Alite Dissolution Research Articles

New Insights into the Dissolution Kinetics of Alite Powder and the Effects of Organic Toughening Materials.

Alite dissolution plays a crucial role in cement hydration. However, quantitative investigations into alite powder dissolution are limited, especially regarding the influence of chemical admixtures. This study investigates the impact of particle size, temperature, saturation level, and mixing speed on alite powder dissolution rate, considering the real-time evolution of specific surface area during the alite powder dissolution process. Furthermore, the study delves into the influence of two organic toughening agents, chitosan oligosaccharide (COS) and anionic/non-ionic polyester-based polyurethane (PU), on the kinetics of alite powder dissolution. The results demonstrate a specific-surface-area change formula during alite powder dissolution: SS0=0.348e1-m/m0/0.085+0.651. Notably, the temperature and saturation level significantly affect dissolution rates, whereas the effect of particle size is more complicated. COS shows dosage-dependent effects on alite dissolution, acting through both its acidic nature and surface coverage. On the other hand, PU inhibits alite dissolution by blocking the active sites of alite through electrostatic adsorption, which is particularly evident at high temperatures.

Lead retardation on cement hydration: Inhibition and re-acceleration of clinker dissolution

Lead-bearing solid wastes strongly retard the hydration of Portland cement. In this work, the effect of Pb(NO3)2 and PbCl2 on the hydration of Portland cement and tricalcium silicate (C3S) was investigated. The results show that both Pb(NO3)2 and PbCl2 prominently delay the end of the dormant period, the setting time of cement paste, and decrease compressive strength especially at lead dosages of 1%. Conductivity, pH, and elemental concentration in C3S suspension underlined a strong effect of lead on C3S reaction kinetics. XRD and SEM-EDS measurements indicated the formation of lead silicates and chloride (hydr)oxides. Alite dissolution is probably retarded by the formation of surface complexes between Pb(II) with silanol groups on clinker surface. The increase of pH and precipitation of less soluble Pb-hydrates lowers Pb concentrations and the inhibition induced by Pb(II).

Enhanced alite dissolution by CAH10 addition

The influence of varying Al addition in the form of CAH10 on the hydration of alite was studied to gain further insights into the alite hydration mechanisms in the presence of increased Al concentration in pore solution. Therefore, pure alite and mixtures of alite and CAH10 were investigated by means of isothermal heat flow calorimetry, XRD and pore solution chemistry analysis. The two occurring hydration events can be attributed to the precipitation of two different silicate-bearing hydrate phases, C2ASH8 and C-(A)-S-H. In this case, the consumption and removal of Al in pore solution initiated the change to C-(A)-S-H-forming hydration. Furthermore, the formation of C2ASH8 and the alite reaction rate and reaction were directly influenced by the amount of Al provided by CAH10 addition, leading to an alite reaction turnover of up to 68–86 % within 48 h.

Phase-selective degree of hydration at setting: An in situ synchrotron diffraction study

Setting of Portland cements is still not well understood. Previous studies used techniques that analyse the pastes as a whole to correlate setting times with degree of hydration. Here, in-situ synchrotron X-ray diffraction analysed by the Rietveld method, with internal standard methodology, is used to derive alite dissolution rate and the crystallization kinetics of ettringite and portlandite in three commercial cements. Moreover, C–S–H gel precipitation pace has also been determined. C–S–H gel precipitation is key to account for setting. For grey Portland cements with w/c = 0.5, initial setting is attained when alite degree of hydration is close to 20% and the C−S−H gel content reached approximately 6 vol%.

The hydration of fast setting spray binder versus (aluminum sulfate) accelerated OPC

The (early) hydration mechanisms of two different binder systems used for shotcrete were investigated: the so far almost unexplored low sulfate binder (spray binder), used in the field of dry-mix shotcrete; and ordinary Portland cement, accelerated by aluminum sulfate, widely used for wet-mix shotcrete. The basis for the fast setting of the spray binder is the rapid dissolution of C3A and the subsequent formation of flaky CO3-AFm phases. Thereby induced high aluminum concentrations in the pore solution lead to a blockage of alite dissolution during the first hours of hydration. At later stages, higher amounts of portlandite are formed in the dry-mix, compared to the wet-mix system. The lower calcium availability for portlandite formation in the wet-mix system is explained by an enhanced formation of C–A–S–H phases with a higher Ca:Si ratio. Additionally, wet-mix systems show lower porosity and higher compressive strength after 1 d of hydration and beyond.Graphical abstract

Effect of citric acid-modified chitosan on the hydration and microstructure of Portland cement paste

In this study, citric acid-modified chitosan (CAMC) is prepared by an acylation reaction to regulate the exothermic process occurring in cement hydration. The thermodynamic process of cement hydration is characterized by isothermal calorimetry. The effects of various CAMC contents of 0%, 0.2%, 0.4%, and 0.6% on the hydration process were systematically studied by X-ray diffraction (XRD), inductively coupled plasma emission spectroscopy (ICP–OES), and proton nuclear magnetic resonance (1H-NMR). The results show that the adsorption effect delays the dissolution of alite (C3S). The content of CAMC is shown to have a significant effect on the formation time and amount of ettringite (AFt), and the competitive adsorption relationship increases the concentration of Ca2+. The increases in Ca2+ and SO4 2- concentrations resulted in an increase in the AFt amount. With prolongation of the hydration, the capillary pores of the cement paste are transformed into gel pores, and the cement slurry exhibits a self-drying phenomenon. Highlights Designed and prepared citric acid modified chitosan. Adsorption delays the dissolution of C 3 S and the formation of hydrated products. Competitive adsorption increases calcium ion concentration. CAMC delays the formation of AFt and increases the amount of AFt formation.

A dissolution model of alite coupling surface topography and ions transport under different hydrodynamics conditions at microscale

Portland cement is the most produced material in the world. The hydration process of cement consists of a group of complex chemical reactions. In order to investigate the mechanism of cement hydration, it is vital to study the hydration of each phase separately. An integrated model is proposed in this paper to simulate the dissolution of alite under different hydrodynamic conditions at microscale, coupling Kinetic Monte Carlo model (KMC), Lattice Boltzmann method (LBM) and diffusion boundary layer (DBL). The dissolution of alite is initialised with KMC. Two Multiple-relaxation-time (MRT) LB models are used to simulate the fluid flow and transport of ions, respectively. For solid-liquid interface, DBL is adapted to calculate the concentration gradient and dissolution flux. The model is validated with experiment from literature. The simulation results show good agreements with the results published in the literature.

Hydration mechanisms of sewage sludge ashes used as cement replacement

The influence of sewage sludge ash (SSA) on the hydration of Portland cement (OPC) was investigated in this study. Blending OPC with SSA was found to influence the kinetic of hydration, notably the alite dissolution. This retardation effect has been attributed to the presence of the orthophosphate ions (PO43−). However, SSA releases only a small fraction of PO43− into solution, which is deemed insufficient to precipitate calcium phosphates compounds on noticeable levels at least. On the other hand, an enhancement of the engineering-scale properties (i.e. mechanical and durability) and portlandite consumption are observed when SSA is used. This is explained by the formation of AFm phases, which leads to changes in the total volume of the solid and thus the properties of the systems. These experimental results were confirmed by thermodynamic modelling, showing that the additional alumina released by SSA results in the formation of higher amount of AFt and AFm phases.

Influence of w/s ratio on alite dissolution and C-S-H precipitation rates during hydration

The first 24 h of hydration of tricalcium silicate (C3S) have been studied at varying water/solid (w/s) mass ratios ranging from 0.5 to 10. Gathered data comprise the reaction degree, pore solution concentrations and specific surface area evolution. Combined with interfacial C3S dissolution rates and C-S-H precipitation rates found in the literature, these data are used to calculate theoretical reaction rates from C-S-H precipitation and C3S dissolution. Compared with the measured reaction rates, theoretical rates calculated from C-S-H precipitation show a good match, indicating a correct kinetic description of C-S-H precipitation. Theoretical rates calculated from C3S dissolution are higher than expected during the first hours of hydration which could be explained by the buildup of a metastable hydrate phase barrier. Furthermore, a comparison of the reaction rates shows no effect of the w/s ratio. The Ca concentration evolution indicates a delay of the first portlandite precipitation with increasing w/s ratio.

Effect of graphene oxide (GO) on the hydration and dissolution of alite in a synthetic cement system

An investigation is about the role of GO on the dissolution behaviours and hydration process of alite (Fe, F-C3S, abb. C3S) in a synthetic cement system, which also contains aluminate (Fe, F-C3A, abb. C3A) and gypsum. The results show that GO can increase the undersaturation levels of starting materials (C3S, C3A and gypsum) and thus promote their dissolution. The dissolution of C3A might be more sensitive than that of C3S due to the fluctuation of Ca concentration in pore solution before 30 min. Trace amount of GO could accelerate the interfacial dissolution rate of C3S throughout 24 h. As for the precipitation behaviours, GO could increase the supersaturation level of portlandite while decrease that of ettringite and C–S–H. In general, GO can improve the hydration degree of C3S by up to 15.0% and accelerate its hydration reaction rate by 33.6%. It is notable that the morphology changes of C–S–H from foil-like for reference to fibrous-like for GO-containing system at 2 h were likely attributed to the supersaturation of portlandite.

Alite dissolution and C-S-H precipitation rates during hydration

An extensive dataset suitable for kinetic calculations was collected for the hydration of monoclinic alite at water to solid ratios of 10 and 0.5. The measurement methods used to investigate the hydration behavior are XRD, TGA, solid state NMR, BET, SEM and ICP-OES. A comparison of the two water to solid ratios shows only minor effects of the w/s-ratio on the reaction kinetics. Reaction rates for the two kinetic steps alite dissolution and C-S-H precipitation are calculated from this dataset and compared to the measured reaction rates. Computed reaction rates from C-S-H precipitation show a good match with the measured reaction rates, indicating that the computation incorporates all relevant factors related to C-S-H precipitation kinetics during hydration.

Acceleration of OPC by CAC in binary and ternary systems: The role of pore solution chemistry

In dry-mortar-formulations calcium aluminate cement (CAC) is often used as a set accelerator for ordinary Portland cement (OPC). However, a critical amount of CAC is able to delay not only the silicate reaction but also the renewed C3A dissolution of an OPC. This delay can be counteracted by adding an appropriate amount of calcium sulfate (C$) to the mix OPC/CAC. The initial hydration period is dominated by fast ettringite formation from CA (CAC) and CaSO4 from OPC generating early strength. High aluminum and low calcium concentrations in the pore solution probably hinder C-S-H seeding and precipitation, thus preventing alite dissolution. Aluminum has to be initially removed by precipitating hydrate phases to change the pore solution chemistry. The start of the silicate reaction is then induced by increasing the calcium concentration in the pore solution.

Reactive transport modelling of a high-pH infiltration test in concrete

A laboratory-scale tracer test was carried out to characterize the transport properties of concrete from the Radioactive Waste Disposal Facility at El Cabril (Spain). A hyperalkaline solution (K-Ca-OH, pH = 13.2) was injected into a concrete sample under a high entry pressure in order to perform the experiment within a reasonable time span, obtaining a decrease of permeability by a factor of 1000. The concentrations of the tracers, major elements (Ca2+, SO42−, K+ and Na+) and pH were measured at the outlet of the concrete sample. A reactive transport model was built based on a double porosity conceptual model, which considers diffusion between a mobile zone, where water can flow, and an immobile zone without any advective transport. The numerical model assumed that all reactions took place in the immobile zone. The cement paste consists of C-S-H gel, portlandite, ettringite, calcite and gypsum, together with residual alite and belite. Two different models were compared, one with portlandite in equilibrium (high initial surface area) and another one with portlandite reaction controlled by kinetics (low initial surface area). Overall the results show dissolution of alite, belite, gypsum, quartz, C-S-H gel and ettringite and precipitation of portlandite and calcite. Permeability could have decreased due to mineral precipitation.

Influence of the reactivity of the amorphous part of mechanically activated alite on its hydration kinetics

The hydration of one mechanically activated alite passing through different drying procedures is examined by heat flow calorimetry and quantitative in-situ XRD analysis. The reactivity of the alite powders is strongly affected by the drying technique. It is shown that the reactivity of the amorphous part of the activated alite sample is particularly affected. Due to the fast initial dissolution of the amorphous “alite” part, the hydration progression is speeded up significantly. However, if the hydration is not speeded up by the amorphous “alite” dissolution, as in the case of surface passivation, the heat released until the transition to the deceleration period will increase. It is discussed that a crystalline alite dissolution by etch pit opening could increase the reactive alite surface and therefore increase the reaction degree at the transition to the deceleration period.

Early age hydration of cement pastes with alkaline and alkali-free accelerators for sprayed concrete

The objective of this study is to evaluate the early age hydration mechanisms of accelerated CEM I pastes. Liquid phase analysis, conductimetry, isothermal calorimetry, in situ XRD and SEM were performed on cement pastes produced with equivalent doses of an alkaline accelerator (sodium aluminate solution) and of an alkali-free accelerator (aluminum sulfate solution). Results showed that the addition of these chemicals first changes the ionic equilibria of the medium by the consumption of Ca2+ and SO42− ions from the liquid phase. The alkali-free accelerator contributes to the rapid formation of ettringite, as well as to a faster rate of alite dissolution and hydration. The aluminate accelerator leads to AFt and AFm formation, rapidly depleting gypsum and filling up the space, inhibiting further alite hydration. According to the results, variations in cement hydration mechanisms caused by accelerators with different chemical compositions are explained.

Morpho-topological investigation of the mechanisms and kinetic regimes of alite dissolution

The pure dissolution of alite MIII polymorph under various flow conditions and initial calcium hydroxide concentrations was investigated in a designed flow through dissolution cell. Dissolution profiles and their associated dissolution rates were measured using vertical scanning interferometry (VSI). Two main characteristic features were observed: the rate of dissolution decreases non-linearly with the amount of calcium hydroxide in the initial solution composition and a transition of rate limiting factor occurs for a threshold undersaturation of alite. This behaviour is deduced from the dependency of the rate of dissolution with the flow rate which decreases drastically at low undersaturation level and further supported by the determination of apparent activation energies of dissolution. The study is complemented with surface imaging of dissolution front from samples submitted to different experimental conditions.

Analytical Model for the Alite (C3S) Dissolution Topography

The surface topography that develops during the dissolution of alite (or C3S) has never been considered as an important aspect of the hydration of portland cement. Like many other minerals, alite dissolution results in the formation of etch pits. In this study, a simple model with a handful of parameters is proposed to explore the kinetic consequences of pitting on the dissolution of alite. The first consequence is an accelerating period during which the reactive surface, that is, the pit, expands. This new feature, to the authors’ opinion never reported previously for alite, is supported by experimental data. The mechanisms leading to the activation of the initial dissolution centers and to the growth of pits could be hindered by some inhibitory species such as aluminum ions or organic molecules. The model indicates that only a few assumptions about the formation of pits in the presence of these species are necessary to introduce an initial period of low dissolution prior to the accelerating phase. Such a low early reactivity is similar to the so‐called dormant period observed during portland cement hydration. The implications of this new model in cement hydration could go beyond this article and shed light on still unresolved fundamental questions on hydration behavior.

Micro‐reactors to Study Alite Hydration

A new approach to study the early hydration reaction of alite is presented. It relies on milling micron‐sized gaps into alite grains using a Focused Ion Beam. This provides for a “micro‐reactor” in which hydration in confined conditions can be followed after introducing aqueous solutions of any desired composition. Hydration is followed by stopping the reaction at desired ages using solvent exchange and imaging the gaps using a Scanning Electron Microscope. These micro‐reactors offer several advantages over conventional methods to monitor alite hydration. In particular, we can study the time evolution of dissolution in a reproducible manner using a setup that mimics particles in close contact. This paper presents this new methodology and applies it to studying the effects of solution composition and selected chemical admixtures on the dissolution of alite. New insights into the role of these factors are obtained and discussed.

Quantitative analysis of C–S–H in hydrating alite pastes by in-situ XRD

In this study about the early hydration of monoclinic alite, a method is presented that allows to follow the formation of the Calcium–Silicate–Hydrates directly and quantitatively over time. The combination of the PONKCS method [20] and the G-factor method [6] allowed the refinement and calibration of a C–S–H phase model which can be used for direct quantification of C–S–H in cementitious pastes.The comparison between the heat flow measured by calorimetry and the heat flow calculated from the phase development of C–S–H revealed that C–S–H formation starts later than is indicated by the measured heat flow. It is therefore assumed that the silicate reaction proceeds the following way: A not XRD-detectable C–S–H phase is precipitated which then evolves into a long-range ordered, XRD-detectable C–S–H. This C–S–H acts as a nucleation site for further long-range ordered C–S–H and leads to the acceleration of alite dissolution and direct formation of C–S–H.

A flow-through method for measuring the dissolution rate of alite and Portland cement clinker

A flow-through method for measuring dissolution rates of cement minerals is described. The quantity of dissolved material is measured after the dissolution experiment from SEM-BSE images. This makes it possible to measure dissolution rates of individual phases in multiphase cements. The SEM images also directly show if precipitation (hydration) occurred and how the surface roughness changed during dissolution. Furthermore, the method is designed for the purpose of determining if dissolution rate is reaction- or diffusion-controlled, by changing water flow rate or by calculating expected diffusion fluxes. Dissolution rates of flat-ground Portland cement clinker and synthesized alite in deionized water were measured to demonstrate the potential of the method. Initial dissolution rates at lower flow rates were transport-controlled as indicated by the flow rate dependency and predicted diffusion fluxes. The highest alite dissolution rate measured in this study was 93μmol/m2/s.