Pozzolanic Reaction Of Metakaolin Research Articles

Synergistic effect of aluminum sulfate and triethanolamine on the hydration of metakaolin blended cement

AbstractShotcrete is regarded as an indispensable component in the construction of tunnels and mines, which can stabilize the tunnel wall and prevent groundwater infiltration. Along with the massive railway construction in China's mountainous region, it is unavoidable to build more tunnels, and then the used amount of shotcrete increases dramatically. Calcined clay blended cement is one promising low‐carbon cementitious material and metakaolin (MK) is the main reactive component in calcined clay. Considering the high demand for shotcrete and the urgently needed carbon neutrality in China, it is necessary to contain more MK in the cement of shotcrete.Beside the cement, accelerator is also necessary for the production of shotcrete. Aluminum sulfate (AS) and triethanolamine (TEA) are two representative components in the alkali‐free liquid accelerator. However, limited researches were conducted regarding the synergistic effect of AS and TEA on the setting and hardening performance of the metakaolin blended cement (OPC‐MK).Hence, in order to clarify the effect of TEA and AS on the hydration of OPC‐MK, different dosages of TEA were added to AS solution and their effects on setting performance, compressive strength and hydration kinetics of OPC‐MK were investigated. It is found that, with the presence of both AS and TEA, a high dosage of TEA facilitates the reaction of the aluminate phase while greatly retards the silicate reaction. More importantly, TEA can promote the dissolution and pozzolanic reaction of MK. Thereby a dense structure can be formed and the compressive strength of pastes at 28 d is enhanced. At last, the interactions between TEA, AS and OPC‐MK is thoroughly analyzed, which could provide a theoretical basis for the wide application of low‐carbon cement in shotcrete.

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.

Mechanical properties and hydration of ultra-high-performance seawater sea-sand concrete (UHPSSC) with limestone calcined clay cement (LC3)

Ultra-high-performance seawater sea-sand concrete (UHPSSC) has attracted increasing attentions over the past few years as it exhibits exceptional mechanical and durability properties similar to conventional ultra-high-performance concrete (UHPC). Comparing to conventional UHPC, UHPSSC has great advantages when applied in marine engineering, particularly in offshore projects where construction materials are not easily accessible, and the transportation is costly. Nonetheless, current production of UHPSSC still necessitates large amounts of cement due to its low water/binder ratio. The larger demands of cement in manufacturing UHPSSC/UHPC than ordinary concrete can lead to substantial increase in the carbon emission. Therefore, an eco-friendly UHPSSC is developed in this paper by incorporating a promising ternary binder system (LC3) that consists of limestone (LS), calcined clay (metakaolin) and cement. The influence of the metakaolin (MK) dosage on the mechanical, hydration and microstructure properties of UHPSSC is comprehensively studied by carrying out a variety of experiments including mechanical test, isothermal conduction calorimetry test, TGA, XRD, SEM etc. The results showed that the UHPSSC mixed with LC3 exhibited lower early-age compressive strength than the reference mix, but its long-term strength was better than the reference group owing to the formation of more hydrates at the late age by the pozzolanic reaction of MK and the synergistic reaction between LS, MK and cement. Based on this study, the coupled replacement rate of cement by LS and MK could reach up to 40% without strength compromise. Nonetheless, 20% MK and 10% LS appeared to be the optimum substitution rate (total substitution rate of 30%) where the highest long-term strength was achieved. The main exothermic peak and total heat of hydration normalized per binder weight decreased with the increase of MK content level, but the main peak occurred earlier. Moreover, the addition of seawater can promote the hydration, leading to a greater main exothermic peak.

Early Strength-Promoting Mechanism of Inorganic Salts on Limestone-Calcined Clay Cement

This study aims to report the early strength effect and hydration mechanisms of limestone-calcined clay cement (LC3) with sodium carbonate, sodium sulfate and sodium chloride. The experimental results show that it is feasible to add three kinds of insoluble inorganic salts to improve the early strength of LC3 through different promotion methods. In comparison to sodium sulfate, the strengthening effects of sodium carbonate and sodium chloride on early strength of LC3 are more significant. The hydration heat evolution, mercury intrusion porosity and a set of tests for microstructural characterization (XRD, FTIR and SEM) were utilized to better understand the enhancement mechanism of inorganic salts in LC3 system. The mechanism by which sodium carbonate promotes the early strength of LC3 is mainly the strengthening of the aluminate reaction and pozzolanic reaction of metakaolin. The mechanism by which sodium sulfate promotes the early strength of LC3 is mainly the additional ettringite. The mechanism by which sodium chloride promotes the early strength of LC3 is mainly the strengthening of the silicate reaction and the generation of Friedel’s salt by alumina from tricalcium aluminate and metakaolin.

Molecular Insight into the Pozzolanic Reaction of Metakaolin and Calcium Hydroxide.

The reaction mechanism of the pozzolanic reaction of metakaolin (MK) from the atomic point of view has not yet been explored. To explain the process and mechanism of the pozzolanic reaction from the atomic point of view, molecular insight into the pozzolanic reaction of MK and calcium hydroxide (CH) was analyzed through the reaction molecular dynamics (MD) simulation. The results show that the pozzolanic reaction of MK and CH can be essentially regarded as the CH decomposition and penetration into MK. Also, the structure evolution after the pozzolanic reaction shows that the water molecules cannot penetrate the MK structure till the participation of Ca2+ and OH- ions of CH. The Ca2+ and OH- ions have strong interaction with MK and drill into the MK structure, followed by the destruction of a part of the MK structure and water penetration. The final structure of CH removed by MK can be regarded as the precursor of the CASH gel structure.

Na2SO4 modified low-carbon cementitious binder containing commercial low-reactivity metakaolin for heavy metal immobilization: Mechanism of physical encapsulation and chemical binding

In this study, a Na2SO4 modified low-carbon cementitious binder based on commercial low-reactivity metakaolin (MK) and cement was utilized for the immobilization heavy metals (Cu2+, Ni2+, Cd2+ and Zn2+). The compressive strengths and leaching concentrations of the immobilization systems were investigated to evaluate the immobilization effect for heavy metals. Meanwhile, the pore structure and chemical compositions of the immobilization systems were characterized to study the immobilization mechanisms. Results demonstrate that the MK-blended systems exhibit excellent heavy metal immobilization effect and low releasing risk under the effect of Na2SO4. The incorporation of Na2SO4 significantly improve the compressive strength of the MK-blended systems due to the promotion effect on the early hydration kinetics of cement, the formation of ettringite and the pozzolanic reaction of MK. Moreover, the incorporation of Na2SO4 decreases the capillary porosity and/or increases the tortuosity of the MK-blended systems, which enhances the physical encapsulation to heavy metals. The MK-blended systems have higher chemical binding capacities to heavy metals than pure cement immobilization systems. This is due to the increase in the Si/Ca and Al/Ca atom ratios of calcium aluminosilicate hydrate that introduces more negative charged silicon and aluminium sites to bind heavy metal cations.

Influence of metakaolin on the hydration and microstructure evolution of cement paste during the early stage

The incorporation of metakaolin into cementitious materials could improve the compressive strength and durability of cement paste, which are correlated well with the hydration behavior of cement-metakaolin blended mixture. The aim of this work is to reveal the coupled effects between the cement hydration and the pozzolanic reaction of metakaolin during the early stage, the methodologies including fluidity, setting time, compressive strength were employed to measure the properties of cement paste, while another methodologies, such as conductivity, Mercury Intrusion Porosimetry (MIP), Thermogravimetric (TG), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were adopted to investigate the hydration behavior and microstructure characterization. The results indicated that the fluidity of fresh cement paste decreased sharply with the increase of metakaolin, and the addition of superplasticizer could improve the fluidity of fresh cement paste effectively. The setting times of fresh cement paste were delayed slightly with the incorporation of metakaolin. The conductivity of cement suspension dropped gradually with the increase of metakaolin during the early stage, meaning the hindrance to the ion dissolution from cement-metakaolin blended mixture. The compressive strength increased slightly with the addition of metakaolin at 1 d, while it increased remarkably at 3 d. The results of MIP presented that the pore structure was refined with the increase of metakaolin at 3 d. The TG results showed that the incorporation of metakaolin promoted the formation of hydration products at 3 d. The XRD results presented that calcium hydroxide (CH) decreased with the addition of metakaolin, which may be consumed by the pozzolanic reaction of metakaolin. SEM observation confirmed that the microstructure of cement paste became much denser with the increase of metakaolin, which was in good agreement with the results of compressive strength and MIP measurement.

Hydration and Microstructure of High-Volume Ground Granulated Blast Furnace Slag Concrete Incorporating Metakaolin

In order to relieve the carbon emission of concrete industry, the application of high-volume ground granulated blast furnace slag (HVGGBFS) concrete is one of low cost and effective methods, but its properties develop slowly which needs to be solved. The impact of metakaolin (MK) on the microstructure and hydration evolution of HVGGBFS concrete has been investigated through the compressive strengths, non-evaporable water, morphology and nanoindentation. The results show that the non-evaporable water evolution of HVGGBFS concrete with MK has a similar trend with the compressive strength evolution. The use of 10% MK increases the compressive strengths of HVGGBFS concrete, and from the beginning of 28 days, its strengths are higher than those of the control concrete without GGBFS and MK. The use of 10% MK replacing cement reduces pores and unhydrated particles of HVGGBFS concrete, and however, increases high density calcium-silicate-hydrate (HD C–S–H) and ultra-high density C–S–H (UHD C–S–H), which may be attributed to C–S–H generated by the pozzolanic reaction of MK largely being HD C–S–H and UHD C–S–H.

Rehydration of ultra-high performance concrete matrix incorporating metakaolin under long-term water curing

This study investigated the late age rehydration of UHPC matrix containing metakaolin (MK) under long-term water curing (90–365 d). The UHPC matrixes were divided into two series including binary mixtures incorporating MK and ternary mixtures incorporating MK and limestone powder (LS). Steam curing (variable curing temperature and duration) and standard curing were performed on the UHPC matrix before long-term water curing. Compressive and flexural strengths, chemically bound water, microstructures, hydration degree were measured to evaluate the late-age rehydration of UHPC matrix. The results indicated that the strength of UHPC matrix generally increased from 90 to 270 d and then decreased from 270 to 365 d. The ultimate strength of UHPC matrix is closely related to the early age curing regime. The rehydration of long-term water cured UHPC matrix is attributable to unhydrated cement clinker as well as pozzolanic reaction of MK. The higher steam curing temperature degraded the rehydration degree of binary matrix while has little effects on that of ternary matrix samples. It was demonstrated that small pores below 20 nm increased with the long-term water curing in UHPC matrix. Additionally, a little increase of large pores above 200 μm was detected for MK added samples, contributing to strength degradation of UHPC matrix under long-term water immersion. Therefore, the rehydration of UHPC was influenced by both cementitious composition and early age curing regime.

Hydration and mechanical properties of UHPC matrix containing limestone and different levels of metakaolin

In this paper, the hydration process and mechanical properties of UHPC matrix incorporated with limestone (LS) and different substitution rates of metakaolin (MK) were investigated. MK was added with five levels of substitution (0, 5%, 10%, 15% and 20%) and LS was kept at 30% of the total binder mass. Mechanical properties, hydration heat, hydration products and pore structure measurements were carried out for every mixture. The results show that the hydration rate as well as the hydration heat peak of UHPC pastes decrease with MK level in the first 24 h possibly due to the higher superplasticizer usage, but the hydration process is accelerated with the incorporation of MK after 24 h. The pozzolanic reaction of MK and synergistic effect with LS consumes portlandite and induces the formation of more CSH gels with the improved microstructure of UHPC matrix. The trace of calcium monocarboaluminate (Mc) phase can be observed in UHPC paste samples with a low MK substitution (5%), and becomes weaker with the higher MK dosage. The pozzolanic reaction of MK with replacement level of 15% results in the best long-term compressive strength of UHPC mortars. Therefore, the incorporation of both MK and LS at an optimal level is favorable to manufacture UHPC with improved mechanical properties.

Autogenous Shrinkage, Pozzolanic Activity and Mechanical Properties of Metakaolin Blended Cementitious Materials

In the literature, there are some conflicting results regarding the influence of metakaolin (MK) addition on autogenous shrinkage behavior of cementitious materials. In this study, with the aim of identifying how the use of MK changes the properties of cementitious materials, cement was partially replaced by MK in different proportions (8%, 16%, and 24%) in pastes produced with variable water/binder ratios (w/b; 0.28, 0.35 and 0.42). The temperature development and calcium hydroxide consumption by thermogravimetric analysis were observed to better characterize the effects of MK on the autogenous shrinkage behavior. The mechanical properties such as compressive strength and flexural strength were experimentally investigated. The results show that the addition of MK has contrasting effects on autogenous shrinkage during early ages depending on the w/b ratio. Thermogravimetric analysis showed that the amount of stratlingite was smaller in pastes with low w/b ratios than in pastes with high w/b ratio. The calcium hydroxide consumption with the pozzolanic reaction of MK was significant even at 2 days, but consumption ratios with respect to the reference paste slowed down at 7 to 28 days.

Analysis of Hydration and Optimal Strength Combinations of Cement-Limestone-Metakaolin Ternary Composite

Metakaolin (MK) is an aluminosilicate pozzolan material and can contribute to property development of concrete due to the pozzolanic reaction. Limestone (LS) powder presents the dilution effect, nucleation effect, and chemical effect on hydration of cement. When metakaolin and limestone are used together, due to the additional chemical reaction between the aluminum phase in MK and limestone, the synergetic benefit can be achieved. This study presents a hydration model for cement-limestone-metakaolin ternary blends. Individual reactions of cement, metakaolin, and limestone are simulated separately, and the interactions among cement hydration, limestone reaction, and metakaolin reaction are considered through the contents of calcium hydroxide and capillary water. The hydration model considers the pozzolanic reaction of metakaolin, chemical and physics effects of limestone, and synergetic effect between metakaolin and limestone. Furthermore, the gel-space ratio of hydrating concrete is calculated using reaction degrees of binders and concrete mixtures. The strength development of ternary blends is evaluated using the gel-space ratio. Based on parameter analysis, the synergetic effect on strength development is shown and the optimal combinations of cement-limestone-metakaolin ternary blends are determined.

Effects of metakaolin on mechanical and microstructural properties of ultra-high performance cement-based composites

The effects of metakaolin (MK) on the mechanical and microstructural properties of ultra-high performance cement-based composite (UHPCC) were studied in this paper. The microstructure of UHPCC was investigated via XRD, SEM, nanoindentation, X-ray CT, and other different analytical testing methods. The results indicated that the mechanical performance of UHPCC was improved with the adding of MK. Blended mortars with 10% MK showed the highest compressive and flexural strengths in comparison to the others. As a result of Ca(OH)2 consumption through the pozzolanic reaction of MK, the interface was strengthened with the increase of curing time and the microstructure of MK blended mortar was denser than the mortar without addition of MK. Nanoindentation results revealed that most hydrated products of MK blended mortar was UHD C–S–H and a large quality of unhydrated cement existed in the mortar. X-ray CT results indicate that MK conduced to the distribution of steel fiber, thus improved the mechanical properties of UHPCC.

Mineralogical and chemical evolution of hydrated phases in the pozzolanic reaction of calcined paper sludge

Abstract Calcined residual paper sludge can be reused and valorized in a safe an environmental way by the construction industry. The highly reactive metakaolin produced by calcination of the paper sludge exhibits good pozzolanic properties and permits its incorporation in cement systems. The pozzolanic reaction of metakaolin in Ca(OH) 2 -saturated solution at 40 °C is reported in this study as a function of time, up to 1 year. The stability of hydrates phases has been evaluated according to the evolving aqueous conditions. Analytical determinations of the solid phase show early formation of C–S–H phases and a later precipitation of laminar minerals, hydrotalcite and stratlingite, at the expense of the C–S–H phases. The thermodynamic calculations confirm the experimental observations.

The effect that the pozzolanic reaction of metakaolin has on the heat evolution in metakaolin-cement mortars

The hydration heat of mortars or concretes may be decreased by means of pozzolanic additions. Materials that exhibit pozzolanic activity can decrease the hydration heat by means of cement substitution, which increases the heat generated in hydration due to the pozzolanic reaction. Thus, the evolution of the hydration heat with time represents a reference point when establishing the pozzolanic activity of materials; by monitoring this heat, the behaviour of mortars or concretes during hydration can be established. In this work, the influence of the pozzolanic activity of the metakaolin (MK) on the hydration heat has been studied in comparison to the behaviour of other traditional pozzolanic materials such fly ash and silica fume. The results revealed that MK mortars produce a slight heating increase when compared to a 100% Portland cement mortar, due to the high pozzolanic activity of MK. With respect to the hydration heat, MK-blended mortar showed closer behaviour to silica fume than to fly ash.