Reaction Of Alite Research Articles

Sulfate consumption during the hydration of Alite and its influence by SCMs

AbstractSCMs with a large specific surface area, such as calcined clays or fine limestone powder, can significantly accelerate the occurrence of the aluminate heat flow peak during the early hydration of blended cements. These systems therefore require a higher amount of sulfate. While this effect has been drawn back to the sulfate incorporation into C‐S‐H, which is formed more rapidly due to the filler effect, it has been recently shown that in systems with high amounts of metakaolin, the sulfate balance is influenced by further mechanisms. This study provides new results on the influence of aluminum‐rich and aluminum‐free SCMs on the early hydration of alite in presence of calcium sulfate. A synthesized alite was blended with 5 wt% of anhydrite and its hydration was investigated by isothermal calorimetry and in‐situ XRD at a w/b ratio of 0.5. It was shown that the formed C‐S‐H is able to take up the calcium and sulfate ions, indicated by a depletion of the solid anhydrite after approximately 20 hours. This process is significantly accelerated by incorporating 30 wt% of a fine limestone filler. The addition of metakaolin leads to an acceleration of the alite hydration compared to the reference as well, but to a significantly lower extend compared to the limestone. However, this system requires a lower degree of alite reaction to reach sulfate depletion. This indicates that in metakaolin‐rich systems, the sulfate demand is influenced by further effects. The results from in‐situ‐XRD reveal that significant ettringite formation is taking place in the metakaolin‐blended system already during the first hours of hydration. Correlations between the degree of alite hydration and the rate of sulfate depletion provide new insights into the mechanisms affecting the sulfate balance and early hydration of blended cements.

Influence of C–S–H seed and sodium sulfate on the hydration and strength of limestone calcined clay cement

Reducing the cement content in blended cements is one of the most promising ways to achieve low carbon dioxide emission and sustainable development of cement industry. However, as the substitution level of cement increases, the early age strength of blended cements decreases significantly, limiting their application in situations where sufficient early age strength is required. In the paper, the influence of 0.5% C–S–H seeds and 2% sodium sulfate (CS) on the hydration and strength of limestone calcined clay blended cement (LC3) with three different cement content were systematically studied, trying to understand the potential of this technique in improving the mechanical properties of LC3. The results showed that the addition of CS significantly enhances the early age strength of LC3, enabling the system with 54% and 38% cement content to reach strength level comparable to the pure cement system at 1 and 2 days, respectively. The enhancement is due to the accelerated reaction of alite and tricalcium aluminate as well as the pozzolanic reaction of metakaolin, which results in a lower porosity of the CS added samples compared to the reference (without adding CS). Nevertheless, CS negatively affects the development of later age strength of LC3, indicating the imperfection of this technique. Therefore, further studies are needed to improve this shortcoming.

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.

Enhancement of early age cementitious properties of yellow phosphorus slag via CO2 aqueous carbonation

Yellow phosphorus slag (YPS) is an industrial waste by-product and has pozzolanic characteristics. Due to its poor early-age reactivity, this study aims to overcome this limitation via CO2 activation of raw YPS (RYPS) and use as a cement substitute. The experimental results showed that carbonation of RYPS under optimal conditions (liquid to solid ratio of 8 ml/g, temperature of 80 °C, pressure of 0.1 MPa, and for 9 h) can form rich amorphous silica gel and hydro calcite-type CaCO3. As a result, carbonated YPS (CYPS) was utilized as a supplementary cementitious material to improve the early age of pozzolanic characteristics in cement paste. This is because the formation of hemi- and mono-carboaluminates and amorphous silica gel played the important role in early-age strength development in CYPS incorporated cement pastes. Moreover, the presence of CYPS can accelerate the hydration rate of cement clinker, together with the intensification of the alite reaction. The microstructure of composite cement paste containing CYPS was denser compared to that of corresponding samples with RYPS.

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.

On the mechanisms for improved strengths of carbon nanofiber-enriched mortars

As nanomaterials, carbon nano-fillers have manyfold benefits in concrete applications, and while of considerable research interest, their impacts on cement phases are still not fully understood. We applied multiple techniques, including mercury intrusion porosimetry, X-ray diffraction, thermal gravimetry, infrared spectroscopy and electron microscopy to investigate in detail the effects of a pre-mixed carbon nanofiber admixture in mortars. The addition of carbon nanofibers was observed to: (i) alter pore structure to a lower total porosity with greater proportion of micropores, (ii) delay the hydration reaction of alite, (iii) shift hydration of alite towards the formation of portlandite and (iv) foster formation of calcium carbonate phases. The ensemble of results indicate that improved performance is unlikely to be due to formation of additional calcium silicate hydrates, as has been suggested in the literature, but instead is due to the combined effect of nano-fiber bridging and pore-filling by portlandite and calcium carbonates.

Influence of sulfur trioxide in clinker on the hydration heat and physical properties of Portland cement

The cement industry has achieved stable usage of waste gypsum board in clinker production. However, few studies have focused on the heat of hydration and physical properties of cements with high-SO3 clinker with different C3A and gypsum contents. Clinker with different amounts of SO3 and coexisting C3A was prepared in an electric furnace. Cement samples were then prepared by adding gypsum. Increasing the clinker SO3 content accelerated the alite reaction and caused aluminate hydrate reaction to occur later. Compressive strength at 3 days increased with SO3 content whereas that after 28 days decreased. The drying shrinkage strain of mortar decreased with increasing SO3 content of the clinker and gypsum at each C3A content. The decrease of drying shrinkage attributed to SO3 in the clinker was smaller than that caused by gypsum SO3. As the clinker SO3 content increases, cement physical properties can be maintained by adjusting the gypsum content.

The influence of the filler effect on the sulfate requirement of blended cements

In some blended cements, the optimum amount of sulfate addition differs from that observed in OPC. This study aims to understand the mechanism behind the impact of two SCMs, namely metakaolin and limestone, on the sulfate balance of blended cements. No significant impact of the aluminum content of these SCMs was observed. Instead, it is observed that the filler effect of the SCM, which accelerates the reaction of alite, is the main factor impacting the sulfate balance. As the rate of precipitation of C-S-H is increased, more sulfate is adsorbed by the C-S-H and consequently, the depletion of gypsum is reached earlier in time during the hydration process. A relationship between heat release at the onset of the aluminate peak and the gypsum content of the system was established.

Chemical activation of hybrid binders based on siliceous fly ash and Portland cement

The hydration of Portland cement (PC) blended with a high amount of a siliceous fly ash (70% fly ash, 30% PC) has been examined. The fly ash contributes significantly to the long-term strength development, when compared to a reference sample with quartz powder. However the long setting time and the poor early strength prevent the use of such binders. Therefore the effect of different activators (sodium carbonate, potassium sodium silicate, potassium citrate and sodium oxalate) on the setting, the hydration kinetics and the strength development of the fly ash-PC blend has been investigated.The addition of the activators increases the pH and decreases thus the calcium concentrations in the pore solution, which leads to a faster reaction of alite and thus to early setting and increased early strength. On the long term, the high alkali concentrations lower the compressive strength and lead to a (partial) destabilization of ettringite.Sodium oxalate and potassium sodium silicate accelerate both the setting of the fly ash-PC blend and increase the early compressive strength. Furthermore, they show better compressive strengths at later ages compared to the other activators. Based on these findings, they can be considered as the most suitable accelerators among the investigated activators.

Structural and nano-mechanical properties of Calcium Silicate Hydrate (C-S-H) formed from alite hydration in the presence of sodium and potassium hydroxide

This research evaluates the effect of sodium and potassium hydroxide on the structure and nano-mechanical properties of Calcium Silicate Hydrate (C-S-H) formed from the hydration of pure alite. Monoclinic (MIII) alite was synthesized and hydrated, using water-to-alite ratios of 0.5 and 0.6 and additions of 10% NaOH and KOH by weight of alite. Based on results of X-ray diffraction, isothermal calorimetry, thermogravimetric analysis, Nuclear Magnetic Resonance and nanoindentation, two different effects of the alkaline hydroxides on the hydration reaction of alite, both at early and later ages, can be identified: (i) a differentiated hydration process, attributed to an enhancement in calcium hydroxide (CH) precipitation and a stimulation of the C-S-H nuclei; and (ii) an increase in the elastic modulus of the C-S-H aggregations, attributed to an electrostatic attraction between positive charges from the alkaline cations and negative charges from the C-S-H structure.

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.

The hydration chemistry of ProRoot MTA.

'Tooth-coloured' ProRoot MTA is an endodontic cement comprising an 80:20 wt% mixture of white Portland cement (WPC) and bismuth oxide. The setting reactions within this cement system are not currently well understood. Accordingly, this research monitors the early hydration chemistry of ProRoot MTA by X-ray diffraction, solid state nuclear magnetic resonance and Fourier transform infrared spectroscopies, and isothermal calorimetry. The initial rate of hydration is rapid with 40% having reacted within the first 24 h; it then slows considerably such that within 3 days the hydration reactions are 58% complete and only increase by a further 1% within one week. The relatively fast reaction of alite to form C-S-H gel and portlandite, and the development of the calcium aluminosulphate phases, are as would be anticipated for the hydration of pure WPC. These findings confirm that bismuth oxide is an inert additive which does not participate in the hydration reactions.

The Influence of Methylcellulose on the Hydration Process of C3S, C3A and Mixture of these Phases

The paper presents the results of investigation over influence of 0.3% addition of hydroxypropyl methylcellulose (MC) of 40 and 70 Pa·s viscosity on the hydration process of main clinker phases: C3S and C3A and also their mixture, both in presence and absence of gypsum.During the research it has been indicated that methylcellulose inhibits the hydration process of clinker phases, analyzed both separately and in their mixture. Moreover, presence of that additive reduces reactivity of calcium sulphatedihydrate what leads to formation of decreased amounts of ettringite. No significant differences resulting from viscosity of used methylcellulose were observed.Investigation over the influence of that admixture on the hydration process of tricalcium aluminate indicated that addition of methylcellulose inhibits hydration process of C3A with water. Similar influence of that admixture was observed in the case of C3A hydration in presence of gypsum. A lot of gypsum remains in the reaction mixture. Little amounts of ettringite are forming, however, there is no monosulphate. Inhibition of sulphate ions reaction with C3A is slightly higher in the case of methylcellulose of higher plastic viscosity.Also in the hydration examinations of tricalcium silicate methylcellulose addition caused a retardation of alite reaction with water. In first hydration hours of alite with MC addition there were no peaks of portlandite in the XRD pattern. In the presence of gypsum the influence of methylcellulose addition on the hydration course was significantly weaker. Simultaneously no significant differences related to the influence of different plastic viscosity of methylcellulose were observed.Similar course of reactions was observed in the case of hydration of alite and tricalcium aluminate mixture in the presence of gypsum and methylcellulose. After one week of hydration significant amounts of unreacted gypsum are still remaining. However, little amounts of ettringite are created.

Influence of limestone and anhydrite on the hydration of Portland cements

The addition of CaCO3 and CaSO4 to Portland cement clinker influences the hydration and the strength development. An increase of the CaSO4 content accelerates alite reaction during the first days and results in the formation of more ettringite, thus in a higher early compressive strength. The late compressive strength is decreased in Portland cements containing higher quantities of CaSO4. The reduced late compressive strength seems to be related to an increase of the S/Si and Ca/Si content in the C–S–H.The presence of calcite leads to the formation of hemicarbonate and monocarbonate thus indirectly to more ettringite. Only a relatively small quantity of calcite reacts to form monocarbonate or hemicarbonate in Portland cement. Although hemicarbonate is thermodynamically less stable than monocarbonate, hemicarbonate formation is kinetically favored. Monocarbonate is present only after 1week and longer independent of the quantity of calcite available and the content of sulphate in the cement.

Interactions between alite and C3A-gypsum hydrations in model cements

This paper focuses on the interactions that occur between cement phases during early hydration, especially between the hydration of alite and C3A-gypsum. This study shows that alite reaction is accelerated in the presence of gypsum. This acceleration was attributed to the interaction of gypsum with Aluminium ions present in alite. Three exothermic peaks attributable to the aluminate reaction could be observed in model cements instead of one in C3A-gypsum systems. The first corresponds to the C3A dissolution when the sulfate ions run out, but the product formed is ettringite. The second corresponds to the formation of calcium monosulfoaluminate, but the third could not be assigned to any specific reaction. The effect of alite/C3A ratio, the dispersion of the phases in the grains and the temperature were also investigated. With respect to temperature, it was found that the activation energy for the C3A-gypsum reaction was higher than that for alite.

RATE OF HYDRATION OF ALITE AND BELITE IN PORTLAND CEMENT

Hydration process of many kinds of cement paste is investigated by Powder X-ray diffraction (XRD)/Rietveld analysis. The parameters of the present study are type of cement (ordinary Portland cement, low heat Portland cement, and Eco-cement), water to cement ratio (0.50, 0.35), and curing temperature (283, 293, 313K). In this contribution, reaction of alite and belite is focused and evaluated. Experimental results show that the reaction of alite is affected by the temperature, but the effect of W/C is rather small. Regarding belite, its reaction is strongly affected by the reaction of alite and temperature. The hypothesis, in which rate of reaction of belite is explained by the ion concentration of CaO and SiO2 in the capillary water and solution equilibrium curve of alite, belite, C-S-H and Ca(OH)2, is proposed. Based on these experimental facts, simple hydration model is proposed.

Effect of Pair-Mineralizer on the Reaction of Alite and Calcium Langbeinite Formation

This paper investigates the effect of the pair-minerializer (CaSO4,-CaF2) on the reaction of alite, belite and calcium langbeinite formation with different alkali and sulfate contents. A set of clinker samples was prepared by adding laboratory grade reagents of (NH4)2SO4, CaF2 and K2CO3 to the cement raw mixes. The mineralogical composition of clinker was analyzed by X-ray powder diffraction, and the quantity of minerals was evaluated by using TOPAS software. As the experimental results, the total amount of calcium silicate minerals was rapidly increased with the addition of F and SO3 components simultaneously as pair-mineralizer with K2O more than the value which mineralizer was added separately. Also, in the case of adding K2O only to the raw mixes, the amount of alite is decreased after clinkering. However, if alkali (K2O) and pair-minerializer (CaSO4,-CaF2) were added simultaneously, the quantity of alite and calcium langbeinite mineral increased because of the formation of stable clinker minerals by the reaction of alkali (K2O) and sulfate.

Burnability of cement raw mixes at 1400°C I The effect of the chemical composition

An empirical expression has been developed, in which the content of uncombined lime of a cement raw mix, heated at 1400°C for 30 minutes, is correlated with the lime saturation factor, the silica ratio, the content of calcite particles >125 μm, and the content of quartz particles >44 μm. Qualitative agreement is demonstrated between this expression and deductions from data on the alite reaction, C+C 2S → C 3S. For raw mixes with constant fineness parameters, the effect of chemical composition, deduced from the empirical expression, is in quantitative agreement with the effects deduced from kinetic data on the alite reaction and from other independent sources of information.

Rate of alite-formation in clinker sandwiches

The reaction rate of the alite reaction, C + C 2S → C 2S, was measured by means of a sandwich technique, for experimental conditions pertinent to burnability studies. The growth of the alite layer followed the parabolic rate law, X 2 = k·t, where X is width of layer and t reaction time. k (cm 2 sec −1) was found to increase with increasing melt content in the structure, with increasing temperature, and with increasing substitution of Fe 2O 3 for Al 2O 3 in the melt. Substitution of MgO for Al 2O 3 up to MgO-saturation had no significant effect on k.