Precipitation Of Portlandite Research Articles

Modifications to reaction mechanisms, phase assemblages and mechanical properties of alkali-activated slags induced by gypsum addition

Alkali-activated slags (AASs) are often reported to exhibit fast settings, undermining their utilisation as sustainable binders from a technological point of view. Although gypsum was shown in previous studies to change the setting time of AASs, understanding the mechanism behind the changes in the reactivity and microstructure of the AASs in the presence of gypsum remains unclear which limits its utilisation in AASs. For the activator conditions used in this research (i.e., Na2O/raw material = 4.5 % and SiO2/Na2O = 1.0), gypsum was observed to dissolve quickly in the system, promoting the precipitation of portlandite and thenardite phases which led to a reduction in the pH of the pore solution causing a slow dissolution of slags. Furthermore, gypsum addition increased the setting time and delayed the compressive strength development of the paste, accompanied by a reduction in the total measured heat. The description of the changes in phase assemblages and kinetic of the system provided here is fundamental for predicting the behaviour of gypsum in AASs and promoting the industrial wide-uptake of AAS technology.

Effect of hydroxide, carbonate, and sulphate anions on the β-dicalcium silicate hydration rate

The effect of fixed (0.8 M) Na+ balanced by either OH−, SO42−, and CO32− anions on β-dicalcium silicate (C2S) reactivity is investigated. The exothermic reaction of varying water-to-solid ratios and chemical activation is monitored. Subsequently, the hydration products are characterized via FTIR, TG/DTG, QXRD, and SEM analysis. The findings showed that the carbonate ions expedited the reactivity up to 55% at 1–7 days due to the simultaneous precipitation of calcium silicate hydrate (C-S-H) and calcite. At 7–28 days, the lack of transportable cations between the solid surface and solution impeded further hydration, as confirmed by in-situ pH and conductivity measurements. The sulphate ions accelerated the reactivity only upon calcium sulphate dissolution at high pH. The hydroxide ions decelerated the hydration due to the earlier precipitation of portlandite than C-S-H. Overall, the β-C2S reaction with water exhibited the highest hydration degree (~67%) after 28 days of hydration.

Effect of elevated Al concentration on early-age hydration of Portland cement

To understand the effect of elevated Al concentration on early hydration in Portland cement, hydration heat, the evolution of solid and aqueous composition in paste blended with reactive alumina were investigated in this study. The increase of Al concentration depletes gypsum rapidly with massive precipitation of ettringite during the pre-induction period, resulting in decreased Ca and S concentration in pore solution. The elevated Al concentration passivated the active sites on C3S surface, and the decreased Ca concentration inhibited the nucleation of C–S–H and precipitation of portlandite, hence prolonging the induction period. Meanwhile, the reaction of C3A has not been accelerated even after the complete depletion of gypsum, which is associated with the lowered undersaturation degree caused by the increased Al concentration. Additionally, the precipitation of large amounts of ettringite and foil-shaped C–S–H reduced available nucleation sites, thus hindering the reaction of both C3S and C3A during the acceleration period.

Effect of alkali and sulfate on early hydration of Portland cements at high water to cement ratio

In this work, the hydration kinetics and the development of concentrations in the pore solution of cement clinker, neat and composite cements pastes were investigated.Early alite hydration is mainly limited by the nucleation and growth of the hydrates (C-S-H, CH, ettringite). Only after 4 to 10 h, when the main alite reaction starts, the dissolution rate of the alite becomes the limiting factor. The kinetics of the reaction of C3A with anhydrite is initially limited by the slow nucleation and formation of ettringite and only later by the dissolution rate of calcium aluminate phase. The change of the mechanisms corresponds to the initiation of the portlandite precipitation. The slower reaction of clinker sample cannot be explained by the high Al concentration alone but rather by the effect of available sulfate; high sulfate concentrations accelerate the hydration. Higher alkali hydroxide concentration does not show an accelerating effect on PC hydration.

Assessment for utilization of tobacco stem ash as a potential supplementary cementitious material in cement-based composites

Present study investigates the effectiveness of utilizing tobacco stem ash (TSA) as the partial replacement of Portland cement (PC) on the properties of cement pastes and mortars. Cement pastes and mortars were fabricated by replacing cement with TSA at four replacement levels i.e., 3.75, 7.5, 11.25, and 15 wt% relative to cement. Setting time, consistency, and hydration activity tests were performed on TSA-based cement pastes. Moreover, water absorption, porosity, compressive strength, ultrasonic pulse velocity (UPV), and electrical resistivity (ER) testing were performed on TSA-based mortars. The pozzolanic behavior of TSA was assessed via Frattini test and strength activity index. Analytical characterizations such as x-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), thermogravimetric (TG), scanning electron microscopy (SEM), and energy-dispersive x-ray (EDX) analyses were also performed. The results indicate lower water absorption and apparent porosity, leading to enhanced compressive strength, UPV, and ER values for TSA-based mortars. The analytical characterizations show that TSA promotes the hydration reaction and formation of a greater amount of cement hydrates. The strength activity index and Frattini test results showed that TSA possesses moderate pozzolanic activity at a higher replacement level. While, for low replacement levels of TSA in cement composites, the hydration reaction is governed by rapid precipitation of portlandite due to higher hydration activity.

Comparative study on mechanisms for improving mechanical properties and microstructure of cement paste modified by different types of nanomaterials

Abstract Filling and nucleation are the mechanisms of modifying cement paste with nanomaterials, as investigated by previous studies, and are difficult to reflect the different effects of nanomaterials, especially on the changes of cement clinker and hydration products in the cement hydration process. In this study, the mechanisms of modifying cement paste with nano-calcium carbonate (NC), nano-graphene oxide (NG), nano-silica (NS), and nano-titanium dioxide (NT) were investigated by determining the mechanical properties of cement paste treated with nanomaterials and analysing the changes in the cement clinker (tricalcium silicate and dicalcium silicate), hydration products (portlandite and ettringite), and microstructure through many micro-test methods. The results indicate that the incorporation of nanomaterials could improve the early strength of cement paste specimens due to more consumption of cement clinker. Meanwhile, different nanomaterials promote the formation of different hydration products at early ages. C–A–S–H gel, flower-like ettringite, and C–S–H seeds are widely distributed in the cement paste with the incorporation of NC, NG, and NS, respectively. NT exhibits insignificant nucleation effect and has inhibitory effect on portlandite precipitation. This study provides key insights into the mechanism of nanomaterials from the perspective of cement hydration, which may promote the further research and application of nanomaterials in the cement and concrete industries.

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.

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.

A new view on the kinetics of tricalcium silicate hydration

C3S hydration is an interesting example of chemical coupling between C3S dissolution, C–S–H and portlandite precipitation. It occurs because Ca2+, OH− and silicate ions are present in C3S, in both hydration products and in the surrounding solution. Various experimental data sets reveal that the undersaturation with respect to C3S always increases when C3S hydration enters into the deceleratory phase, leading to the conclusion that C3S dissolution is at the origin of this deceleration, not C–S–H growth. In addition, as soon as portlandite precipitates, the dissolution limits the hydration already in the acceleratory hydration step. The evolution of the undersaturation cannot account for the hydration peak. Rather, it results from an extension and subsequently a decrease of the C3S reactive surface area. The formation and coalescence of dissolution etch-pits can provide a reasonable explanation for the C3S hydration kinetics.

Swelling behavior of ion exchange resins incorporated in tri-calcium silicate cement matrix: I. Chemical analysis

This paper presents the first part of a theoretical and experimental work aiming at modeling the chemo-mechanical behavior of composites made up of ion exchange resins (IER) solidified in a tri-calcium silicate cement paste (C3S). Because of ion exchange processes, the volume change of the IER may cause internal pressures leading to the degradation of the material. In this study, a predictive modeling is developed for describing the chemical behavior of such material. It is based on thermodynamic equilibria to determine the evolution of the ion exchange processes, and the potential precipitation of portlandite in the composite. In parallel, a phenomenological study has been set up to understand chemical phenomena related to the swelling mechanisms. The model created has been finally implemented in a finite elements software; the simulation of a laboratory test has been performed and the results compared to experimental data.

Calorimetric study of the early stages of the nanosilica - tricalcium silicate hydration. Effect of temperature

The pozzolanic reaction of nanosilica in contact with C3S was monitored via isothermal conduction calorimetry. Since that reaction is affected by heat, its variations with curing temperature (25, 40 and 65 °C) were also analysed. The findings showed that while C3S hydration was accelerated by both nSA and higher curing temperature, the former impacted the reaction more intensely. Moreover, the presence of nSA was observed to shorten the acceleration stage during C3S hydration, inducing earlier precipitation of portlandite and earlier C-S-H precipitation and nucleation.

A Multi‐Ionic Continuum‐Based Model for C3S Hydration

A computationally efficient strategy for modeling tricalcium silicate hydration based on through‐solution‐phase kinetics is demonstrated. This study extends a recently introduced advanced continuum‐based single particle model by including rigorous multi‐ionic transport, nonlinear reversible reaction kinetics and portlandite precipitation. Model parameters were either fixed based on known values, estimated using experimental measures or extracted by model fitting to benchmark experimental datasets. The model is now able to generate calorimetric hydration and evolution of pore solution chemistry responses that are in good agreement with available experimental results and predictions of other multiphysical modeling platforms. Oncecalibrated, the model was tested to see if it could predict the effect of water to cement ratio (w/c) and particle size on hydration outcomes. The findings support the need for a mechanism that limits the volume into which product can form.

Alite-C<sub>3</sub>A-Gypsum系の水和反応における相互作用機構の解明

For understanding the cement hydration composed of high C₃A content, pure clinker phases such as Alite, Tricalcium aluminate (C₃A) were synthesized and five different compositions of mixtures were prepared. The hydration of samples were followed by calorimetry. Quantitation of the crystalline and amorphous products were conducted by X-ray diffraction and Rietveld analysis. It was found that the hydration between C₃A and Gypsum was accelerated by hydration of Alite. It was implied that amorphous materials produced from the hydration between C₃A and Gypsum as well as the hydration of Alite. Rise of pH due to precipitation of portlandite from Alite hydration accelerated the hydration between C₃A and Gypsum. It could be explained on the basis of thermodynamic equilibrium theory that AFm phase with low sulfate content precipitated in the vicinity of C₃A particle. In the bulk solution far from the surface of C₃A, AFt with high sulfate content precipitated. Subsequently, because of low sulfate content in AFm, mass transfer into C₃A particle kept proceeding and hydrating C₃A continuously progressed. That is why hydration reaction of C₃A proceeds when added Alite in contrast to the hydration without Alite.C₃A含有量の多いセメントの水和反応について理解することを目的とし、その主たる反応を司るAlite、C₃A、Gypsumを混合したモデルセメントの水和実験を行った。発熱量測定、XRD・リートベルト解析、TG-DTA測定の結果、Aliteの水和反応がC₃A-Gypsum系の水和反応を促進させることを確認した。さらに、その要因はAliteの水和反応からPortlanditeが生成することによるpH上昇であり、その際C₃A粒子付近に析出するAFm相に含まれるSO₄²⁻量が少ないため、未水和C₃Aへの物質の透過が継続するからであることが示唆された。

Cement paste prior to setting: A rheological approach

The evolution of cement paste during the dormant period is analyzed via small amplitude oscillation rheological measurements. Cement paste, from the very first moments after mixing cement and water, shows the formation of an elastic gel whose strength is rapidly increasing over time. Up to the onset of Portlandite precipitation G′(t) increases by more than 2 orders of magnitude and in the acceleratory period G′(t) continues steadily to increase. A microstructural modification is likely to occur between the dormant and the acceleratory period. At low deformations in the linearity domain the storage modulus G′(ω) exhibits a negligible frequency dependence. At higher deformations cement paste shows a yield stress which increases on increasing paste concentration.The presence of superplasticizers decreases the yield stress and increases the gelation threshold of the paste. Above the gelation threshold the evolution of cement paste with superplasticizers follows similar trends to the neat paste.

Sequential precipitation of a new goethite–calcite nanocomposite and its possible application in the removal of toxic ions from polluted water

This study proposes a simple and innovative synthesis route for a goethite–calcite nanocomposite. This synthesis is summarised by three sequential precipitation reactions: (1) precipitation of nanosized acicular goethite (α-FeOOH) using a high OH/Fe molar ratio (=5); (2) instantaneous precipitation of portlandite (Ca(OH)2) by adding CaCl2 salt to a goethite alkaline suspension (2NaOH+CaCl2→Ca(OH)2+2NaCl) and; (3) sub-micrometric calcite precipitation by injection of CO2 into a goethite–portlandite alkaline suspension (Ca(OH)2+CO2→CaCO3+H2O). The XRD patterns have confirmed the goethite and calcite mineral composition in the composite precipitated at 30 and 70°C. FESEM and TEM observations have revealed the formation of nanosized goethite particles well dispersed with sub-micrometric calcite particles, leading to an orange–brown colour nanocomposite with high specific surface area of around 92m2/g for a composite synthesized at 30°C and 45m2/g for a composite synthesized at 70°C. Both values were determined using the conventional BET method on N2 sorption isotherms. Finally, a goethite/calcite weight ratio equal to 0.8 in the composite was determined by Thermogravimetric Analysis (TGA). Additionally, some adsorption experiments carried out at two different pH values revealed that the goethite–calcite composite has a good sequestration capacity for Cu>Cd>As(III)>Se(IV)>As(V). Conversely, the Se(VI) did not show any chemical affinity with the goethite–calcite composite under the physico-chemical conditions studied. In practice, the goethite–calcite composite can neutralise acidic wastewater by slight calcite dissolution, enhancing the removal of heavy metals (e.g. Cu and Cd) at the calcite–solution interfaces.

Pore-scale modeling of reactive transport in wellbore cement under CO2 storage conditions

A modular pore-scale model is developed to assess the response of wellbore cement to geological storage of CO2. Numerical formulations for modeling of solute transport are presented and a methodology for coupling with geochemical processes is discussed, which includes: (1) advective and diffusive fluxes of solutes within the pore space, (2) aqueous phase speciation, (3) mineral dissolution–precipitation kinetics, and (4) the subsequent changes in pore space geometry. A Complex Pore Network Model (CPNM) is used to discretize the continuum porous structure as a network of pore bodies and pore throats, both with finite volumes. CPNM allows for a distribution of pore coordination numbers ranging between 1 and 26. This topological property, together with a geometrical distribution of pore sizes, enables the microstructure of porous media to be mimicked. For each pore element, transport of solute is calculated by solving the governing mass balance equations. Chemical reaction of the fluid phase with the main reactive solid components (portlandite and calcite) is incorporated through coupling with a geochemical reactive simulator. Average values and properties are obtained by integration over a large number of pores. Using this approach, we investigate how chemical reaction between water-bearing wellbore cement and supercritical CO2 can create a distribution of porosity in a direction parallel to the CO2 concentration gradient and transport path, at 50°C. The dynamics of this process involve interaction between diffusion dominated mass transport and the kinetics of dissolution and precipitation of portlandite and/or calcite. Simulation of unconfined chemical degradation, in a fluid of constant composition, shows development of different regions: (1) a zone adjacent to the inlet face, which is characterized by an increase in porosity due to extensive dissolution, (2) a carbonation zone with decreased porosity, (3) the carbonation front which made a thin layer with the lowest porosity due to calcium carbonate precipitation, and (4) dissolution zone. These results are in agreement with laboratory observations under similar conditions. This provides confidence that this pore-scale approach can ultimately be applied to model the progress of coupled CO2 transport and cement degradation at critical points along the length of cemented wellbore sections at CO2 storage sites.

The influence of sodium and potassium hydroxide on alite hydration: Experiments and simulations

The basic nature of alkali hydroxides (NaOH, KOH) when added to mixing water, increases the pH in proportion to the level of salt addition. For alite (impure tricalcium silicate; MIII-Ca3SiO5) hydration, this pH increase accelerates the rate of hydration and reduces the duration of the induction, acceleration and deceleration regimes. This study evaluates alite hydration in solutions of varying compositions and alkalinities (0.1M, 0.2M and 0.5M NaOH and KOH) in context of their heat release behavior and analysis of the solid/liquid phases. The modeling platform, μic, is used to simulate, describe and discriminate the impact of the pore solution chemistry and reaction product formation parameters on alite hydration (Bishnoi and Scrivener, 2009 [1]). Numerical predictions of the solid and liquid phase compositions and the heat release response show good agreement with experimental determinations. The simulations indicate that the effects up to the end of the induction period follow directly from a change in the pore solution composition under a solution controlled dissolution mechanism, which leads to the faster precipitation of portlandite. The changes in the main heat evolution peak appear to be related to an increase in the nucleation density of C–S–H in alkali hydroxide solutions. Examination under the SEM did not indicate significant difference in C–S–H morphology and composition in the presence of NaOH/KOH.

Procedure to use phosphogypsum industrial waste for mineral CO 2 sequestration

Industrial wet phosphoric acid production in Huelva (SW Spain) has led to the controversial stockpiling of waste phosphogypsum by-products, resulting in the release of significant quantities of toxic impurities in salt marshes in the Tinto river estuary. In the framework of the fight against global climate change and the effort to reduce carbon dioxide emissions, a simple and efficient procedure for CO 2 mineral sequestration is presented in this work, using phosphogypsum waste as a calcium source. Our results demonstrate the high efficiency of portlandite precipitation by phosphogypsum dissolution using an alkaline soda solution. Carbonation experiments performed at ambient pressure and temperature resulted in total conversion of the portlandite into carbonate. The fate of trace elements present in the phosphogypsum waste was also investigated, and trace impurities were found to be completely transferred to the final calcite. We believe that the procedure proposed here should be considered not only as a solution for reducing old stockpiles of phosphogypsum wastes, but also for future phosphoric acid and other gypsum-producing industrial processes, resulting in more sustainable production.

Changes in C 3S hydration in the presence of cellulose ethers

The influence of cellulose ethers (CE) on C 3S hydration processes was examined in order to improve our knowledge of the retarding effect of cellulose ethers on the cement hydration kinetics. In this frame, the impacts of various cellulose ethers on C 3S dissolution, C-S-H nucleation-growth process and portlandite precipitation were investigated. A weak influence of cellulose ethers on the dissolution kinetics of pure C 3S phase was observed. In contrast, a significant decrease of the initial amount of C-S-H nuclei and a strong modification of the growth rate of C-S-H were noticed. A slowing down of the portlandite precipitation was also demonstrated in the case of both cement and C 3S hydration. CE adsorption behavior clearly highlighted a chemical structure dependence as well as a cement phase dependence. Finally, we supported the conclusion that CE adsorption is doubtless responsible for the various retarding effect observed as a function of CE types.

The interaction of magnesium in hydration of C3S and CSH formation using 29Si MAS-NMR

Hydration of tricalcium silicate in hydrothermal conditions in the presence of magnesium oxide has shown changes in the formation of CSH gel structure (Calcium silicate hydrates). The new CSH incorporates magnesium ions, brucite, but a weak presence of portlandite. The magnesium oxide would hinder the precipitation of portlandite. The characterization of CSH gel by 29Si MAS-NMR with various CaO/SiO2 ratios would point out that: (1) A dreierketten structure of the CSH for low CaO/SiO2 1, magnesium ions would be incorporated in the silicate chains of the CSH gel in a tetrahedral coordination. Although, the low MgO/CaO ratios of CSH gels indicate that the magnesium incorporation in CSH chain is low.