Microstructure Of Cementitious Materials Research Articles

Effect of Microwave Pretreatment on the Properties and Microstructure of Low-Concentration Carbon Dioxide Early Cured Cement-Based Materials

The utilization of microwave drying technology has expanded across various sectors due to its rapid processing speed, reduced operation time, lower sample temperatures, and consistent heating. In this research, microwave pretreatment was implemented prior to carbonation curing with low concentrations, and an array of tests including moisture content, compressive strength, carbonation depth, CO2 absorptivity, thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) were utilized to investigate the effect of microwave pretreatment on the properties and microstructure of cementitious materials under early carbonation curing with low CO2 concentrations. The findings reveal that microwave pretreatment significantly decreases the moisture content within the test specimens, expediting the ingress of CO2 and improving the compressive strength of the specimens. At the same time, the effectiveness of microwave pretreatment in reducing moisture content diminishes as the pretreatment time increases. The absorption of CO2 is relatively rapid in the early stage of carbonation curing, with over 50% of the CO2 absorption occurring within the 0–6 h period of carbonation curing. The hydration products and microstructure of the uncarbonated part inside the specimens are generally consistent with the normal curing state. The formation of CaCO3 contributed to the densification of the specimen by infilling its internal voids, thereby enhancing its compressive strength. Although carbonation curing enlarges the average pore size of the samples, it also serves a filling function, making the samples more compact and reducing the porosity.

Effect of steel slag powder and stone powder by-product from manufactured sand on the mechanical properties and microstructure of cementitious materials

Incorporating steel slag (SS) and stone powder (SP) as supplementary cementitious materials (SCMs) in cement and concrete products is an effective approach to promoting environmental protection. Recently, the effect of using steel slag alone is not satisfactory and the performance of blended cementitious materials is expected to improve after compounding with other admixtures. In this study, steel slag powder (SSP) and SP by-products from manufactured sand were employed with a total dose of less than 30% as a replacement for cement. The results revealed that the blended cementitious materials exhibited excellent mechanical strength and pore structure with the synergistic effect of SSP and SP. Compared to SSP or SP doping alone, the activity index of the composite 25% SSP and 5% SP was higher. SSP offers a considerable retardation effect on cement, while SP has little effect. Conversely, the effect of steel slag on mortar fluidity is neglected and SP decreases the fluidity. When 5% SSP is combined with 5% SP, the initial and final setting times of cement are increased by 39% and 25%, respectively. The reduction in mortar fluidity amounted to an insignificant reduction of only 0.7%, and the strength of the mortar at 28 d and 180 d was only lost by 7% and 5%, respectively. The coaction of 15% SSP and 15% SP was observed to promote the wollastonite generation, enhance the crystallization degree of xonotlite phases, decrease the microcrack widths, and optimize the pore structure of the blended cementitious materials.

Effect of fineness on the hydration and microstructure of cementitious materials with high-volume steel slag and blast furnace slag

Effect of fineness on the hydration and microstructure of cementitious materials with high-volume steel slag and blast furnace slag

Effects of urea on carbonation properties and microstructure of cementitious materials

Urea has been utilized in concrete technology to reduce the temperature cracking and promote winter construction property. In this work, the influence of urea on cement hydration and carbonization resistance of cement-based materials was studied through carbonation depth experiment, isothermal conduction calorimetry, XRD, BET and water vapor sorption. It is indicated that the reduction of rate in the hydration acceleration period can be attributed to the increasing pH and lower supersaturation when urea is added to the cement mixture. In addition,the existing of NH4+in pore solution andpH profile results in the decrease of decalcified C-S-Hdeterioration rate,which prolongs carbonation duration to asimilar depth. This study provides a new admixture way to improve the durability of cement-based materials to prolongthe life of reinforced concrete.

Water transport channel of cement paste modified by hydrophobic agent: X-ray nanotomography based analysis

ABSTRACT Hydrophobic treatment for cementitious material is one of the effective methods to improve the durability of concrete structures. In this study, the pore system of cementitious materials modified by a new type of carboxylic acid hydrophobic agent is reconstructed by X-ray computed tomography technology. The changes in porosity, the connectivity degree in 2D / 3D space, the tortuosity and permeability of samples with hydrophobic agents are investigated. The experimental and simulation results show that the addition of a hydrophobic agent will tremendously increase the volume of a large pore but the solid phase observed in pore space can be confirmed to be the calcium carboxylate flaky crystals which modify the water transportation channels. In conclusion, the change of transportation channel from the surface can explain the hydrophobicity of cement pastes in the presence of a hydrophobic agent. This study provides a new perspective to reveal the hydrophobic mechanism of cement-based materials and guidance to design the new microstructure of cementitious materials

Mechanical properties, durability and microstructure of cementitious materials with low-calcium circulating fluidized bed fly ash

To enhance the utilization of circulating fluidized bed fly ash in construction materials, this study investigated the influence of low-calcium circulating fluidized bed fly ash (LCFA) on the properties of cement-based materials. In addition, an alkali activator was added to improve the properties of cement-based materials mixed with LCFA and the synergistic effect was also investigated. The results show that an appropriate amount of LCFA can improve the compressive strength of cement-based materials in underwater curing conditions. The addition of LCFA can improve the physical properties and carbonization resistance of cement-based materials. Whereas, the addition of water glass (WG) can appropriately shorten the setting time and effectively increase the compressive strength of cement mixtures with LCFA. In addition, adding WG can effectively improve the resistance of chloride ion penetration and carbonation. It is speculated that adding appropriate amount of LCFA enhanced the amount of hydration products generated, filled the coarse pores, and made the pore structure of samples more compact. Appropriate addition of LCFA and WG increases the concentration of hydroxyl groups in the system, accelerates the rupture of Si-O and Al-O bonds on the surface of LCFA particles, and forms free and unsaturated active bonds on the surface of LCFA, which is easier to react with Ca(OH)2 to form hydration products such as C-S-H and C-A-S-H. At the same time, gelatinous hydration products migrate and diffuse with the change of ion concentration in the system, and gradually fill and compact the internal structure.

Experimental investigation on the effect of high-pressure water on tensile strength and microstructure of cementitious materials

The strength of cementitious materials is strongly related to their microstructures. This paper aims to investigate the effect of high-pressure water on the static and dynamic tensile strength of cementitious materials and interpret the influence through the view of microstructure. Specimens were first put in different water pressure (0 MPa, 1 MPa, 3 MPa, and 5 MPa) then their strength and microstructures were measured through splitting tensile tests, SEM tests, and MIP tests. Results showed that as water pressure increased, the strength of mortar decreased with a maximum loss of 32.54 %, while strain rate sensitivity increased. The static strength of saturated specimens was higher than the dried ones under the same water pressure, whereas the opposite results were obtained for dynamic strength. The observed microstructure morphology indicated that higher water pressure would induce more microcracks around pores and around aggregate-matrix interfaces, which led to the loss of strength. The water content and total porosity increased with increasing water pressure. Predictive static and dynamic tensile strength models concerning water pressure, porosity, and saturation states are proposed. The predicted values are in good agreement with the test values.

Photocatalytic Cementitious Material for Eco-Efficient Construction—A Systematic Literature Review

Photoinduced processes governed by light activated TiO2 have been studied in many ways. One of the most active areas is the development of TiO2 photocatalysis technologies on their application for reducing environmental impacts. The immobilization of TiO2 on solid support, such as cementitious materials, greatly enhances its use in practical applications. In this review, a wide range of applications for achieving eco-efficient building using cementitious composite materials containing TiO2 photocatalyst was presented. The basic mechanism of photocatalysis, such as electron excitation, charge transfer process, reactive oxygen species (ROS) generation, and its role to oxidize the pollutant and microorganisms were extensively discussed. Unlike self-cleaning and air purification systems, the study on the antibacterial function of a cement-based surface containing TiO2 is very limited. In photocatalytic cementitious materials, the key element affecting the photocatalytic performance is the accessible active surface area. However, microstructure of cementitious materials changes with age due to hydration and surface carbonation. Hence, surface area reduction and mass transfer limitation become the main drawbacks of incorporating TiO2 in cementitious materials. This review, therefore, provides the state of the art in photocatalytic cement-based composite materials and identifies the areas in which future improvement is needed.

Improvement of Mechanical Properties and Microstructure of Cementitious Materials in a Hot-Dry Environment

The hot-dry environment in high geothermal tunnels negatively affects the development of properties of cementitious materials. In order to improve the mechanical properties of cementitious materials in such environments, a proper curing system was developed, the material compositions were taken into account, and the related mechanism was analyzed by microscopic tests. The results showed that the mechanical properties and microstructure of cementitious materials can be improved effectively by adopting film curing and an adequate dosage of fly ash in a hot-dry environment. Compared with standard curing, a hot-dry environment accelerates the evaporation of water in fresh cementitious materials and the early hydration rate of cementitious materials, resulting in lots of unhydrated cement particles and an uneven distribution of hydration products, seriously decreasing the properties of the cementitious materials. Film curing keeps the water from evaporating and ensures enough water is available for hydration at an early age. The addition of FA reduces the amount of cement clinker and decreases hydration reaction rate in high temperature environments at an early stage, avoiding the unevenness of hydration products. High temperatures stimulate the pozzolanic activity of fly ash and promote the degree of secondary hydration reaction, producing more hydration products, as well as contributing to the improvement of the mechanical properties and microstructure of cementitious materials. In this study, in a hot-dry environment, the mechanical properties of mortar with 25% fly ash and 2–3 d film curing are better than those of other experimental groups, including those of standard curing.

Experimental Studies on Mechanical Properties and Microscopic Mechanism of Marble Waste Powder Cement Cementitious Materials

The resource utilization of waste stone powder is a meaningful way to realize sustainable development. This paper aims to study the influence of marble waste powder particle size and replacement cement dosage on the mechanical properties of cementitious materials and evaluate its microstructure and mineral characterization by SEM and XRD. The results show that the early strength of cementitious materials is obviously improved when the dosage of marble waste powder is in the range of 0–15%, and the lifting effect of marble waste powder with a particle size of 600 mesh instead of cement on the strength and microstructure of cementitious materials is the most obvious. The replacement of cement with different particle sizes of marble waste powder found that it had low chemical activity and participated in the hydration reaction of cement, but the reaction degree was low. The smaller the particle size of marble waste powder instead of cement, the denser the early microstructure, the more obvious the nucleation phenomenon, and the more serious the agglomeration between particles. In addition, the mechanism model of marble waste powder replacing cement cementitious materials was proposed. The strength prediction function model between the material dosage and compressive strength was constructed, and the accuracy of the model was verified.

Interaction between composition and microstructure of cement paste and polymeric carbon nitride

As a metal-free and visible-light-responsive photocatalyst, polymeric carbon nitride (PCN) is an ideal alternative to traditional titanium dioxide employed in photocatalytic cementitious materials (PCM) for environmental remediation. However, the interaction between the composition and microstructure of cementitious materials and PCN is far from clear. Herein, the hydration behavior, phase composition, microstructure, compressive strength and nitrogen oxides removal of the PCN-modified paste with different dosage of PCN at varying curing ages were investigated and the key factors governing the photocatalytic activity of the PCM were studied. The results show that the porosity of the PCN-modified pastes presented an increasing trend compared to the reference, but the micro-filler effect of PCN contributed to the varying reductions in the volume of harmful pores, which are mainly responsible for the highest compressive strength of 0.5% PCN-added pastes at 3 days and 28 days. Although the kinds of hydration products were not affected with the addition of PCN, the carbonates could be regulated to the well-crystallized and flower flaky calcites in the presence of PCN at 28 days. The well-crystallized calcite would be regarded as potential acceptors of photo-induced holes to promote the separation of electron-hole pairs on the PCN. The promoting effect of electron transfer could outperform the negative effect of the decreased porosity caused by ongoing hydration and carbonization, resulting in an enhanced photocatalysis of PCM with the increase of curing ages. The findings from this study would provide a novel understanding on the improvement of photocatalytic efficiency for PCM against the shielding effects of hydrates and carbonates.

Microstructure of Cementitious Materials Under the Coupling Effects of Cl- and Mg2+ in a Marine Tidal Environment

Microstructure of Cementitious Materials Under the Coupling Effects of Cl- and Mg2+ in a Marine Tidal Environment

Study on strength test and hydration mechanism of phosphogypsum based cemented backfill

In view of low early strength of phosphogypsum based slag cementitious material, exploration test of filling body strength is developed to get the optimal ratio of cementitious materials. Firstly, the physicochemical analysis of the test material was carried out; secondly, the cementitious material ratio and strength of waste rock coarse aggregate filling body was tested by orthogonal test and range analysis. In the end, the hydration products and microstructure of cementitious materials were analyzed by XRD and SEM. The results show that, for the slurry with waste rock coarse aggregate, 1:4 cement sand ratio and 80% mass concentration, the optimal ratio of cementitious materials is determined as phosphogypsum 30~33%, quicklime 5.5~6.0%, NaOH 1.5~2.0%, mirabilite 2.5~3.0%. The average particle size of cementitious material is - 16μm, and the early strength agent or synergist must be added, which could meet the requirements of the Jinchuan mine.

Shock wave propagation in micro-mechanics based Representative Volume Element of cement paste

Shock wave propagation in a synthetic representative volume element (RVE) of Type-I Portland cement paste has been analyzed. The existence of unhydrated water, nano/micro pores, and other hydrated and unhydrated products such as calcium silicate hydrate (C-S-H) and four main clinkers (tricalcium silicate, dicalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite) create a complex microstructure in cementitious materials. The objective is to analyze the heterogeneity induced shock wave decay phenomenon under compression caused by the plate impact of a copper flyer. The shock wave response of cement paste as a heterogeneous material is highly influenced by these constituents and their relative amount. The effective material properties of the RVE have been obtained by imposing appropriate boundary conditions. The stress wave propagation of both heterogeneous RVE and homogeneous media are presented and compared when the effective material properties are considered for all phases in homogenous RVE. The results clearly show the effect of heterogeneity on the stress wave propagation as well as attenuation and shock wave decay phenomena. Lastly, the effects of spatial distribution, material properties and volume fraction of particles on velocity profiles at the rear surface of an RVE are investigated and results show that how they affect the heterogeneity that lead to attenuation.

Effects of leaching and chloride migration on the microstructure and pore solution of blended cement pastes during a migration test

This paper investigates the influence of the multispecies diffusion and leaching phenomena on the thermodynamic equilibrium and microstructure of cementitious materials containing mineral additions during a chloride migration test. Because of a lack of data on these effects, the aim of this study was to quantify changes in the material’s microstructure during an experimental program based on mercury intrusion porosimetry before and after chloride exposure, scanning electron microscopy and chemical analyses of the materials. The experimental results highlight a modification of pore size distribution and a reduction of up to 60% in total porosity after chloride transfer. This was due to chloride interactions which led to the formation of new compounds in the porosity range of 7–70 nm. These compounds were investigated by elemental analyses and quantified.Moreover, a chloride migration test was performed in order to investigate the leaching of pore solution ions in the two compartments of the migration cell. The results enabled us to: (i) quantify the concentration of ions that diffused into/from the material, and (ii) estimate the kinetics of calcium release and the quantity of portlandite dissolved in the material. The chloride diffusion coefficient of the materials ranged between 2.38 × 10−12 and 5.96 × 10−12 m2/s, the kinetics of calcium, expressed as a flux, were between 1.8 × 10−2 and 1.2 × 10−1 mmol.m−2.s−1.

Effect of temperature rise on the microstructure of cementitious materials: A study through X-ray computed microtomography (µ-CT)

The microstructure of a mortar that was submitted to elevated temperatures undergoes many degradation processes in terms of the morphology of crystalline phases, fissuration and consequently, of its porous structure. As a result in macrostructural scale, many of its physical characteristics and mechanical properties may be affected. This article has the objective of evaluating the effects of temperature rise on the microstructure of cementitious materials, considering the changes on porosity and deterioration of the cement mass hydration products. To do so, mortars with 1:3 trace and (20 × 40) mm dimension were submitted to elevated temperatures of 150 °C, 300 °C, 600 °C e 900 °C, being compared to mortars maintained on ambient temperatures, i.e., 23.7 °C. The cooling regime was not controlled. For the microstructural evaluation, the samples were tested through Computed microtomography (µ-CT), thermogravimetry and derived thermogravimetry (TG/DTG) to verify the alteration on the initial porosity and to identify the peaks on the mass loss, respectively. The samples showed that the pores size distribution is altered depending on the temperature rise, however, it is not possible to confirm that these pores increase every time. Furthermore, the fissuration inducted by the thermic incompatibility between the paste and aggregates may have contributed for the increase on the porosity values, specially from 600 °C. The highest mass loss peak registered by the thermogravimetric analysis occurred on 418 °C and can be associated to the CH decomposition. It is concluded that the behavior of the mortars submitted to elevated temperatures is not linear, given the most expressive results occurring from 600 °C.

Hydration process and microstructure of magnesium potassium phosphate cement with nitrate solution

Magnesium phosphate potassium cement (MKPC) can be potentially used for solidification/stabilization (S/S) treatment of hazardous wastes. The influence of inorganic salts on the hydration process and microstructure of cementitious materials must be considered, especially to assess the performance of the S/S treatment of wastes with a high nitrate content. Hence, in this study, the hydration process and microstructure of MKPC specimens were investigated, along with their variations in compressive strength setting time, pH, and conductivity. The pore structure, phases composition, and elemental composition of the specimens were investigated using the Brunauer-Emmett-Teller method, X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results showed that although the potassium nitrate (KNO3) solution did not affect the formation of struvite-K, it affected the crystallization degree of struvite-K, where its microstructure changed from dense, plate-like and prismatic crystals into loose, cluster-like crystals at higher amounts of nitrate. Furthermore, the addition of nitrate delayed the setting time and slowed the pH growth during the hydration process of the MKPC. The nitrate solution hindered the hydration process of the MKPC specimens and increased their porosity. When the amount of nitrate was less than 5 wt%, the effect of nitrate on the hydration and hardening of the MKPC was small. However, at higher amounts of nitrate, the nitrate ions had a negative effect on the compressive strength development of the MKPC and the hydration process was delayed.

Comparison of different techniques to study the nanostructure and the microstructure of cementitious materials with and without superabsorbent polymers

It is difficult to study the microstructural porosity in time without destroying the samples and stopping hydration. Current techniques mainly involve mercury intrusion porosimetry and microscopic analysis. These destructive techniques are able to give information on the microstructure, but the nanostructure is affected due to sample preparation. Dynamic vapour sorption is a technique which is not often used to study the nanostructure of cementitious materials and requires the application of different models. Furthermore, nuclear magnetic resonance can be applied to non-destructive study not only the total water content but also the pore size distribution by comparing the T2 relaxation times, and can be combined with cryoporometry. In this paper, these different measuring techniques are compared and linked to each other. The obtained nano- and microstructures are compared to different models found in literature. Pore sizes in the range of 1.5–2 nm and of 8–12 nm are found, reflecting the gel pores. In addition, some bigger capillary pores are found. The measuring techniques are complementary to each other as they study different pore size ranges and are based on different phenomena.

A new approach for the mix design of (patch) repair mortars

The failure of most repairs in concrete structures is mainly manifested in the form of cracking and/or debonding of the repair layer. These two repair mechanisms could lead to the continuous corrosion of reinforcing steel in concrete. Cracking and debonding have been attributed to the high differential shrinkage that exists between the concrete substrate and repair layer as well as the lack of adherence, which commonly results from poor workmanship. It is crucial that restrained shrinkage mechanisms of patch repairs be understood so that the abovementioned effects can be prevented. The performance of patch repairs can be improved through the optimization of their mix design. Thus, it is common to use admixtures, non-cementitious compounds and polymers. However, promising technologies such as expansive admixtures that counteract shrinkage have found little application to date.Polymers do not chemically react with the cementitious matrix. They, however, have been reported to modify the microstructure in cementitious materials, consequently, resulting in improved properties with respect to mechanical, chemical and durability aspects. They increase chemical interactions with mineral phases, which improves the bond between the mortar and the substrate. Besides this, expansive cements are types of cement whose volume increases, unlike plain Portland cement where the volume typically decreases with time. This property relates to the use of expansive admixtures like quicklime, which exhibits an enormous volume increase upon hydration. This property leads to the reduction of the shrinkage, which is increased by the combination of the quicklime to a shrinkage-reducing admixtures (SRA).Using polymer technology and expansive admixtures, it is possible to develop a new approach for the mix design of repair mortars that combines the positive effects of polymer latex (increasing of bond strength) and expansive admixture (reducing the shrinkage) properties to reduce cracking and debonding failures in patch repairs.

Effect of sodium sulfate and nano-SiO2 on hydration and microstructure of cementitious materials containing high volume fly ash under steam curing

Generally, high volume fly ash would considerably reduce the early strength of cement-based materials, and because of this disadvantage, the added amount of fly ash in steam-cured precast concrete is very limited. In this study, an attempt to enhance the early strength of steam-cured cement-high volume fly ash (40 wt%, CHVFA) system has been done with addition of nano-SiO2 (NS) and sodium sulfate (SS). Effect of these two chemicals on hydration, pore structure, and microstructure of interfacial transition zone (ITZ) were investigated. The results show that both NS and SS can improve the compressive strength, and the reason is mainly because these two chemicals can accelerate the hydration of cement minerals and can also hasten the pozzolanic reaction of fly ash. By contrast, because of the high activity of pozzolanic reaction and the filling and nucleation effects, NS has much stronger effect on refining pore structure and promoting the hydration of cement; while SS can exert much stronger effect on accelerating the hydration of fly ash than that of NS. An interesting thing can also be seen that with co-doped NS-SS, the enhancement in compressive strength is much stronger than that of the sum of these single systems with the same dosage, indicating that NS and SS can exert the synergistic effect; this effect is involved in the accelerated hydration of CHVFA, the refined pore structure, and the improved microstructure of ITZ. Such results would be expected to provide guidance on enhancing the early strength of steam-cured precast concrete with high volume fly ash in practical engineering.