Hydration Of Cement Particles Research Articles

The Relationship between Molecular Structure and Foaming of Poly(ethylene glycol)—Poly(propylene glycol) Triblock Surfactants in Cementitious Materials

This study investigates the relationship between the molecular structure and foaming of poly(ethylene glycol) and poly(propylene glycol) triblock copolymers in Portland cement pastes. Four copolymers with different molecular structures were studied at varying concentrations. All copolymers showed a reduction in surface tension of the cement pore solution; however, only some of them demonstrated foaming and air entraining in cement paste. The results indicated that the molecular structure parameter, hydrophilic-to-lipophilic balance (HLB), has a direct relationship with the foaming and air-entraining performance of the copolymers. The total organic carbon measurements showed very small adsorption of these non-ionic copolymers on hydrating cement particles due to the lack of surface charge needed to interact with the heterogeneously charged surface of hydrating cement. In addition, these copolymers did not seem to affect the flow of cement paste due to a lack of adsorption on cement particles. The cement paste modified with the copolymers showed increased water sorption compared to the control paste due to the increased capillary porosity and slight increase in pore surface hydrophilicity. However, the freeze-thaw resistance was shown to improve with an increase in the number of air voids in the modified cement pastes. The findings establish the relationship between molecular properties of copolymers and their air-entraining performance in cement paste to mitigate the damages caused by freeze-thaw action.

Numerical modeling of microstructure development and chloride diffusion coefficient of cement paste with ellipsoidal particles

In view of the important role of cement profile in hydration process, pore structure characteristic, porosity and chloride ingression of cement paste, this paper aims to propose a numerical method for predicting the chloride diffusion coefficient of cement paste by considering the particle shape. By introducing periodic boundary conditions and contact function, the three-dimensional fresh cement paste with ellipsoidal particles is firstly constructed. By taking into account the interferences induced by overlapping of hydration layers between adjacent hydrated cement particles and calcium hydroxide precipitation, the vector hydration model is proposed. Each hydrated cement particles is composed of inner and outer hydration layers, unhydrated cement particle and partial calcium hydroxide embedded in the hydration layer. After the validity of hydration model is verified by the experimental data of hydration degree and the content of hydration products, the effects of aspect ratio on hydration degree, pore size distribution and porosity are evaluated. Secondly, based on first-passage theory, the Brownian motion algorithm applicable to voxel model is proposed. In order to improve the simulation efficiency, the first-passage radius and Brownian number are determined. The accurancy of this approach is fully verified by comparing with sufficient experimental data. Finally, resorting to this numerical algorithm, the effects of the aspect ratio and the diffusion coefficients of inner and outer hydration layer on chloride diffusion coefficient are analyzed.

Performance assessment of seawater fly ash cement paste with integrated physicochemical- geochemical simulation platform and experimental validation

The shortage of river sand and fresh water for concrete production has become a severe issue, accompanied by a significant increase in infrastructural constructions, especially for coastal areas. Therefore, the possibility of utilizing seawater and sea sand as alternative raw materials for concrete components has attracted broad research interest in recent decades. This paper presents experimental, and simulation investigation to comprehensively study the material performance of seawater fly ash cement paste (SFCP). An integrated simulation platform is adopted to achieve the performance assessment, in which the physicochemical and geochemical processes can be addressed, such as the hydration of cement particles, formation of micro-pore structure, multi-species migration and equilibria, activity effect, the thermodynamic balance between the matrix skeleton and pore solution. A comparison of experiment and simulation showed that the properties of SFCP from micro to macroscale are consistently associated and fairly explained through the integrated simulation platform for further practical realization.

Statistical Modeling of Pore Structures in Fresh Cement Paste Based on Dispersion Theory of Random Two-Phase Media

Pore structures are critical for most properties of cementitious materials. To describe the initial pore structure of fresh cement paste, which is the foundation of the evolution of a pore network induced by hydration of cement particles in hardened concrete, statistical modelling of pore structure is carried out based on a concentric-shell sphere model, by taking cement paste as a random two-phase media. Calculations by the dispersion models agree well with the pore size distributions tested by the low-field H1 nuclear magnetic resonance (NMR) technique. For cement paste that disperses completely with the help of a dispersant, a hard sphere model is practical. However, in most real conditions, an agglomeration of cement particles will occur both before and after mixing with water, which can be characterized by the redistribution of pore size determined by the critical grain size and the hardness parameter in the concentric-shell model, respectively. Calculation results suggest that agglomeration of particles leads to coarser pores. This work will promote the understanding of the formation mechanisms of pore networks and further offers a foundation for modelling the evolution of pore structures in cementitious materials.

Experimental Study on Performance Optimization of Grouting Backfill Material Based on Mechanically Ground Coal Gangue Utilizing Urea and Quicklime.

Previously conducted studies have established that grouting backfill in mining-induced overburden bed separation and mined-out areas with broken rocks provides an efficient strategy to control strata movement and surface subsidence caused by underground mining. Grouting backfill materials (GBMs) based on coal gangue (CG) are highly desirable in coal mining for accessibility to abundant CG and urgent demand for CG disposal. However, CG is generally employed as coarse aggregate due to rather rigid and inert properties, limiting its application in GBMs. Herein, to reduce reliance on fine aggregates, such as fly ash and clay, cemented GBM formulations using ground CG powder as a dominant component were proposed. Urea and quicklime were utilized as additives to optimize slurry transportability and compressive strength. Besides typical grinding without additives, CG powder was also prepared via grinding with urea, intending to enhance the hydrogen bonding (HB) interaction between urea and minerals contained in CG. The effect of grinding time and urea on CG particle size and phase composition was investigated. Then, the dependence of slurry transportability and compressive strength on grinding time, solid concentration, urea, and quicklime dosage were revealed. It has been experimentally proved that grinding for 30~90 min significantly decreased CG particle size and even induced crystal deformation of dolomite and kaolinite. For GBMs, urea improved slurry flowability, possibly caused by decreased water absorption on the CG surface and the release of water encapsulated in hydrated cement particles. Moreover, quicklime strengthened GBM bodies, which could be explained by an accelerated pozzolanic reaction between CG powder and additional CH supplied by quicklime hydration. G60U3-based GBM-B2 with 5% quicklime provided a stable and smooth slurry with a bleeding rate of 1.25%, a slump flow of 205 mm, and a hardened body with a seven-day UCS of 1.51 MPa.

The long-term suppression effects of fly ash and slag on the expansion of heat-cured mortar due to delayed ettringite formation

The long-term resistance of mortar containing fly ash and slag against expansion due to delayed ettringite formation (DEF) was investigated. DEF expansion of heat-cured mortar containing fly ash and slag at various replacement levels was monitored over 10 years. The results indicate that fly ash and slag suppress DEF expansion effectively over the long term, even when there is excessive SO3 in the cement. In the DEF-expanded mortar, the cracks and gaps were filled with ettringite, and fully hydrated cement particles were observed. In contrast, a substantial amount of unhydrated cement particles and high S/Ca ratio of inner products remained over long term, since the incorporation of a sufficient amount of fly ash and slag possibly facilitated the development of a dense microstructure in the paste. A large amount of monosulfate was also present throughout the paste. These results suggest that fly ash and slag eliminate the transport of moisture and ions in the mortar, which considerably extends the latency time of DEF expansion.

Simulation of Permeation of Saturated Cement Paste Based on a New Meso-scale Pore Network Model

This paper presents the simulation of the permeation of saturated cement paste based on a novel pore network model. First, a 2D hydration model of cement particles was developed by extending the work of Zheng et al. 2005 to provide the background for the network construction. Secondly, the establishment of the pore network model and simulation of permeation of saturated cement paste were carried out. The irregular pores between any two hydrated cement particles were linearized with clear distances as the diameters of pores. The straight tubular pores were interconnected with one another to form the network model. During this process, the weighted Voronoi diagram was employed to operate on the graphical expression of the hydrated cement particles. Water permeation in saturated cement paste was simulated to verify the pore network model. Finally, the factors including water–cement ratio, reaction temperature, reaction time and cement particle size that would influence water permeation were numerically investigated.

Properties of Concrete Modified by Ultrafine Cement Admixture

This article describes possibility using hydrated cement as additions for batching new concrete. Time of cement hydration was 2, 4 and 6 hours. Quantity of pre-hydrated cement for batching admixture was equal 5, 10, 15, 30% dry matter content in relation to cement for control mixture. Influence of addition on normal consistency, concrete unit weight and compression resistance was determined. Cement particles distributions of control and pre-hydrated specimens was studied. The article shown need for increasing water quantity to getting normal consistency which led to decreasing unit weight. As a result of adding hydrated cement particles an early strength increased with amount of admixture less than 10%. Received results were discussed according to cement hydrations mechanism.

Preparation and characterization of ultra-lightweight fly ash-based cement foams incorporating ethylene-vinyl acetate emulsion and waste-derived C-S-H seeds

Recycling of accumulated fly ash (FA) from the coal-burning power plant positively avoids the severe environment problems. Also, to achieve the energy-savings in building facades, the excellent fire-resistant and heat-insulating high-porosity fly ash-based foamed cement (ULFC) with dry density less than 160 kg/m3, was formulated in this work. Ethylene-vinyl acetate (EVA) emulsion was introduced into ULFC slurry, and a lab-made hardening accelerator named A-3 and waste-derived calcium silicate hydrate (C-S-H) seeds from the discarded cement foams were also developed and mixed to regulate the early hydration rate of this green FA-based binder incorporating FA up to 50 wt%. Results revealed that under the optimum preparation parameters, the dry density, compressive strength and heat conductivity of ULFC at 28 days can approach to 154.7 kg/m3, 0.47 MPa and 0.0514 W/(m·K), respectively. Morphological analysis elucidated that the polymeric EVA film solidified at the gas–solid interface and well-interwined with hardened cement stone to improve the compressive strength when the specimens was cured at a relative humility of 60–65%. Moreover, the foam stability of ULFC was promoted by the increased bubble confinement force with an incremental content of EVA. Also, an optimized air-void structure was observed in the sample containing synthetic C-S-H from X-CT due to its nucleation effect on the hydration of cement particles. From the energy consumption analysis in practical application, more than 50% energy consumed for heat supply was conserved in ULFC-embedded exterior wall.

Test and simulation of cement hydration degree for shotcrete with alkaline and alkali-free accelerators

In this paper, adiabatic temperature rise test was used to evaluate the effect of the type and content of accelerator on cement hydration in shotcrete. Two kinds of accelerators, alkaline and alkali-free accelerators were used in the experiments. For each type of accelerator, two different concrete mixture were used, and the dosage of the accelerator was 3% and 7% respectively. The experimental and theoretical results show that the use of alkaline or alkali-free accelerator can significantly improve the hydration rate in the first and second stage for shotcrete, but the alkaline accelerator will reduce the final hydration degree obviously, however the effect of alkali-free accelerator is small or even slightly increase. With the increase of the content of accelerator, the final hydration degree of concrete decreases, whether for alkali-free or alkaline accelerators. The mechanism may present in two aspects. For alkali-free accelerator, the hydration products are mainly rod-shaped AFt which will make concrete lose fluidity but do not completely cover the surface of the cement particles, and the acid substances in alkali-free accelerator will accelerate the dissolution of C3S, thus promoting the hydration reaction. However, for the alkaline accelerator, the hydration products are mainly composed of C-A-H, AFm and a small amount of C–S–H gel which will wrap on the surface of cement particles, thus affect the further hydration of cement particles. The proposed cement hydration rate model can well predict the development process of the hydration degree for shotcrete, especially for the early age concrete.

Function Synergy of a New Type of Fluid Loss Additive Formula for Comprehensive Performance of Oil Well Cement

With 2-acrylamido-2-methylpropanesulfonic acid (AMPS) units on polymeric additive, additive showed high effectiveness used for oilwell cement. However, due to chemical absorption and chelation mechanism of AMPS units to Ca2+ hydrating cement particles, adding of AMPS type additives caused delay of cement hydration process. In this research, AMPS type fluid loss additive, named as FLA A additive, was studied for its hydration delay side effect to class G Portland cement. Furthermore, polyvinyl alcohol (PVA) polymer, modified by glyoxal and boric acid, called as PVAGB was used as a synergistic functional additive to AMPS type polymer fluid loss additive to research on hydration delay problem of AMPS type additive to cement and the improvement for the effectiveness of AMPS type fluid loss additive. When AMPS type additive showed functional drawbacks, with more disordered chemical absorption and chelation behaviors to Ca2+ hydrated cement particles rather than constituting a completed and superior fluid loss control system, and this kind of modified PVA polymer was utilized for making up its failure. New compound additive formula, PVAGB/FLA A fluid loss additive formula, was investigated, which showed superior and more stable fluid loss control ability, i.e. about 50 mL at 30°C and 108 mL at 80 °C with just 0.2 % BWOC (weight percentage by weight of cement) PVAGB and 0.5 %BWOC (weight percentage by weight of cement) FLA A addition. In addition, within 28-day curing period, cement samples showed a healthy compressive-strength development with no less than 28MPa after 7-day curing period rather than failure due to cement strength retrogression. With scanning electron microscope (SEM) analysis, PVAGB showed accelerating effect to cement hydration process, in which hexagonal plate Ca(OH)2 crystal and aggregated product of C-S-H gel were formed when compared with pure cement and cement with FLA A additive added.

Function Synergy of Cross-Linked Cationic PVA Polymer to AMPS-Type Fluid Loss Additive Used for Cement-Based Material

In this research, a kind of 2-acrylamido-2-methylpropanesulfonic acid sodium salt- (AMPS-Na-) type copolymer additive, the fluid loss additive (FLA), named as FLA A additive, was used for research. The performance of FLA A was tested and found to fail in the effective control of free water and to hinder the hydration process for delaying the breaking of the early hydration shell. The reason for it was the absorbed behavior and chelating effect of the AMPS-Na unit to Ca2+ hydrating cement particles. Thus, a cationic polyvinyl alcohol (PVA) polymer, modified by glyoxal and boric acid, was discovered due to its excellence in associating with the FLA A additive for controlling the free cement-based material water amount and preventing the chelating effect of FLA A chains on the surface of the cement-based material. Glyoxal/boric acid-modified polyvinyl alcohol, abbreviated as PVAGB or PVA-G-B, was with special molecular properties, i.e., positive ZETA (ζ) potential characteristics and cross-linked molecular structure. Due to competitive absorbed behavior of glyoxal-modified hydroxyl groups and free Ca2+ released by the hydration product, the chelating effect of AMPS-Na units to Ca2+ was weakened and the possibility of FLA A chains being absorbed to the surface of the cement-based material was decreased. Then, the formation of a complete fluid loss system was obtained; i.e., the fluid loss volume decreased to less than 50 mL at 30°C and 108 mL at 80°C with 0.2 percentage by weight of cement (%BWOC) of PVAGB and 0.50%BWOC (percentage by weight of cement) of FLA A. Besides, the hydration process of cement-based material was accelerated due to formation of more C-S-H gels in the early hydration period. As a result, the cement-based material not only showed no worse compressive-strength retrogression but also showed a stable 28-day compressive strength of 28 MPa.

Partial Replacement of Microsilca on Compressive Strength Concrete: Applying Predictive Model

Monitoring the rate of micro-silica variation was carried out to determine different rate of strength developed applying various dosage, these are the pozzolanic reaction which occurs between silica fume and CH producing additional CSH in many of the voids around hydrated cement particles. This is due to high surface area of silica fume that normally affect the mobility of water within concrete segregation, this include bleeding of concrete that are virtually eliminated . This application detailed on the reactions from micro-silica reflection of it are influential factors on rapid increase of concrete strength, these are based on the mixed designed output, such application reflects the significant growth rate of concrete performance in the study, mathematical model application using analytical solution generated derived model for the study, this derived model techniques developed predictive values through simulation, the predictive were applied on the reflection of various water cement ratios including variation of micro-silica dosage numerically to ninety days of curing, water cement ratios influence of strength development were expressed in the study, variation of concrete permeability and void ratios are also detailed as an influential factor in concrete formations, the study has express its significant influence based on the developed strength within seven and ninety days of curing.

Analysis methodology of XCT results for testing ingress of substances in hardened cement paste: Explained with chloride immersion test

X-ray computed tomography (XCT) can non-destructively detect microstructure of cement-based material. However, its raw results involve errors induced by the equipment and external factors. Once studying changes taken place in a sample, such as durability related studies, which need testing on a same sample at intervals, these errors will result in misleading conclusions. At the moment, in literature, no method focusing on effectively and reasonably eliminating or minimising these errors has not been observed. After testing and analysing ten series of XCT results for samples experiencing various types of exposures, authors established a rational data-processing method to minimise the influence of these errors on test results, and is introduced in this paper. To assist readers’ understanding, the method and relevant information are illustrated with a case of immersion of hardened cement pastes in NaCl solution. Obtained results prove that diffusion of substances in the hardened cement paste can be properly revealed with the data processing method introduced; trend and depth of microstructure change caused by the exposure can be determined; the immersion of samples in NaCl solution led to a gradual decrease in density and a further hydration of cement particles, which should be related to leaching of Calcium from the sample into immersion solution; once immersed in same NaCl solution, the change of microstructure in the 0.35 w/c was slower than that in the 0.5 w/c sample and the influence depth had an approximate linear relationship with square root of immersion duration.

Investigating the influence of volumetric hydrophobization on the formation of phase composition of cement stone and its physical-mechanical properties

We have examined the influence of the most widely used silicon-organic surface water repellent GKZH-11K on the physical-mechanical properties and the phase structure formation of cement is the fact that the introduction of the surface water repellent GKZH-11K to the composition of cement slurry leads to the formation of dispersive water-repellent films of various levels (size). These films adsorb at the surface of cement grains, as well as at the surface of hydrated components of cement particles, Са(ОН)2, calcium sulphoaluminates. It was determined that the disperse hydrophobic films partially block the penetration of water into the surface of cement grains and alter the kinetics of hydration.This leads to a decrease in the degree of hydration and the content of Са(ОН)2 in cement stone, resulting in the reduced shrinkage at hardening. At the same time, the disperse films adsorbed on neighboring hydrated cement particles within the contacts occurred in the process of their condensation can form hydrogen and chemical relationships. This interaction between the particles' films, despite the reduction in the degree of hydration, leads to a decrease in water absorption, improving the strength of cement stone, which in this case is also determined by the number and area of contacts per unit volume of cement stone. The overlap of pores with hydrophobic chains decreases the permeability of cement stone throughout its entire volume, which contributes to the growth of operational reliability and durability of structures, particularly thin-walled. Using this admixture for volumetric hydrophobization could greatly prolong repair-free operation of thin-walled products and structures, as well as bring down cost as there is no need to apply materials to protect the surface of concrete.

Adsorption interaction between cement hydrates minerals with fluid loss additive investigated by fluorescence technique

The mechanism of synthetic polymer fluid loss additive (FLA) was based on adsorption of FLA on the surface of hydrating cement particles and the formation of a complex network structure, resulting in decline of cement slurry filtrate. However, due to the complexity of early hydration environment of cement slurry, there was lack of effective and intuitive way to characterize the adsorption interaction between FLA and cement particles. In this study we described our quest in furthering understanding of the adsorption action of fluid loss additive on the cement minerals. By fluorescence staining technique (combined polymer additives with organic fluorescent dyes), we observed that the skin of the hydrated particles was coated with dyed polymer, which led to a successful fluid loss additive. More importantly, the analysis of the fluorescence intensity of luminous polymer in cement pore solution distinguished the respective adsorption capacity of various admixtures (dyed and undyed) on the surface of the cement particles, so as to solve the problem of the traditional method in characterizing the competitive adsorption between polymers, thus gave a convenient way to analyze the interaction mechanism between polymer admixture and cement particles from the theory and technology.

Influence of triethanolamine on reactivity of hydrated matrix in sodium silicate self-healing system and the mechanism

A suitable binding agent proposed in literatures or patents is sodium silicate, which achieves self-healing by formation of calcium silicate hydrate (C-S-H) in cracks. We systematically investigated the influences of triethanolamine (TEA) on the stimulated C-S-H, sodium silicate/hydrated cement and real self-healing system. Results suggest that TEA addition induces the C-S-H to arrange in order and forms sponge-like structure. In sodium silicate/hydrated cement sample, the morphology of C-S-H is simultaneously affected by the heterogeneous nucleation and restriction effect of hydrated cement particles, and the reticulate structure is observed. It is believed that the formation of TEA-Ca2+ facilitated the dissolution of solid Ca from hydration products, hence increases the chemical reactivity of hydrated cement matrix and the actual binding ability is improved due to the increment of high density C-S-H. Finally, the improved performance of self-healed area was demonstrated by optical microscope, SEM and Raman analysis, meanwhile the promotion in micromechanical properties of C-S-H was studied by AFM.

Effect of further water curing on compressive strength and microstructure of CO2-cured concrete

This study aims to investigate the effects of further water curing on the compressive strength and microstructure of CO2-cured concrete. The results showed that concrete with a residual w/c ratio of 0.25 showed the most rapid strength development rate upon further water curing due to hydration of uncarbonated cement particles. Thermogravimetric, IR-spectrophotometric and scanning electron microscope examinations indicated that further hydration of the cement particles could form C-S-H gel and ettringite crystals. The results showed that the calcite formed during the initial CO2 curing was consumed during the further hydration of C3A, and produced calcium monocarbonaluminate hydrate. Also, Ca(OH)2 was not detected due to its reaction with the formed silica gel. Mercury intrusion porosimetry test results indicated that the porosity and pore size of the CO2 cured mortar decreased further after water curing.

Estimation of the Applicability for the Filler Produced by Recycling of Concrete and Reinforced Concrete Used in Heavy Concrete

The estimation of the secondary filler applicability showed that secondary rubble and sand on all indicators satisfy the standard requirements (except the content of dust particles and clay particles) and can be used as filler for construction works. The content of particles sized less than 0.16mm is 8%, which exceeds the requirements. However, the chemical analysis shows that these particles consist of hydrated and non-hydrated cement particles. The amount of non-hydrated cement, based on cement consumption is about 10%. Consequently, in the course of cement materials production using the secondary rubble and sand you can either reduce the cement consumption or increase the strength of products while maintaining the cement consumption. In turn, the hydrated cement particles can act as additives - the substrates for crystallization. Thus, the screening is a multimineral active additive to cement-based materials.

English

The effect of individual and combined addition of both nanosilica (NS) and polycarboxylate ether plasticizer (PCE) admixtures on cements pastes was studied. The sole incorporation of NS increased the water demand, as proved by the mini-spread flow test. An interaction between NS and hydrated cement particles was observed in fresh mixtures by means of particle size distribution studies, zeta potential measurements and optical microscopy, giving rise to agglomerates. On the other hand, the addition of PCE to a cement paste increased the flowability and accelerated the setting process. PCE was shown to act in cement media as a deflocculating agent, reducing the particle size of the agglomerates through a steric hindrance mechanism. Mechanical strengths were improved in the presence of either NS or PCE, the optimum being attained in the combined presence of both admixtures that involved relevant microstructural modifications, as proved by pore size distributions and SEM observations. The results indicate also the effectiveness of NS and polycarboxylate superplasticizer in producing high packing density and in accelerating the pozzolanic activity to produce more C-S-H gel by consuming calcium hydroxide Ca(OH)2 in order to improve the mechanical properties of cement pastes.   Key words: Cement pastes, polycarboxylate plasticizer, nanosilica, optical microscopy, rheological and electrokinetical properties.