Hydration Behavior Research Articles

Icelike water molecules with single hydrogen bond donor on the surface of nano anatase and rutile particles by IR spectroscopy

The hydration behavior of TiO2 nanoparticles was studied by molecular dynamics simulations and a novel IR ratio spectroscopy method. It was found that the hydration water at the titanium dioxide interface contains molecules with a large number of single hydrogen bond donors, and approximately five water layers were confined within the nano-grooves of the nanoparticles. The confinement effect was observed to enhance the strength of the hydrogen bonds and to slow down the movement of the water molecules around the surface of the nanoparticles.

Experimental measurements and thermodynamic modeling of dissociation pressure of CO2 hydrate in sulfate solutions

Carbon capture and storage (CCS) has been a popular strategy to mitigate climate change and has attracted significant attention from both industry and academia. CO2 can be stored in the form of CO2 hydrate in deeper locations in an ocean, which makes it a potential option for CO2 sequestration. Dissolved salts in the ocean brine can significantly affect the dissociation pressure of CO2 hydrate. Sulfate salts are one of the most common salt species in the oceanic brine. Although various experimental studies have been conducted to investigate the phase behavior of CO2 hydrate in brine, few studies focus on the effect of sulfate salts on the dissociation pressure of CO2 hydrate. In this study, we build an in-house experimental setup to investigate the effect of monovalent and divalent sulfate salts on the dissociation pressure of CO2 hydrate. The dissociation pressure of CO2 hydrate in Na2SO4, K2SO4, MgSO4, and CuSO4 aqueous solutions is measured using the isochoric pressure search method at different concentrations over the temperature range of 274.36–––282.15 K and the pressure range of 1.50–––4.03 MPa. A hybrid methodology incorporating the Soave-Redlich-Kwong Equation of State (SRK EOS), the van der Waals-Platteeuw (vdW-P) model, and the Pitzer model is applied to predict the dissociation pressure of CO2 in sulfate solutions. In addition, the dissociation enthalpies of CO2 hydrate in these sulfate solutions are calculated using the Clausius-Clapeyron equation based on the measured dissociation points. The experimental results show that the dissociation pressure of CO2 hydrate in Na2SO4, K2SO4, and MgSO4 solutions is higher than that in pure water, and the dissociation pressure of CO2 hydrate in sulfate solutions increases with an increasing salt concentration. Conversely, CuSO4 barely affects the dissociation pressure of CO2 hydrate, which is mainly attributed to the lower molar concentration of the ions compared with the other salt solutions and the ion specificity of Cu2+. The prediction results are in alignment with the experimental data measured in this study, which proves the feasibility of the thermodynamic model in predicting the dissociation pressure of CO2 hydrate in sulfate solutions. Additionally, the calculated dissociation enthalpies of CO2 hydrate show a dependence on both temperature and salt concentration. It is also revealed that Cu2+ exhibits ion specificity in affecting the dissociation enthalpy of CO2 hydrate, likely due to its distinct ability to impact the cage occupancy of CO2 hydrate compared with the other ions. These findings enhance our understanding of the impact of sulfate salts on the dissociation behavior of CO2 hydrate and offer valuable insights for CO2 sequestration.

Revealing the hydration mechanisms in iron-rich calcium sulfoaluminate cements with elevated bauxite residue content

The goal of the study was to investigate the reactivity at different Blaine, hydration behavior, and performance of calcium sulfoaluminate-ferrite cement (CSA-F). The raw meal incorporated bauxite residue (38 wt%) fired at 1260 °C and 1300 °C for 30 and 60 minutes. Based on multi-objective optimization, an optimum dosage of citric acid ranges between 0.4<x<0.6 wt% with a w/b ratio of 0.45 was chosen for the mortar evaluation and hydration studies. At an early age, monosulfates-AFm only started appearing for 1300_30. In the later age, an increase in ettringite formation was only reported at 1300°C for 60 min due to the dissolution of the Ternesite phase after 7d of hydration. Later-age studies after 28d reported ettringite, monosulfates, hydrogarnets (except for 1300_60), and strätlingite had formed. The 28d highest compressive strength was reported for 1300_60 corresponding to the highest bound water and amorphous content from the pore filling ettringite and amorphous stratlingite.

Fluidity and hydration evolution of cement-LS binder in the function of Mg/Al-LDH-PCE

In order to improve the early properties of lithium slag (LS) blended binder, Mg/Al-LDH/polycarboxylate (PCE) nanocomposites as chemical admixture were adopted. In this research, effects of Mg/Al-LDH/PCE nanocomposites on the hydration behavior of Portland cement including LS powder (20 wt%) were evaluated. The rheological properties and compressive strengths were studied. And the micro-analysis, such as hydration heat, hydration products and micro-morphology were also analyzed to reveal the potential hydration mechanism of LS-cement binder in the function of LDH-PCE. Results showed that LDH-PCE was beneficial for improving the rheological and mechanical performances of the fresh binder. Adding 1 wt% LDH-PCE contributed to the highest compressive strength, leading to 14.66 %, 32.62 %, and 24.88 % enhancement at 3 d, 7 d, and 28 d, respectively. LDH-PCE acted as nucleation sites and promoted the hydration process of LS blended binder. With the addition of LDH-PCE, the second exothermic peak of hydration heat evolution and cumulative hydration heat elevated obviously. The amount of CH enhanced at the early stage (3 d) due to the enhanced hydration of cement clinker and decreased at a later time (28 d) because of enhanced pozzolanic reaction. Enhanced cement hydration and pozzolanic reaction of LS generated more hydration products in the matrix, resulting in denser microstructure as well as enhanced mechanical strength. The addition of LDH-PCE significantly improved the rheological properties of the LS blended binder, shortened the setting time, increased the compressive strength of the mortar, and promoted the hydration process.

Hydration behavior and thermodynamic modelling of ferroaluminate cement blended with steel slag

This study investigates the effect of steel slag (SS) content on hydration of ferroaluminate cement (FAC). The physical and mechanical properties, hydration properties and microstructure of the cement paste mixtures were also evaluated. The results reveal that the setting time initially shortens and then prolongs while the fluidity, electrical resistivity at 24 h, and chemical shrinkage at 3 days decrease with the increase of SS from 0 % to 30 %. In addition, the analysis of hydration products shows that the incorporation of SS promotes the growth of strätlingite in the SS-FAC paste, which is responsible for strength recovery of FAC at a late period. The compressive strength of the paste with 20 % SS at 90 days exceeded that of the control group. The addition of a small amount (≤20 %) of SS can improve the pore distribution of the SS-FAC paste and have a certain improvement effect on its microstructure in the late periods of hydration. Thermodynamic modelling shows that the growth of AFt and strätlingite (C2ASH8) in the SS-FAC paste is promoted, which is consistent with the experimental results.

Mechanical properties and micro-mechanism of seawater cementitious materials reinforced by in-situ polymerization

Understanding the potential mechanism of in-situ polymerization of acrylamide (AM) for modifying seawater cementitious materials is crucial for designing high-strength and durable marine concrete. Herein, the acrylamide (AM) in-situ polymerization was investigated for its effects on the hydration behavior, micro-morphology, and pore structure of cementitious materials mixed with seawater and freshwater through a series of elaborately designed microscopic characterization methods. The results reveal that the hydration process of cementitious materials proceeds simultaneously with in-situ polymerization. However, compared with freshwater mixtures, seawater provides a large number of metal ions and SO42- ions, which can cross-link with the generated polyacrylamide (PAM) during in-situ polymerization to form a three-dimensional network structure. The synergistic effect of the hydration, in-situ polymerization, and cross-linking processes of cementitious materials can improve the pore structure of seawater-mixed paste, enhance erosion resistance, and improve the stability and toughness of microstructure. These findings were further confirmed by comparing infrared spectroscopy results, hydration products, pore size, and micro-morphology analysis as well as flexural performance tests. This is of great significance to guide the design of novel materials in marine infrastructure.

Enhancing the emulsification and demulsification efficiency of switchable surfactants through molecular dynamics simulation: The roles of surfactant concentration, salinity, and structure

This study aims to investigate the impacts of a surfactant structure, surfactant concentration, and salt content on switchable emulsification processes through molecular dynamics (MD) simulations. Specifically, we focus on assessing the properties and behaviors of water/tetradecane systems containing CO2-switchable acetamidine surfactant N’-dodecyl-N, N-dimethylacetamidine (C12DMAA) and C18 naphthalene sulfonate (C18PS), both of which are relevant to enhanced oil recovery processes. Utilizing MD simulations, we comprehensively explore the influence of the molecular composition of switchable surfactants, salinity, and surfactant concentration on the reversible processes of emulsification and demulsification in a complex oil/water/C18PS/C12DMAA system. This system can be activated through the injection of CO2 or N2 gas. Various analyses, including molecule mobility, hydration behavior, void volume analysis, a solvent accessible surface area (SASA), a diffusion coefficient, and relative concentration profiles, are employed to gain insights into the emulsification and demulsification processes. Our study reveals that lower surfactant concentrations result in the formation of partial emulsions, while the presence of salt disrupts surfactant hydration and weakens emulsification properties. Additionally, we observe that the impact of hydrogen bonding interactions is less pronounced at lower surfactant concentrations. Furthermore, the MD simulations provided insights into the interplay of a surfactant monomer number and alkyl phenyl introduction with a solvent-accessible surface area (SASA) and a void volume. Understanding these factors is crucial for designing and optimizing emulsion systems, particularly in oil recovery processes. The findings advance our understanding of CO2/N2-switchable surfactants, offering insights into their potential for sustainable development in the petroleum industry. This research contributes to the optimization of switchable surfactants, providing a foundation for improved emulsification processes in enhanced oil recovery applications.

Choline chloride and amino acid solutions taste and hydration behavior with experimental thermodynamic properties and COSMO-PC-SAFT calculation

Aqueous amino acid solutions have been introduced as dietary supplements for both animals and humans. This study investigates the physicochemical properties of the solutions containing amino acids (l-glycine, d,l-alanine, l-proline), choline chloride, and water at temperature range of 288.15 to 318.15 K. The results show that increasing concentrations of amino acids and choline chloride lead to higher solution densities. Analysis of apparent molar volume (Vφ) and apparent molar isentropic compressibility (κφ) reveals that Vφ values increase with choline chloride concentration and temperature, indicating enhanced solute–solvent interactions, while κφ values decrease, suggesting increased solution compression. Thermodynamic analysis using the Redlich-Mayer model and COSMO-based modeling provides insights into molecular interactions. However, COSMO-based parameters show high average relative deviation percentage (ARD %) values, indicating poor predictive performance for the density of these systems. In contrast, the ePC-SAFT equation of state effectively predicts the densities, particularly for l-proline-based solutions, which show very low ARD % values, indicating high accuracy. The ePC-SAFT model also performs reasonably well for l-glycine solutions but shows poorer results for d,l-alanine-based solutions. The study also examines the sweetness and saltiness criteria (ASV and ASIC) of these solutions. The ASV values, which serve as a sweetness criterion, are higher than the ideal range of 0.5 < ASV < 0.7, suggesting an overly sweet taste. The ASIC values follow a similar trend, indicating increased saltiness. To achieve an appropriate grade of sweetness and saltiness, dilution to lower concentrations of the solution is recommended. Furthermore, the use of choline chloride is found to increase salt intake and enhance the taste of salt, which can be beneficial in amino acid supplements used in animal food.

A structural determinant of the behavior of water at hydration and nanoconfinement conditions.

The molecular nature of the phases that conform the two-liquid scenario is elucidated in this work in the light of a molecular principle governing water structuring, which unveils the relevance of the contraction and reorientation of the second molecular shell to allow for the existence of coordination defects in water's hydrogen bond network. In turn, such principle is shown to also determine the behavior of hydration and nanoconfined water while enabling to define conditions for wettability (quantifying hydrophobicity and predicting drying transitions), thus opening the possibility to unravel the active role of water in central fields of research.

Experimental Study and Neural Network Predictions of Early-Age Behavior of Microexpansion Concrete in Large-Diameter Steel Tube Columns

The improved mechanical performance of concrete-filled steel tube (CFST) components has led to their widespread application in megastructures. Therein, CFST hybrid arch bridge has become an optimal selection of large span bridges. Nonetheless, for massive concrete poured in CFST members with large diameter, the thermal cracking and sustained rising temperature caused by early hydration heat of core concrete are urgently required to be studied. The two large-diameter CFST columns in this work, each having a diameter of 2.1 m, had their temperature and strain fields recorded in-situ. An existing CFST arch bridge served as the model for the two CFST columns’ design. Additionally, early-age characteristics of several scaled CFST columns with varying diameters were documented. A multi-field finite element (FE) model that combines linked chemical (hydration), heat, and mechanical fields is suggested in order to properly characterize the evolutions of temperature and strain fields. The model is validated by comparing the in-situ measurements with the numerical results. Finally, to investigate the affect factors on the hydration temperature of core concrete in CFST columns, early-age hydration behaviors of CFST columns was simulated using the validated FE models input parameters as water to cement ratio (w/c), cement dosage, heat release of cement and diameter of CFST columns. Based on the numerical results under the input parameters mentioned, the LSTM neural network was constructed, and the hydration temperature variances computed by FE models were selected as the input dataset. Afterwards, the temperature variance of core concrete of CFST columns was predicted using the established LSTM network. It is discovered that the LSTM neural network that was previously constructed was able to predict the peak temperature of CFST columns as well as the hydration temperature of CFST specimens with respect to time.

Exploring the effect of self-combustion gangue powder particle distribution on rheological behavior and hydration mechanism in cement-based systems

Solid waste-based mineral admixtures are widely employed as a partial replacement for Portland cement due to their superior performance and minimal carbon and environmental footprint. This study involves the production of self-combustion gangue powder (SCGP) with varying particle sizes achieved through crushing, screening, and grinding. The SCGP is subsequently blended with cement to form a self-combustion gangue powder-cement (SCGP-C) system with diverse particle size distribution. Rheological tests and hydration exothermic tests were conducted to investigate the rheological behavior and degree of hydration of the SCGP-C mixed system. The results show that SCGP enhances the plastic viscosity and thixotropy of the mixed slurry. However, when a finer SCGP (d50=1.36 μm) was used to replace 15 % of the cement for the same mass, the yield stress and rheological index of the slurry were reduced by 28.1 % and 15.4 % respectively compared to the control group, while the closed-packing of the SCGP-C system was obtained. When the cement was replaced by 15 % SCGP (mass substitution), the yield stresses of finer and coarser SCGP (d50=18.24 μm) and pure cement slurries were increased by 289.1 %, 67.3 %, and 29.2 %, respectively, at T=60 min compared to T=0 min. This suggests that the fineness of SCGP has a greater effect on the yield stresses of mixed slurries over time. Mixed slurries with different finenesses of SCGP all prolonged the hydration induction period and reduced the total exothermic heat of hydration. Incorporating 15 % finer SCGP extended the induction period of the mixed slurry by 27 min and increased the peak value of the main hydration curve. Appropriate incorporation of SCGP helps to improve the fluidity and early strength of mortar in the SCGP-C system and promotes the physical filling effect and the positive role of SCGP volcanic ash in the cement system.

Non-Destructive Monitoring of Hydration Characteristics in Alternative Materials and Seawater-Based Cementitious Pastes Using Electrochemical Impedance Spectroscopy

This study investigates the hydration behavior of cementitious materials incorporating fly ash, limestone and calcined clay (LC2), and seawater, aiming to understand the individual and synergistic effects of these components on hydration kinetics. The motivation behind this research lies in the growing interest in enhancing the performance and sustainability of cement-based materials by incorporating supplementary materials and utilizing seawater. To achieve this, the hydration process was meticulously examined using electrochemical impedance spectroscopy (EIS). An innovative equivalent circuit model was developed to analyze the results. The experimental data indicated that, with ongoing hydration, the diameter of the impedance arc in the high-frequency range gradually increases. A noteworthy observation is that increasing the proportion of fly ash and LC2 in the cement paste leads to a corresponding enlargement of the high-frequency arc, indicating a significant influence of these supplementary materials on the hydration process. Additionally, LC2 was found to be more effective in accelerating the hydration process compared to fly ash.

Effect of Stöber Nano-SiO2 Particles on the Hydration Properties of Calcined Coal Gangue-Blended Cement.

This study focuses on the calcined coal gangue (CCG)-blended cements containing Stöber nano-SiO2 (SNS) particles. The effects of SNS particles on the workability, hydration behaviour, mechanical properties and microstructure evolution of the blended cements were comprehensively investigated at curing ages ranging from 1 to 28 d. The hydration behaviour was studied via isothermal calorimetry test, X-ray diffraction (XRD) and thermogravimetric (TG) tests. The microstructural evolution was studied using mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The results show that the incorporation of SNS led to a significant reduction in fluidity, particularly at an SNS content of 3%. The SNS significantly increased the compressive strength of the CCG-blended cement at all curing ages, and the optimum SNS content was found to be 2%. SNS significantly accelerated not only the early cement hydration but also the pozzolanic reaction of CCG at later curing ages, resulting in a decrease in portlandite, as evidenced by the isothermal calorimetry, XRD and TG analysis. Microstructural analysis shows that the incorporation of SNS effectively refined the pore structure of the CCG-blended cement, resulting in the formation of a dense microstructure. All these beneficial effects of SNS provides advantages in the development of the compressive strength of the CCG-blended cement at all curing ages.

Mechanical properties, hydration behavior and shrinkage deformation of ultra-high performance concrete (UHPC) incorporating MgO expansive agent

To reduce the extremely high autogenous shrinkage deformation of ultra-high performance concrete (UHPC) caused by low water-to-binder ratio and high binder materials content, this work aimed to designed and prepared low shrinkage UHPC containing MgO-based expansive agents (MEA). The effects of UHPC with different reactivity of MEA (R-MEA and S-MEA) were compared in terms of fluidity, mechanical properties, hydration characteristics, shrinkage deformation, pore structure and microstructure. The results indicated that the incorporated of MEA decreased the fluidity and mechanical properties of UHPC, while the autogenous shrinkage deformation was compensated significantly. When 6 % R-MEA was incorporated, the flow and compressive strength decreased by 16 % and 7.9 %, respectively, but the autogenous shrinkage at 7 days also decreased by 70.1 % at the same time. The same amount of S-MEA showed lower compensation for the autogenous shrinkage of UHPC than R-MEA, but with the same reduction in compressive strength loss. This was due to the highly reactive R-MEA showed higher hydration degree and larger swell increment, which caused more compensation for shrinkage in UHPC, while it had slight adverse effect on the strength development. It is worth noting that the compensation effect of autogenous shrinkage is very limited when the doping is higher than 6 % for either MEA, but the loss of mechanical properties is serious. In addition, the microstructure showed that the crystal growth pressure of brucite forced the pastes to expand and thus resulted in increase on the porosity for UHPC. This study provides a feasible and effective way for mitigating the autogenous shrinkage of UHPC.

Hydration mechanism of hydrated magnesium citrate: The effects of MgCl2

AbstractIn this study, the evolutions of kinetic parameters, such as hydration temperature, pH, and electrical conductivity, were observed to investigate the hydration mechanism of hydrated magnesium citrate (HMC). Additionally, the effects of MgCl2 on the hydration behavior of HMC and the working performance of its bonded castables were also investigated. The results show that the hydration mechanism of HMC is dissolution–precipitation, and its hydration product is magnesium citrate tetrahydrate [Mg(H2O)6] [MgC6H5O7(H2O)n]2∙(8‐2n)H2O. The hydration process of HMC is controlled by the concentration of the C6H6O72− ions. MgCl2 could inhibit the ionization of HMC, thereby delaying the hydration progress of HMC. The second exothermic peak was delayed from 1 to 4.8 h with increased MgCl2 content to 1.0 wt.%. The working performance of HMC‐bonded castables could be improved by adding MgCl2. After adding MgCl2, the setting time, flow value, and cold modulus of rupture of HMC‐bonded castables increased by 26.7%, 25.2%, and 8.8%, respectively, while the porosity decreased by 12%.

Research on the effect of tuff powder on the properties of moderate-heat portland cement-based materials and the methods for evaluating pozzolanic activity

Tuff powder (TP) has attracted significant attention due to its abundant resources, low carbon emissions, and potential enhancements in concrete property. This study investigates the effect of TP on the mechanical properties, hydration behavior, pore structure, microstructure morphology, economic and environmental analysis of moderate-heat portland cement (MHPC), comparing it with MHPC-based materials containing fly ash (FA). The synergistic morphological effect and micro-aggregate filling action of FA and TP result in higher early strength in the fly ash-tuff powder (FT) group mortar. The CH content and chemically bounded water content of TP paste are higher, showing a better early hydration promotion effect. The pore structure of the composite cement mortar is changed due to the addition of the admixture. Furthermore, the multi-factor response strength and strength activity index (SAI) model were developed based on response surface methodology (RSM), linking to TP specific surface area, dosage, and specimen curing age. This provides a reference for more accurate and comprehensive prediction of cement mortar property, quantification of effects, optimization of mix proportions, and evaluation methods for the activity of pozzolanic materials.

Enhanced hydrate formation at the liquid CO2-brine interface with shear flow for solid CO2 sequestration

The hydrate-based CO2 sequestration provides an alternative solution for reducing CO2 emissions. However, the hydrate film that forms preferentially at the CO2-water interface can severely restrict hydrate conversion rate. How to enhance CO2 hydrate formation remains a crucial question. Here, we constructed static and shear flow contact models of liquid CO2 and brine in a cubic cavity and investigated their hydrate growth behavior at different subcoolings from perspectives of morphology and kinetics. For static contact, nucleation usually occurs at the CO2-brine interface, and high subcooling increases the degree of hydrate film wrinkling and the CO2 hydrate formation amount. The shear flow can delay the explosive growth of hydrates and tends to nucleate in bottom water; meanwhile, the continuous flow causes the overall extrusion deformation of hydrate film until the injection hole is blocked. Compared to static contact, this forced CO2-H2O contact caused by film deformation enhanced the CO2 hydration amount by 2.68–3.62 times at the subcoolings of 6.94–3.25 K. Increasing the flow rate can further delay hydrate appearance and enhance hydrate growth. We also speculate on the hydration patterns of CO2 in sediment pores under static and flowing conditions, and think that maintaining long-term flow can be an effective means to improve CO2 hydrate storage and prevent blockage occurring in the pore throat. Our work provides new insights into the growth behavior of CO2 hydrate for future CO2 storage in submarine sediments.

New insights into the hardening mechanism of calcium silicate hydrates under creep deformation: A reactive molecular simulation study

Understanding the time-dependent behaviour of calcium silicate hydrates (C-S-H) under stresses is crucial for unraveling the microscopic creep behavior of concrete and its consequences on the evolution of nanostructure of C-S-H. In this study, the structural evolution of C-S-H nanostructure under stress was investigated using reactive molecular dynamics and stress perturbation technique. Under sustained stress, C-S-H nanostructure undergoes a relaxation process leading to a creep deformation and transitioning the system towards a more energetically stable state. The process was comprehensively examined under shear stress through a detailed analysis of the system's potential energy. A hardening phenomenon was observed within calcium silicate layers throughout this relaxation process, categorizing in three distinct stages. in Stage I, structural reorganization occurred in the silicate chains, progressing towards a stable state by reducing non-bonded interactions. The calcium-oxygen ionic bonds within the intralayer were strengthened in Stage II, leading to a decrease in bonding energy. Finally, the interface between calcium silicate layers and the interlayer contributed to overall structural stability in StageIII.

Dissociation Behavior of Methane Hydrate Inside Different Saturation Sediments in a Sustained Depressurization Process

Summary Methane hydrate is one of the important energy storage sources, naturally distributed in marine porous sediments. However, the dissociation behavior of hydrate inside different saturation sediments during sustained depressurization remains unclear. In this study, methane hydrate-bearing sediments were synthesized using initial water saturations varying between 9.5% and 56.9% and subsequently dissociated at a gas exhaust rate of 0.77 Ls/min. The results indicate that the dissociation of hydrates is closely related to the initial hydrate distribution until the sediments get iced when the sediment pressure declines below 2.5 MPa. Due to the exothermic reaction of ice formation, the dissociation of hydrates after icing accelerates significantly, and its limiting factor becomes the gas exhaust rate. In addition, both production and monitoring pipes were used in this study to evaluate the possible plugging within the sediments, and the plugging zone within the sediments can be located by examining the thermodynamic correlation between pressure and temperature responses. It was found that all experimental cases with high saturations (47.4% and more initial water) easily induce plugging between sediments and production/monitoring pipes, with the maximum pressure gap reaching up to 2.5 MPa. These findings may aid in ensuring the safety and efficiency of the hydrate exploitation process in the future.

Influence of Coal Char Addition on the Heat of Hydration and Rheological Behavior of Cement Grout

Influence of Coal Char Addition on the Heat of Hydration and Rheological Behavior of Cement Grout