Complete Hydration Research Articles

Synchronous hot-pressed metakaolin-fly ash based geopolymer: Compressive strength and hydration products

The dense structure of geopolymers typically requires long curing times, resulting in excessively lengthy production cycles for related products. To address this issue, synchronous hot-pressed curing technology is employed to accelerate the preparation of geopolymers. A comparative experiment was contrived to analyze the impacts of high-temperature curing and synchronous hot-pressed curing of metakaolin-fly ash geopolymer, examining how the silicon-aluminum ratio, alkali equivalent, and curing time influence the compressive strength and hydration products of synchronous hot-pressed cured metakaolin-fly ash geopolymer (SHPMFG) through orthogonal experiments. The outcome indicate that the compressive strength of SHPMFG test specimens can reach 80 % of their final strength within 20 min, with a more complete hydration reaction observed in SHPMFG, resulting in a denser microstructure. The alkali equivalent emerged as the dominant factor affecting SHPMFG strength, with a sample exhibiting a denser matrix structure and lower porosity achieving a maximum compressive strength of 95.33 MPa at 28 days when the alkali equivalent was 8 wt%, the silica-to-alumina ratio was 2.0, and the curing time was 30 min.

Foam Stabilization Process for Nano-Al2O3 and Its Effect on Mechanical Properties of Foamed Concrete.

Foamed concrete is increasingly utilized in engineering due to its light weight, excellent thermal insulation, fire resistance, etc. However, its low strength has always been the most crucial factor limiting its large-scale application. This study introduced an innovative method to enhance the strength of foamed concrete by using nano-Al2O3 (NA) as a foam stabilizer. NA was introduced into a foaming agent containing sodium dodecyl sulfate (SDS) and hydroxypropyl methylcellulose (HPMC) to prepare a highly stable foam. This approach significantly improved the foam stability and the strength of foamed concrete. Its drainage volume, settlement distance, microstructure, and stabilizing action were investigated, along with the strength, microstructure, and hydration products of foamed concrete. The presence of NA effectively reduced the drainage volume and settlement distance of the foam. NA is distributed at the gas-liquid interface and within the liquid film to play a hindering role, increasing the thickness of the liquid film, delaying the liquid discharge rate from the liquid film, and hindering bubble aggregation, thereby enhancing foam stability. Additionally, due to the stabilizing effect of NA on the foam, the precast foam forms a fine and uniform pore structure in the hardened foamed concrete. At 28 d, the compressive strength of FC0 (0% NAs in foam) is 2.18 MPa, while that of FC3 (0.18% NAs in foam) is 3.90 MPa, increased by 79%. The reason for this is that NA promotes the formation of AFt, and its secondary hydration leads to the continuous consumption of Ca(OH)2, resulting in a more complete hydration reaction. This study presents a novel method for significantly improving the performance of foamed concrete by incorporating NA.

Study on the Effect of Silica–Manganese Slag Mixing on the Deterioration Resistance of Concrete under the Action of Salt Freezing

The use of silico-manganese slag as a substitute for cement in the preparation of concrete will not only reduce pollution in the atmosphere and on land due to solid waste but also reduce the cost of concrete. To explore this possibility, silico-manganese slag concrete was prepared by using silico-manganese slag as an auxiliary cementitious material instead of ordinary silicate cement. The mechanical properties of the silico-manganese slag concrete were investigated by means of slump and cubic compressive strength tests. The rates of mass loss and strength loss of silico-manganese slag concrete were tested after 25, 50, and 75 cycles. The effect of the silica–manganese slag admixture on the microfine structure and properties of concrete was also investigated using scanning electron microscopy (SEM). Finally, the damage to the silica–manganese slag concrete after numerous salt freezing cycles was predicted using the Weibull model. The maximum enhancement of slump and compressive strength by silica–manganese slag was 17.64% and 11.85%, respectively. The minimum loss of compressive strength after 75 cycles was 9.954%, which was 34.96% lower than that of the basic group. An analysis of the data showed that the optimal substitution rate of silica–manganese slag is 10%. It was observed by means of electron microscope scanning that the matrix structure was denser and had less connected pores and that the most complete hydration reaction occurred with a 10% replacement of silica–manganese slag, where an increase in the number of bladed tobermorite and flocculated C-S-H gels was observed to form a three-dimensional reticulated skeleton structure. We decided to use strength damage as a variable, and the two-parameter Weibull theory was chosen to model the damage. The final comparison of the fitted data with the measured data revealed that the model has a good fitting effect, with a fitting parameter above 0.916. This model can be applied in real-world projects and provides a favorable basis for the study of damage to silica–manganese slag concrete under the action of salt freezing.

Enthalpic Classification of Water Molecules in Target-Ligand Binding.

Water molecules play various roles in target-ligand binding. For example, they can be replaced by the ligand and leave the surface of the binding pocket or stay conserved in the interface and form bridges with the target. While experimental techniques supply target-ligand complex structures at an increasing rate, they often have limitations in the measurement of a detailed water structure. Moreover, measurements of binding thermodynamics cannot distinguish between the different roles of individual water molecules. However, such a distinction and classification of the role of individual water molecules would be key to their application in drug design at atomic resolution. In this study, we investigate a quantitative approach for the description of the role of water molecules during ligand binding. Starting from complete hydration structures of the free and ligand-bound target molecules, binding enthalpy scores are calculated for each water molecule using quantum mechanical calculations. A statistical evaluation showed that the scores can distinguish between conserved and displaced classes of water molecules. The classification system was calibrated and tested on more than 1000 individual water positions. The practical tests of the enthalpic classification included important cases of antiviral drug research on HIV-1 protease inhibitors and the Influenza A ion channel. The methodology of classification is based on open source program packages, Gromacs, Mopac, and MobyWat, freely available to the scientific community.

Impact of Water-Cement Ratio on Concrete Mechanical Performance: Insights into Energy Evolution and Ultrasonic Wave Velocity.

The water-cement ratio significantly affects the mechanical properties of concrete with changes in porosity serving as a key indicator of these properties, which are correlated with the ultrasonic wave velocity and energy evolution. This study conducts uniaxial compression tests on concrete with varying water-cement ratios, analyzing energy evolution and ultrasonic wave velocity variations during the pore compaction stage and comparing damage variables defined by dissipated energy and ultrasonic wave velocity. The results indicate the following findings. (1) Higher water-cement ratios lead to more complete hydration, lower initial porosity, and a less pronounced pore compaction stage, but they deteriorate mechanical properties. (2) In the pore compaction stage, damage variables defined by dissipated energy are more regular than those defined by ultrasonic wave velocity, showing a nearly linear increase with stress (D = 0~0.025); ultrasonic wave variables fluctuate within -0.06 to 0.04 due to diffraction caused by changes in the pore medium. (3) In the pre-peak stress stage, damage variables defined by ultrasonic wave velocity show a distinct threshold. When the stress ratio exceeds about 0.3, the damage variable curve's growth shows clear regularity, significantly reflecting porosity changes. In conclusion, for studying porosity changes during the pore compaction stage, damage variables defined by dissipated energy are more effective.

Experimental Study on the Effect of Polycarboxylate Superplasticizer on the Performance of Cement-Based Grouting Materials.

Polycarboxylate superplasticizers BMC-L and BMC-S were utilized as modifiers in the formulation of novel cement-based grouting materials. Indoor tests were conducted on 32 groups of cement slurries, varying by water-cement ratio (0.5:1 and 0.6:1) and modifier content (0, 2‱, 4‱, 6‱, 8‱, 10‱, 12‱, and 14‱), to test their density, funnel viscosity, water separation rate, and stone rate. Four groups of slurry modified with BMC-L were selected as the preferred slurry, and the apparent viscosity test, uniaxial, and triaxial compression test of the slurry stone body were conducted. The study investigated the influence of BMC-L on various properties of the slurry, including its apparent viscosity, uniaxial compressive strength, stress-strain relationships, shear strength parameters, and elastic modulus. Ultimately, the pore structure and phase composition of the slurry stone body were detected by Nuclear Magnetic Resonance (NMR) and X-ray Diffraction (XRD), and the impact of BMC-L on slurry performance was examined from a microstructural perspective. Results indicate that the two polycarboxylate superplasticizers exert minimal influence on the density and water separation rate of the slurry. Within the effective incorporation range of the polycarboxylate superplasticizer, increasing the dosage correlates with a decrease in both the stone rate and viscosity of the slurry. BMC-L significantly enhances the mechanical properties of the slurry stone body by promoting more complete cement hydration and reducing porosity. The uniaxial compressive strength of slurry stone body with a 6 ‱ BMC-L dosage reached 29.7 MPa after 7 days and 38.5 MPa after 28 days of curing, representing increases of 118.4% and 64%, respectively, compared to masonry with 0 BMC-L dosage. The shear strength parameters and elastic modulus of the slurry stone body also showed corresponding increases.

The molecular basis for hydrodynamic properties of PEGylated human serum albumin

Polyethylene glycol (PEG) conjugation provides a protective modification that enhances the pharmacokinetics and solubility of proteins for therapeutic use. A knowledge of the structural ensemble of these PEGylated proteins is necessary to understand the molecular details that contribute to their hydrodynamic and colligative properties. Because of the large size and dynamic flexibility of pharmaceutically important PEGylated proteins, the determination of structure is challenging. In addition, the hydration of these conjugates that contain large polymers is difficult to determine with traditional methods that identify only first shell hydration water, which does not account for the complete hydrodynamic volume of a macromolecule. Here, we demonstrate that structural ensembles, generated by coarse-grained simulations, can be analyzed with HullRad and used to predict sedimentation coefficients and concentration-dependent hydrodynamic and diffusion nonideality coefficients of PEGylated proteins. A knowledge of these concentration-dependent properties enhances the ability to design and analyze new modified protein therapeutics. HullRad accomplishes this analysis by effectively accounting for the complete hydration of a macromolecule, including that of flexible polymers.

STUDY OF THE INFLUENCE OF MODIFIERS ON THE HYDRATION OF CEMENT BY THE METHOD OF ISOTHERMAL CALORIMETRY

The effect of modifiers on the hydration of cements was studied by isothermal calorimetry. Thermochemical experiments were carried out using a TAM III isothermal calorimeter (TA Instruments). For the TAM III calorimeter, a method was developed for conducting experiments on monitoring the thermal activity of solid-phase samples with the possibility of repeated use of steel ampoules with a volume of 20 ml. The processes of hydration of Portland cement grade CEM I 42.5B D0 LLC "Holcim Rus" without a modifier and with modifiers polycarboxylate type S (POLYPLAST LLC and «Glass Powder) were studied. Heat release thermograms were recorded, which showed good reproducibility of the results and their similarity with those available in the literature for similar studies carried out according to standard methods using a TAM AIR isothermal calorimeter (TA Instruments). This indicates the reliability of the obtained data and the possibility of using the method proposed in this article to study the processes of hydration of cements. The maximum heat release on the thermograms of samples with a polycarboxylate modifier is observed approximately 20 h after the start of heat release monitoring. Samples without the modifier and with the glass powder modifier show the maximum heat release approximately 10 h after the start of the experiment. Analysis of the results showed that the presence of modifiers leads to increased heat release during cement hydration in relation to its initial composition. Accordingly, it takes less time to complete hydration. However, it is also necessary to take into account the possible risks of violation of the technological modes of operation of the corresponding equipment due to excessive heat release during the hydration of modified cements. For citation: Usacheva T.R., Krainova A.A., Vinogradova L.A., Oganyan V.V., Kokurina G.N., Myshenkov M.S., Anufrikov Yu.A., Shasherina A.Yu., Adamtsevich A.O. Study of the influence of modifiers on the hydration of cement by the method of isothermal calorimetry. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 6. P. 88-93. DOI: 10.6060/ivkkt.20246706.6979.

Passive direct air capture using calcium oxide powder: The importance of water vapor

Calcium oxide (CaO; lime) looping is a proposed technology with the potential to capture gigatonnes of carbon dioxide (CO2) from the atmosphere to help mitigate climate change. The importance of water in carbonation reactions is widely understood as it is needed for mineral and CO2 dissolution and carbonate precipitation. However, the effects of water vapor on CaO carbonation pathways and rates have yet to be elucidated in a systematic manner. Here, we examine the impact of relative humidity (RH) on CO2 removal using CaO powder at 20% to 95% RH. Higher RH resulted in faster hydration rates, forming Ca(OH)2 (portlandite); however, passivation limited carbonation in all experiments, with the greatest carbonation occurring at 80% RH (65% CaCO3; calcite). Thus, CaO powder is highly prone to passivation when using RH to drive hydration and carbonation. In RH swing experiments, RH was changed at different times (hours or days) and by different amounts (e.g., 40–99%). Humidity swings can yield >85% CaCO3 when complete CaO hydration at a low RH (<40%) occurs before carbonation at a high RH (∼99%). Importantly, separating these two processes yields nearly complete carbonation. In this case, we achieved a CO2 removal rate of 1 t CO2 for every 1.95 t of Ca(OH)2 per day.

Study on the calcium dissolution of self-compacting recycled concrete

Considering the effects of the replacement rate of recycled coarse aggregate (RCA), content of fly ash (FA), and solution temperature, the accelerated calcium dissolution test of self-compacting recycled concrete (SCRC) was simulated by using ammonium chloride solution. The results show that the physical properties (the mass loss ratio and the dissolution depth) and the strength degradation of SCRC increase with the increase of the replacement rate of RCA and solution temperature. With the increasing content of FA, the physical properties decrease approximately linearly, while the strength degradation first decreases and then increases. For the SCRC with 50% RCA and a dissolution age of 56 days, the dissolution depth of SCRC with the FA contents of 10%, 20%, and 30% decreased by 3.85%, 10.00%, and 15.56% respectively compared with that of SCRC without FA, and the degradation rate of compressive strength also decreased by 11.60%, 25.91%, and 15.01%, respectively. The peak stress of the stress-strain curve of SCRC under axial compression is negatively correlated with the dissolution age, replacement rate of RCA, and solution temperature, while the peak strain is positively correlated with these variables. When the dissolution age is more than 28 days, the calcium silicate hydrate (C-S-H) gels produced by low-volume (10%, 20%) FA after complete hydration lead to an increase in peak stress and a decrease in peak strain of SCRC, while high-volume (30%) FA has the opposite effect due to incomplete hydration. Finally, the performance degradation model of SCRC under calcium dissolution was established.

A study on the effect of microspheres on the freeze–thaw resistance of EPS concrete

Abstract This study investigated the influence of microbead dosages (0, 5, 10, 15, and 20%) on the frost resistance of expanded polystyrene (EPS) concrete. Five groups of EPS concrete specimens were prepared and subjected to rapid freeze–thaw tests. The freeze–thaw deterioration of EPS concrete was assessed using macroscopic indicators, including mass loss, strength loss, and dynamic elastic modulus loss. The underlying deterioration mechanism was revealed through the analysis of the EPS particle–matrix interface. A concrete damage plasticity model of EPS concrete based on damage mechanics theory was established. The results indicate that the addition of microbeads increased the strength of EPS concrete by 38–53%, reduced the strength attenuation after freeze–thaw damage by 8.1%, and improved the frost resistance level by 10–60 grades. The optimal dosage of microbeads is 15% of the cementitious material. The interfacial transition zone gaps in EPS concrete with added microbeads after freeze–thaw cycles are smaller, contributing to a more complete hydration reaction. The freeze–thaw damage model established in this study accurately reflects the freeze–thaw damage law of EPS concrete and provides a reference for studying the mechanical properties of EPS concrete under freeze–thaw cycles. The research findings of this study can enhance the strength and service life of EPS concrete, expanding its application scope as a structural material. The study provides valuable insights for future research and engineering applications related to the frost resistance of EPS concrete.

Structure and size of complete hydration shells of metal ions and inorganic anions in aqueous solution.

The structures of nine hydrated metal ions in aqueous solution have been redetermined by large angle X-ray scattering to obtain experimental data of better quality than those reported 40-50 years ago. Accurate M-OI and M-(OI-H)⋯OII distances and M-OI(H)⋯OII bond angles are reported for the hydrated magnesium(II), aluminium(III), manganese(II), iron(II), iron(III), cobalt(II), nickel(II), copper(II) and zinc(II) ions; the subscripts I and II denote oxygen atoms in the first and second hydration sphere, respectively. Reported structures of hydrated metal ions in aqueous solution are summarized and evaluated with emphasis on a possible relationship between M-OI-OII bond angles and bonding character. Metal ions with high charge density have M-OI-OII bond angles close to 120°, indicative of a mainly electrostatic interaction with the oxygen atom in the water molecule in the first hydration shell. Metal ions forming bonds with a significant covalent contribution, as e.g. mercury(II) and tin(II), have M-OI-OII bond angles close to 109.5°. This implies that they bind to one of the free electron pairs in the water molecule. Comparison of M-O bond distances of hydrated metal ions in the solid state with one hydration shell, and in aqueous solution with in most cases at least two hydration shells, shows no significant differences. On the other hand, the X-O bond distance in hydrated oxoanions increases by ca. 0.02 Å in aqueous solution in comparison with the corresponding X-O distance in the solid state. A linear correlation is observed between volume, calculated from the van der Waals radius of the hydrated ion, and the ionic diffusion coefficient in aqueous solution. This correlation strongly indicates that monovalent metal ions, except lithium and silver(I), and singly-charged monovalent oxoanions have a single hydration shell. Divalent metal ions, bismuth(III) and the lanthanoid(III) and actinoid(III) ions have two hydration shells. Trivalent transition and tetravalent metal ions have two full hydration shells and portion of a third one. Doubly charged oxoanions have one well-defined hydration shell and an ill-defined second one.

Contribution to a better understanding of long-term hydration, structuration and mechanical properties of slag based cementitious materials: Experimental and modeling approaches

Blast Furnace Slag (BFS), a by-product of the steelmaking process, can be used as a mineral addition in blended cement. In addition to reduce CO2 emissions in the construction field, the use of BFS in cementitious materials also contributes to the improvement of their mechanical and durability properties. However, despite the numerous studies proposed in the literature about the hydration properties of BFS-blended cement, some aspects remain unclear, such as the influence of slag substitution rate on hydration kinetics and on the stoichiometry of hydrates formed and their impacts on both microstructural and mechanical properties. For these reasons, the present study aims firstly to better understand how BFS-blended cement hydrates based on a complete hydration model that considers both kinetics and chemical reactions. The only required input data are the formulation parameters (i.e. water-to-binder ratio (W/B), substitution rate of clinker by slag (fs)), and curing. This study does not introduce the heat of hydration, which limits the validity of other existing models to short-term hydration. Analytical laws involving calibration parameters, which are assumed to vary with the substitution rate of clinker by slag, represent the hydration kinetics of clinker and slag while a stoichiometric approach is adopted to describe the chemical formation of hydrates. In order to calibrate and validate the parameters involved in the hydration model, four different cement paste mixes formulated with 0%, 30%, 50%, and 80% (by mass) of slag are studied in terms of hydration degrees and microstructure evolutions, and completed with data from the literature. The amounts of hydrated phases and those of dry densities, particularly at later ages, are well reproduced by the hydration model. Porous structure evolutions are also investigated in terms of partition between capillary and CSH gel pores by comparing the capillary porosity predicted by the model with the total porosity measured experimentally. Finally, predictive modeling of mechanical properties (compressive strength and Young modulus) of tested materials are finally proposed based on hydration model outputs. A good agreement between experimental and predicted data is observed.

Clinker Portland with iron ore tailing and its characterization by integrated laboratory methods

The large volume accumulation of iron ore tailings (IOT) in Brazil poses significant risks of accidents, highlighting the need for strategies for their recycling. The Brazilian's IOT possesses mainly iron oxide content (HI-IOT) and can be used as raw materials alternatives in clinker production. Based on a fixed lime saturation factor (LSF = 0.96), we incorporated 5 to 10 wt% of HI-IOT in the raw meal to produce Portland clinkers. The mineralogical phases (alite, belite, aluminate, and ferrite) were evaluated by quantitative X-ray diffraction. Optimal results were obtained with 7.5 wt% of HI-IOT in the clinker Portland, i.e., approximately 32 %wt. of clay substitution. The mechanical strength evolution was evaluated by a reduced-size test and obtained compressive strength of 34.8 MPa at 3 days-age, 54.10 MPa at 7 days-age, and 71.20 MPa at 28-days values compatible with standardized high initial strength Portland cement. By monitoring the complete hydration process using TG/DTG and assessing the volumetric composition over time, valuable insights were gained regarding the potential application of these Portland cements at an industrial scale.

Development of a Low-Shrinkage-Lightweight Engineered Cementitious Composite Based on Heavily Doped Zeolites

In recent years, there has been a growing utilization of lightweight engineered cementitious composites (LECC) for the reinforcement and restoration of contemporary building structures. This study focuses on the incorporation of zeolite, serving as an internal reservoir for moisture maintenance, and examines its impact on various performance indicators, including apparent density, compressive strength, tensile strength, and autogenous shrinkage. Additionally, the influence of zeolite on the tensile and ductile properties of LECC is elucidated with the aid of scanning electron microscopy (SEM). The findings reveal that the addition of zeolite enables the preservation of excellent mechanical properties of LECC while further reducing its density. Notably, the introduction of a substantial amount of zeolite leads to a decrease in matrix density, average crack width, and ultimate tensile strain. The ultimate tensile strain exceeds 8% to reach 8.1%, while the decrease in compressive and tensile strengths is marginal. Zeolite’s internal curing capability facilitates the complete hydration of unhydrated cement, concurrently alleviating the autogenous shrinkage of LECC. Consequently, the durability and reliability of the material are enhanced. The ability of zeolite, with its porous framework structure, to significantly improve the ultimate tensile strain of the matrix can be attributed to the amplified occurrence of active defects and a shift in the pull-out mode of PE fibers from “pull-out” to “pull-through”. This study presents a promising alternative material in the field of engineering, holding potential for diverse building and infrastructure projects, as it enhances their durability and reliability.

Design of a strength model for foamed lightweight soil

Foamed lightweight soil has been widely used in civil engineering, and the model for foam concrete has been employed to predict its strength. However, various previous strength models of foam concrete are generally based on the assumption of complete hydration, which is not the case for foamed lightweight soil. For example, Nambiar’s model (reference), a simple and widely used model, sets the ratio of hydrated water to cement by weight as an empirical constant, which is actually related to the water-cement ratio before the completion of hydration for foamed lightweight soil. To address this issue, an improved strength model tailor-made for foamed lightweight soil was proposed to establish a relationship between the ratio of hydrated water to cement and the water-cement ratio. The improved model and the original Nambiar’s model were compared with actual strength data, which shows that this improved model is more effective for foamed lightweight soil.

Adsorption behavior of chemical warfare agent simulants on doped and hydroxylated MgO nanotubes: A DFT study

The present study investigates the adsorption of three chemical warfare agent simulants (DMMP, CEES and paraoxon) on MgO and Ti-doped MgO nanotubes. Simulated annealing has been used to identify all low energy modes of adsorption. The study reveals that both DMMP and paraoxon interact with the metal oxide surface via their phosphoryl oxygens, but in the case of CEES, the adsorption via Cl is highly exothermic on the Ti-doped surface and results in the dissociation of the CCl bond. The effect of hydroxylation on the properties of the nanotube surface has also been investigated. The results reveal that as the number of water molecules attached to the nanotube is increased, the properties are also modified. Moreover, it is found that partial hydroxylation of the MgO surface favors maximal adsorption. This may be because complete hydration leads to blocking of all sites, hence the hydration should be partial, as the presence of a few hydroxyl groups assists the decomposition of the molecule.

Experimental study of pure Class G cement hydration up to 150 °C and 50 MPa

A comprehensive experimental investigation of Portland cement hydration kinetics in a wide temperature range from 35 °C to 150 °C with pressure up to 50 MPa was performed by heat of hydration, chemical shrinkage and ultrasonic measurements. Increasing curing temperature was found to have a stronger acceleration effect during the main hydration period while increasing curing pressure had a stronger acceleration effect during the pre-induction period of cement hydration. By comparing chemical shrinkage test results with other methods of estimating cement degree of hydration, such as quantitative X-ray diffraction, non-evaporable water content measured by thermogravimetric analysis, as well as heat of hydration, it was observed that total chemical shrinkage of cement at complete hydration decreased with curing temperature from 35 °C to 75 °C and then stayed stable between 75 °C and 150 °C. Similarly, the P-wave velocity of cement (which is an indicator of physical and mechanical properties) as a function of hydration degree decreased with increasing curing temperature in the range between 35 °C to 75 °C, then remained relatively stable between 75 °C and 130 °C. The reduction of gel water content in C–S–H from 35 °C to 75 °C is believed to be the primary cause of these observed phenomena.

Using diatomite as a partial replacement of cement for improving the performance of recycled aggregate concrete (RAC)-Effects and mechanism

This work proposes that diatomite can be used to improve the performance of recycled aggregate concrete (RAC). A calcined diatomite sample with an amorphous silica content over 90% was added to the RAC mixture as a partial replacement of the ordinary Portland cement (OPC) at varying rates from 0 to 20 wt%. Test results show that both the strength and impermeability of RAC can be obviously enhanced with the greatest enhancement observed at a replacement level of 15 wt% for which the 28-day compressive/splitting tensile strengths were improved by 29.2% and 33.9%, respectively, and the chloride diffusion coefficient was reduced by 86.27%. The microscopic measurements confirmed the nucleation effects and the high pozzolanic reactivity of diatomite which both contributed to the more complete hydration of cement in RAC. It was also found that the proportion of large capillary pores in the new mortar was reduced by using diatomite, meanwhile the average porosity of the new interfacial transition zone (ITZ) became smaller. This work suggests that the calcined diatomite used for improving RAC performance should be no larger than 15 wt% of cement, as the scarcity of CH at a higher replacement ratio would hamper the pozzolanic reactions, meanwhile the diluted cement clinkers would have a negative effect on the pore structure of RAC.

Analysis of two processing techniques applied on powders from recycling of clay bricks and concrete, in terms of efficiency, energy consumption, and cost

Two processing techniques, i.e., grinding and calcination taken on fine powders from recycling of concrete and clay bricks (RCP/RBP) as construction and demolition wastes (CDW) were evaluated in terms of their efficiency, energy consumption, and cost. It was found that replacing OPC with 20 % non-processed RCP/RBP led to up to 22 % loss of compressive strength of mortar, though the fluidity and flexural strength were similar to that of the OPC mortar. By conducting the grinding treatment, the RCP/RBP particles were refined; cement hydration was found to be more complete in the blended mortar meanwhile the pore structure was improved. Calcination led to the formation of new active phases in RCP/RBP which facilitated the early-age cement hydration and contributed to more complete hydration at later ages. Test results on mortar strength showed that the compressive/flexural strengths can be enhanced by both treatments though the 28-day compressive strength of RCP mortar was still 11 % lower than that of OPC. The optimal grinding duration and calcination temperature in terms of enhancing mortar strength was 20 min and 600 ℃/800 ℃ (for RCP and RBP respectively), under which the 28-day compressive strength of the blended mortars can be enlarged by up to 10.11 % and 14.33 %, respectively. Basically, calcination can enhance the mortar strength (28 or 90 days) better than grinding and the enhancement was more significant for the RCP mortar than for the RBP. Further analysis on extra power consumption and cost performance suggests that the calcination treatment was more energy-efficient and cost-effective than grinding to improve the mortar strength so it should be a better option to process CDW powder for preparing lower-carbon cementitious materials with satisfactory fluidity and strength.