃. The structure of aluminum-oxide and silicon-oxide polyhedron is characterized by ²⁷Al and ²⁹Si solid state NMR spectrum method, which shows that tetrahedron and octahedron are the main forms of aluminum-oxide polyhedrons in SBR-modified cement pastes. There are only [SiO₄]⁴⁻ tetrahedron monomer and dimer in the modified pastes hydrated for 3 d, but there appears three-tetrahedron polymer in the modified pastes hydrated for 28 d. The effect of low SBR dosage on the structure of aluminum-oxide and silicon-oxide polyhedron is slight. However, the combination of Al³⁺ with [SiO₄]⁴⁻ is restrained when P/C is above 15%, and the structure of Al³⁺ is changed obviously. Meantime, the polymerization of the [SiO₄]⁴⁻ tetrahedron in C-S-H gel is controlled.     

The role of alumina on performance of alkali-activated slag paste exposed to 50 °C

The strength and microstructural evolution of two alkali-activated slags, with distinct alumina content, exposed to 50 °C have been investigated. These two slags are ground-granulated blast furnace slag (containing 13% (wt.) alumina) and phosphorous slag (containing 3% (wt.) alumina). They were hydrated in the presence of a combination of sodium hydroxide and sodium silicate solution at different ratios. The microstructure of the resultant slag pastes was assessed by X-ray diffraction, differential thermogravimetric analysis, and scanning electron microscopy. The results obtained from these techniques reveal the presence of hexagonal hydrates: CAH₁₀ and C₄AH₁₃ in all alkali-activated ground-granulated blast-furnace slag pastes (AAGBS). These hydrates are not observed in pastes formed by alkali-activated ground phosphorous slag (AAGPS). Upon exposure to 50 °C, the aforementioned hydration products of AAGBS pastes convert to C₃AH₆, leading to a rapid deterioration in the strength of the paste. In contrast, no strength loss was detected in AAGPS pastes following exposure to 50 °C.     

The hydration of Portland cement,C3S and C2S in the presence of a calcium complexing admixture (EDTA)

The effect of EDTA, a calcium chelating agent, on the early hydration of Portland cement, C₃Sand β-C₂S has been studied by solution analysis and electron microscopy. EDTA is a retarded of cement hydration. Under normal conditions of hydration, the silica levels in solution are very low (<0.05 M) but in the presence of EDTA an initial flush of silica appears in the bulk aqueous phase. On continued hydration, following the saturation of EDTA with calcium, the appearance of ‘free’ calcium causes precipitation of C-S-H gel from the bulk solution and changes in microstructure of the colloidal gel around clinker particles in C₃S and β-C₂S pastes are observed. The action of EDTA as a retarding admixture is explained in terms of the membrane model of cement hydration.     

A Reaction Zone Hypothesis for the Effects of Particle Size and Water‐to‐Cement Ratio on the Early Hydration Kinetics of C3S

The hydration kinetics of tricalcium silicate (C₃S) has been the subject of much study, yet the experimentally observed effects of the water‐to‐cement (w/c) ratio and particle size distribution have been difficult to explain with models. Here, we propose a simple hypothesis that provides an explanation of the lack of any significant effect of w/c on the kinetics and for the strong effect of the particle size distribution on the amount of early hydration associated with the main hydration peak. The hypothesis is that during the early hydration period the calcium–silicate–hydrate product forms only in a reaction zone close to the surface of the C₃S particles. To test the hypothesis, a new microstructure‐based kinetics (MBK) model has been developed. The MBK model treats the C₃S particle size distribution in a statistical way to save computation time and treats the early hydration as essentially a boundary nucleation and growth process. The MBK model is used to fit kinetic data from two published studies for C₃S with different size distributions, one for alite (impure C₃S) pastes and one for stirred C₃S suspensions. The model is able to fit all the data sets with parameters that show no significant trend with particle size, providing support for the reaction zone hypothesis.     

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.