Microstructure Of Paste Research Articles

Impacts of graphene nanoribbon dispersion and stability on the mechanical and hydration properties of cement paste: Insights from surfactant-assisted ultrasonication

In this study, we aim to evaluate the different dispersion methods that affect the dispersion behavior of graphene nanoribbons (GNRs) and to conduct a contrastive analysis of the different surfactants that influence the dispersion and stability of GNRs in deionized (DI) water and alkaline solution. Moreover, we studied uniformly dispersed GNRs with respect to the hydration, mechanical properties, and microstructure of the cement paste. Owing to the surface defects of the GNRs, electrostatic repulsion was sufficient to overcome the van der Waals force and provide a long-term period of high stability. The results show that by using SPs and 60 min of ultrasonication, the dispersibility of GNRs improved by 8.5 % and 25.7 % in DI water and alkaline solution, respectively. Uniformly dispersed GNRs accelerate the hydration and C-S-H polymerization, resulting in significant increases in compressive strength and splitting tensile strength by 17 % and 33 %, respectively, and reduced porosity by 14.6 % after 28 days of curing.

Hydration performance of OWC pastes incorporating GO with various oxygen contents and sizes

This paper explores the reinforcement and toughening of oil well cement (OWC) by incorporating various oxygen content and size of graphene oxides (GO) at 50 °C, and then explores the mechanism of GO in OWC. Hydration heat evolution of OWC paste was tested using isothermal calorimetry. The hydration products and microstructure of OWC paste were examined using X-ray diffraction, thermogravimetric analysis, mercury intrusion porosimetry and scanning electron microscopy. Results suggest that the dispersion performance of GO in Ca(OH)2 solution (CH) enhanced by increasing of oxygen content and enlarging of size in GO. Higher oxygen content and larger size of GO result in lower fluidity and shorter thickening time of OWC paste. The hydration reaction of OWC paste accelerated by incorporating GO with larger size. A more densely packed microstructure and enhanced mechanical properties in GO-bearing OWC pastes are achieved, particularly in early hydration, which can be attributed to the formation of C–S–H with a comb-like structure and denser aggregated calcium hydroxide by incorporating of GO. The increased in oxygen content and enlargement in size of GO further enhanced this effect. Incorporating GO with higher oxygen content and larger size led to further improvements in the strength performance of OWC hardened paste.

Effect of 60 °C sustained temperature conditions on the mechanical properties and microstructure of alkali-activated fly ash-slag pastes and mortars

The temperature effect has been proven to affect the mechanical properties and microstructure of alkali-activated materials greatly. The research on the long-term complex high-temperature environment for alkali-activated materials remains blank. In this paper, the actual working temperature is selected to study the effects of long-term exposure at 60 °C on the mechanical properties of static/impact load, quality, shrinkage deformation, hydration products, pore structure, and microscopic morphology of alkali-activated fly ash-slag (AAFS). The results show that the mass loss rate of AAFS tends to be stable, the shrinkage deformation rate decreases, and the compressive strength decreases after 28d of sustained high-temperature action. The sustained high temperature causes changes in the hydration product phase, mean chain length (MCL) of the gel phase, pore structure, and micro-morphology. When the duration for 7d and 28d at 60 °C temperature, the compressive strength under static and impact loads increases, which is related to the increase of diffraction peak and MCL of the gel phase; when the duration is 60d and 90d, the compressive strength under static/impact loads decreases, which is related to the increase of cumulative pore volume and the decrease of MCL. Sustained high temperature refines the pore diameter of AAFS samples.

Piezoresistive properties of well-dispersed carbon nanotubes (CNTs) modified cement paste

When developing smart cement-based sensors with carbon nanotubes (CNTs) as conductive fillers, CNTs with good dispersion can enhance the conductivity and piezoresistive properties of cement-based materials. In this study, a combination of sonication and dispersant was used to disperse CNTs, and then investigated the piezoresistive properties of well-dispersed CNTs modified cement paste. The experimental results showed that the dispersant used was polyvinylpyrrolidone (PVP), and when the mass ratio of PVP to CNTs was 4:1, the sonication time was 25 min, and the standing time did not exceed 3 h, CNTs had higher dispersion quality. The resistivity of the cement paste decreased by two orders of magnitude with increasing CNTs dosage and exhibited visible percolation phenomenon with percolation thresholds ranging from 0.1 % to 0.3 %. The fractional change in resistivity, stress sensitivity and strain sensitivity of cement paste under cyclic loading reached the maximum values of 10.95 %, 1.146 %/MPa and 123.450 %, respectively, with high repeatability (92.63 %) and low hysteresis (8.40 %) when the content of CNTs was 0.4 %. At the same time, CNTs improved the microstructure of the cement paste, which positively affected the mechanical, electrical conductivity and piezoresistive properties of the material.

Mechanism of recalcification and its effects on the microstructure and transport properties of cement paste

Calcium leaching involves the removal of calcium ions from the pore solution and hydration products, leading to a decrease in pH, coarsening of pore structure, and, in severe cases, loss of cohesion, shrinkage, and cracking. Recalcification is a restoration process aiming at repairing degraded C-S-H structures by externally supplying calcium ions. However, our understanding of recalcification mechanism, its potential to reverse calcium leaching, and its effects on deteriorated cementitious matrices remains limited. This study aims at enhancing the understanding of the impact of recalcification on calcium leached cement pastes. Our findings demonstrate that calcium leaching can be partially reversed by immersion in a saturated calcium hydroxide solution. Recalcification functions by introducing calcium ions to the degraded matrix, triggering a reaction with the degraded C-S-H. This process reverses silicate polymerization, increases the Ca/Si ratio in the degraded zone’s, and densifies the pore structure. After only 14 days of recalcification, the degraded portion experienced a 50 % reduction in porosity, an increase in pH. By day 28, recalcification halved the permeability of the treated sample and completely consumed all silica gel. Additionally, our findings reveal that higher temperatures adversely affect the structure of C-S-H, hindering the beneficial structural modifications facilitated by recalcification.

Development of 3D-printed magnesium silicate hydrate cement mixes involving metakaolin as a substitute for silica source

ABSTRACT The printability of MgO-SiO2 cement pastes involving the use of microsilica (MS) and metakaolin (MK) as a SiO2 source was thoroughly investigated. Fresh properties, including rheology and early-age strength, mechanical properties and microstructures of pastes incorporating different contents of MK were presented and analyzed. Amongst samples studied, those containing 5% MK as a substitute for MS exhibited the highest static yield stress, and fastest re-flocculation and structuration rates, resulting in the best buildability. Although the inclusion of 10% MK resulted in enhanced paste strength at 7 and 28 days, increased MK content impeded paste printability due to diminished formation of hydrate phases. Use of 5% MK led to higher M-S-H generation compared to other samples at 28 days, contributing to subsequent strength development. Results demonstrated that optimisation of the MK content in MgO-SiO2 pastes can yield satisfactory mechanical strengths without compromising printability.

Effect of nano-silica on mechanical properties and microstructure of high-volume ground granulated blast furnace slag cement paste

Effect of nano-silica on mechanical properties and microstructure of high-volume ground granulated blast furnace slag cement paste

Mechanical property evaluation of 3D multi-phase cement paste microstructures reconstructed using generative adversarial networks

This study proposes an artificial intelligence based framework for reconstructing the 3D multi-phase cement paste microstructure to evaluate its mechanical properties using simulation. The reconstruction of cement paste microstructures is performed using modified generative adversarial networks (GANs) based on microstructural images from micro-CT. For computational efficiency, 2D microstructures are first reconstructed and then extended to 3D microstructures. The reconstructed microstructures exhibit the same microstructural features as the original microstructures when characterized by probability functions. Mechanical properties such as stiffness and tensile strength are evaluated for the original and reconstructed specimens using a phase-field fracture model, and similar behaviors are observed. The results confirm that the reconstructed virtual microstructures can be used to supplement the real microstructures in evaluating the mechanical properties of 3D multi-phase cement paste. This approach thus provides a critical element of a data-driven approach to correlating its microstructure and properties.

Research on the effect of different content of rice husk ash on the microstructure of concrete

Mercury injection porosimetry test is used to study the microstructure of rice husk ash cement paste with different content. The influence of rice husk ash on the microstructure of cement paste is analyzed through porosity, pore size distribution, the most probable pore size, pore size gradation and critical pore size analysis. The results show that with the increase of rice husk ash content, the porosity of cement paste decreases, and the pore size distribution and the most probable pore size decrease, which indicates that the addition of rice husk ash can improve the pore structure of concrete and improve the durability of concrete. With the increase of rice husk ash content, the number of harmful holes of cement paste decreases, the number of harmless holes increases, and the critical pore diameter decreases, which indicates that the addition of rice husk ash can improve the strength and impermeability of concrete.

Microstructure and microanalysis of portland cement pastes with high w/c ratios

This study examines the influence of an increasing water-to-cement (w/c) ratio on the hydration mechanism of portland cement. Portland cement pastes at three w/c ratios of 0.5, 0.8 and 1.2 were prepared and cured in hermetically sealed vials at 40 °C for 14 days. Compressive strength, bulk electrical resistivity and bound water measurements were conducted. Thermogravimetric analysis was performed on hydrated pastes. Hydrated microstructures of fractured surfaces of dried pastes representing the extreme w/c ratios of 0.5 and 1.2 were investigated using scanning electron microscopy for morphology and energy dispersive X-ray spectroscopy for chemical composition and elemental mapping of hydration products. The results indicate that an increase in the w/c ratio benefits the hydration of portland cement; however, the advantageous magnitude is limited beyond a certain w/c ratio. The microstructural characteristics of pastes and the types of hydration products formed are highly governed by the w/c ratio. The effects of w/c ratio on the morphology and composition of hydration products, particularly calcium silicate hydrate and calcium hydroxide, are discussed.

Macro- and microstructural evolution of cement paste modified with MWCNTs under thermal shock conditions

The study investigates the influence of multi-walled carbon nanotubes (MWCNTs) on the macro- and microstructure of cement paste (CP) subjected to thermal shock conditions. CPs with 0–0.3 % MWCNTs content, exposed to a sudden temperature load in a range 50–600 °C, were analyzed in terms of mechanical properties, chemical and phase composition, air pore structure, and microstructure of cement hydration products (Si/Ca, Al/Ca, portlandite, unhydrated part of cement). The research found the optimum MWCNT range to be 0.05–0.1 %, enhancing CP's thermal performance by strengthening cement hydration products and their cohesion, by more the nucleation effect than bridging effect. With the application of MWCNTs, the density of the solid cement phase increased, and the amount of the unhydrated part of cement decreased by up to 21.5 %, at 0.1 % MWCNTs content. Unfortunately, the increase in the MWCNTs content resulted in an increase in the pore volume in the worst case, up to 12.7 %, but it did not negatively affect the strength parameters. The MWCNTs effect caused an increase in tensile strength (fcf) by up to 41.0 % at temperatures above 400 °C, where in the most favorable case improvement in compressive strength reached 16.7 %. The study showed that MWCNTs as an admixture to cement composites is suitable for environments where there is a high variability in terms of thermal loads.

NMR investigations on Cl− and Na+ ion binding during the early hydration process of C3S, C3A and cement paste: A combined modelling and experimental study

Using materials containing sodium chloride(e.g., seawater) to make concrete is sometimes unavoidable. However, the associated ion binding mechanisms and structural evolution during the early hydration process are not fully understood. This study investigated Cl− and Na + binding behaviour and microstructure of C3S, C3A and cement paste within the first 48h using a high-field NMR setup. Results show that during hydration, KOH can reduce Cl− binding but enhance Na + ion binding in C3S paste, while increasing gypsum dosage in C3A paste retards both Cl− and Na+ binding. Retarders reduce Cl− and Na+ binding, while accelerators slightly promote Cl− binding but reduce Na+ binding in cement paste. It is also found that boundary nucleation and growth(BNG) modelling is the most suitable for fitting the ion binding. Finally, the microstructure assessed by the T1 relaxation rate is related to ion binding in various ranges, indicating that hydrates during hydration have various binding efficiencies.

Compressive Strength and Microstructure of Carbide Slag and Alkali-Activated Blast Furnace Slag Pastes in China

The alkali-activated blast furnace slag is attracting significant attention in replacing Portland cement due to several characteristics similar to cement hydration. However, there are a few practical problems with commercial alkali activators, such as the fast setting time, relatively high costs, and significant CO2 emissions during preparation. Thus, discovering industrial residues possessing inherent alkalinity are urgent. This study proposes the use of carbide slag at levels of 0%, 5%, 10%, 15%, 20%, and 30% and alkali at levels of 1%, 2%, 3%, 4%, 5%, 6%, 8%, and 10% activated blast furnace slag. The compressive strength and microstructure of carbide slag and alkali-activated blast furnace slag (CAB) pastes were examined using X-ray diffraction analysis (XRD), Differential Scanning Calorimetry/Thermogravimetric Analysis (DSC/TG), Fourier transform infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM). The results revealed that the addition of carbide slag produced more hydrotalcite-like phase as well as decreased the content of ettringite (AFt) and the calcium–silicate–hydrate (C-S-H) gel, which decreased the compressive strength of the CAB pastes. At the age of 28 days, when the dosage was 5%, 10%, 15%, 20%, and 30%, the compressive strength of CAB mixes decreased by 2.1%, 7.1%, 9.2%, 9.8%, and 28.1%, respectively. The addition of NaOH promoted the formation of AFt, and there was an optimum level of NaOH corresponding to the high compressive strength of paste. At the age of 3 days and 7 days, the compressive strength reached its maximum at the dosage of 6% NaOH, which was 24.8 MPa and 36.3 MPa, respectively. However, at the ages of 14 days and 28 days, the compressive strength increased as the dosage of NaOH increased to 5%, which was 43.3 MPa and 44.5 MPa, respectively. The water curing could both enhance the early and later strength, the compressive strength of 23.3 MPa was gained at 3 days, and this increased by 16.3%, 24.0% and 36.9% at 7 days, 14 days and 28 days, respectively. Therefore, water curing was suitable for the strength development of CAB pastes.

Effect of adsorption interactions of Arabic gum with cement

This study seeks to investigate the influence of cement and Arabic gum on the physico-mechanical and microstructural properties of cementitious composites. The influence of varying quantities of Arabic gum on the hydration, fluidity, mechanical performance and microstructure of cement paste was investigated. The influence of Arabic gum on slant shear performance and capillary water absorption was also investigated. The results indicate that the workability of cement was diminished as a result of the ability of Arabic gum to make the cement paste cohesive. It is evident that when the gum Arabic concentration increases from 147 to 174 mm, the resultant slump value for various w/b ratios drops. The adsorption characteristics showed that for a 15 mg g−1 dosage at 60, 45, 30, and 15 min, respectively, 1.43, 1.32, 1.25, and 1.03 mg g−1 are achieved. For 1% gum Arabic substitution, the highest flexural strength percentage growth is achieved at 38.46%, 23.74%, and 17.29% at 7, 14, and 28 days, respectively. In addition, the inclusion of Arabic gum improved the slant shear strength of cement composite, making it ideal for use as a building repair material with significant application potential. Experiments on the bonding behavior of the produced cementitious composite with the old mortar reveal that the shear bond strength was greatly increased, demonstrating the compatibility between the old and new cement composites. The microstructure and the porosity of the cement matrix also showed denser and compact matrix making them durable to attain better service life.

Strength evolution and microstructure of alkali-activated ultra-fine slag in low-temperature environments

At low temperatures, the hydration rate of Portland cement significantly slows down, resulting in a decrease in its strength. Additionally, the pore solution freezes easily. As a kind of low-carbon cementitious material, alkali-activated slag (AAS) material has a short setting time at room temperature and a high early strength, meanwhile the alkali solution does not freeze at low temperatures. This study investigates the evolution mechanism of the strength and microstructure of AAS pastes cured at 0℃. Slag with different fineness was activated by a sodium silicate solution with different moduli, whose strength was compared with that of an AAS paste cured at 20 °C. The setting time, compressive strength, hydration process, pore structure, freezing point, and microstructure of the AAS paste were studied, whose results showed that the setting time of the AAS paste was obviously prolonged, and the compressive strength decreased under low-temperature curing. After the activation of slag ground, the early compressive strength of the AAS paste was significantly improved, whose reason was that the hydration degree of the paste was improved after slag ground, and that a dense pore structure was formed. The freezing point of the AAS paste was approximately −26 °C, which, remarkably, even at −40 °C, was still capable of undergoing a weak chemical reaction, producing an exothermic enthalpy. This characteristic makes alkali-activated ultrafine slag a highiy-promising cementitious material for curing at low temperatures.

Peridynamic simulation of micro-internal damage and macro-mechanical properties of cement paste under freeze-thaw cycles

Concrete is a multi-phase, multi-component, heterogeneous composite material and a typical porous medium with a complex microstructure. Under freeze-thaw cycles (FTCs), pressures from the freezing and migration of water within the pores of the concrete cause freeze-thaw damage, which is influenced by its microstructure. To effectively simulate and understand the freeze-thaw damage mechanism, it is crucial to use a geometric structure that reflects the actual pore distribution. This study employs the μic model to create a geometric structure of hardened cement paste, capturing the spatial distribution of pores. The study utilizes the peridynamics (PD) method based on ice formation and crystallization thermodynamics to investigate the freeze-thaw damage evolution and the deterioration of mechanical properties in cement paste under FTCs. The simulations revealed the entire freeze-thaw damage process of cement paste, including internal crack initiation and propagation, as well as surface freeze-thaw spalling. The freeze-thaw damage of cement paste intensifies with increasing FTCs, and the extension of cracks diminishes the interconnection among the skeletons constituting the cement paste microstructure, leading to decreased tensile strength and elastic modulus. Additionally, the stress-strain evolution behavior of freeze-thaw cement paste under uniaxial tensile loads was obtained. The tensile strength and elastic modulus of cement paste decrease with increasing FTCs, and the higher the water-cement (w/c) ratio, the greater the reduction in strength and modulus.

The Effects of Calcium Nitrite on the Mechanical Properties and Microstructure of Early-Age Frozen Cement Paste.

The objective of this paper is to investigate the effect of calcium nitrite (CN) on improving the mechanical properties and microstructures of early-frozen cement paste. Cement pastes containing 1%, 1.5%, 2%, 2.5%, and 3% CN were prepared. One batch of samples was frozen at -6 °C for 7 days and then cured at 20 °C, and the other batch of samples was directly cured at 20 °C as a control. The compressive strength, ultrasonic pulse velocity, and resistivity of all specimens at different target ages were measured under these two curing conditions. The hydration products and microstructures of typical samples were observed using XRD and scanning SEM. The results showed that the addition of 1.5% CN could promote cement hydration and enhance slurry densification, thereby increasing the compressive strength, ultrasonic pulse velocity, and electrical resistivity of the slurry, and positively affecting the early freezing resistance of the slurry. However, when the CN dosage exceeded 1.5%, the internal structure of the slurry was loose and porous due to the generation of a large amount of nitrite-AFm, which negatively affects the properties of the cement paste. In addition, the effectiveness of CN is only limited to temperature environments above -6 °C. Concrete antifreeze suitable for lower temperatures still requires further research.

Properties and Microstructure of Alkali-Activated Slag Paste Modified by Superabsorbent Polymers

Properties and Microstructure of Alkali-Activated Slag Paste Modified by Superabsorbent Polymers

Performance of cementitious materials prepared with magnesium slag and concrete slurry waste

Concrete slurry waste (CSW) and magnesium slag (MS) have similar compositions to cement, and both pose challenges for conventional reuse. Considering the carbonation activity of MS due to the presence of γ-C2S, MS and CSW were utilized to produce a type of low carbon cementitious material through CO2 curing. The carbonation products and microstructure of the mixture pastes were analyzed using XRD, XPS, FTIR, SEM, MIP, TG-DSC and NMR. The mixture pastes, comprising 26.6 % CSW by weight, demonstrated robust strength, achieving a compressive strength of 52.5 MPa and a flexural strength of 10.7 MPa, with a calculated CO2 absorption rate of 11.26 %. CSW effectively influenced the CO2 absorption rate in mixture pastes, actively participated in their matrix structure construction, and contributed to the formation of C-(A)-S-H gel. Interwoven carbonation products, such as SiO2-gel, amorphous calcium carbonate, calcite, etc., contributed to the increase in strength. The stability of the mixture pastes was demonstrated by the sustained increase in compressive strength after curing in water for 56 days, which acted as a softening coefficient with a value higher than 1.0. After being cured in water, the hydration of residual silicate minerals in the specimen following carbonation, combined with the reaction of CO32− released from calcium carbonate with gels, including C–S–H and C-(A)-S-H, promoted the evolution of strength. Water curing promoted the development of finer pores and resulted in a denser matrix structure within the slurries.

Influence of steam curing on hydration and microstructure of cement paste containing high-volume lithium slag

The application of supplementary cementitious materials (SCMs) of concrete reduces the carbon emissions due to the reduced cement. But the early properties of mortar or concrete with high-volume SCMs are degraded, which hinders the expanded applications of concrete. However, early properties of mortar or concrete containing high-volume SCMs concrete can be effectively improved by steam curing. The influence of different steam curing systems on the microstructure and strengths of cement paste with high-volume of lithium slag (LS) are studied. The results manifest the early-compressive strengths, the content of chemically bound water and the hydration degree of the specimens are improved due to the different curing systems. The early-compressive strengths of specimens under steam curing-II is better than that under steam curing-I, but the compressive strengths at later ages is not as good as the steam curing-I, which is because of the maximum steam curing temperature. The scanning electron microscope (SEM) images show that cement paste with LS have a denser microstructure under the action of steam curing. This is not only due to the filling effect of LS, but also because LS is further activated under high temperature conditions, which promotes generation of hydration products and fills the internal pores. The data of mercury intrusion porosimetry also proves the above phenomenon.