Hydration Of Pastes Research Articles

Microstructural study of concrete performance after exposure to elevated temperatures via considering C–S–H nanostructure changes

Abstract Evaluation of the concrete behavior in elevated temperatures is important in terms of first, structural construction safety under specified loadings, and second, measuring the loading capacity to continue operation. Furthermore, concrete behavior at high temperatures is strongly affected by microstructure. The calcium silicate hydrate (C–S–H), a nanostructure which is produced by hydration of cement paste, plays a significant role in enhancing the concrete strength under the impression of thermal fluctuations. Hence, this study investigates the microstructural performance of concrete exposed to high temperatures with a special focus on C–S–H nanostructure. Accordingly, 300 samples were cured for 1, 3, 7, 14, and 28 days in a moist room and then exposed to temperature range of 25–900°C for 2 h to investigate changes in their weight, length, compressive strength, and cracking behavior. Besides evaluating the microstructural behavior of the specimens in different temperatures, several techniques such as SEM, EDX, and XRD have been employed. Based on the results, any changes in the samples’ length, weight, and compressive strength depend on the C–S–H nanostructure behavior. In fact, following water decomposition from the C–S–H nanostructure at 900°C, the structure is partially converted to porous ceramic. These structural changes have caused a decline of 79–100% of the compressive strength. The compressive strength has decreased from 27.6 MPa to about 6 MPa. Also, the weight loss percentage of 28-days-old sample at this temperature was 18.84%. Based on the SEM and XRD results, this decline under high temperature arises due to the collapse of C–S–H nanostructure and formation of calcium oxide in the cement structure.

Comparative study on the early properties of cement modified with different ionic polyacrylamides

Polyacrylamide (PAM) has been widely applied as a viscosity modifier to optimize the workability of cement-based materials, but PAMs with different ionic types may show various effects, which is often ignored in practical engineering. In this paper, the early properties including the rheology, flowability, and setting of cement pastes modified by three ionic PAMs (anionic APAM, cationic CPAM, and nonionic NPAM) were contrastively studied. The absorption and hydration of cement pastes with different ionic PAMs were further investigated. Moreover, the contributions of physical flocculation and chemical hydration on the structural build-up of cement pastes were quantified by combining the static yield stress and heat release at the same time scale. The results indicated that three ionic PAMs showed different effects on the structural build-up, fluidity and setting of fresh cement pastes, which were dominated by the different physical flocculation and hydration retardation effects of three ionic PAMs on the cement pastes. The order of flocculation and hydration retardation effects of these three ionic PAMs was APAM ≫ NPAM > CPAM, which was concentration-dependence and evolved over times. The reason is mainly due to the distinct molecular structures and physicochemical properties of three ionic PAMs and their different adsorption behaviors with cements particles.

Rheological Properties and Early-Age Microstructure of Cement Pastes with Limestone Powder, Redispersible Polymer Powder and Cellulose Ether.

In order to study the synergistic effects of organic and inorganic thickening agents on the rheological properties of cement paste, the rheological parameters, thixotropy cement-paste containing limestone powder (LP), re-dispersible polymer powder (RPP), and hydroxypropyl methylcellulose ether (HPMC) were investigated using the Anton Paar MCR 102 rheometer at different resting times. The early-age hydration process, hydration products, and microstructure were also analyzed with scanning electron microscopy (SEM) and thermogravimetry analyses (TGA). The results showed that the addition of LP, RPP, and HPMC affected the rheological properties of cement paste, but the thickening mechanism between organic and inorganic thickening agents was different. The small amount of LP increased the plastic viscosity but decreased the yield stress of cement paste due to its dense filling effect. Adding 1% of RPP improved the thixotropic property of cement paste by 50%; prolonging the standing time could improve the thixotropic performance by as much as two times. Only 0.035% HPMC added to the cement paste increased the plastic viscosity by 20%, while the yield stress increased nearly twice. The more HPMC added, the more significant effect it showed. Cement paste compounds with LP, RPP, and HPMC balanced the yield stress and plastic viscosity and improved the thixotropy. The C-L6-R1.0-H0.035 paste presented as a pseudoplastic, its rheological indexes were close to one, and it was hardly affected by the resting time. The composite superposition effect of organic and inorganic thickening agents reduced the impact of resting time for all pastes. As the organic thickening component inhibited the hydration more than the LP promoted the hydration of the cement paste, indicating that the C-L6-R1.0-H0.035 paste remained in the particle fusion stage after curing for three days, as shown by the SEM images.

High-ferrite Portland cement with slag: Hydration, microstructure, and resistance to sulfate attack at elevated temperature

Among the four mineral components in cement, C4AF has been demonstrated to have satisfactory corrosion resistance to sulfate attack. Here, high-ferrite Portland cement (HFPC) is used to prepare high corrosion-resistance cement-based materials. The hydration mechanisms and pore structures of HFPC pastes containing different slag dosages are studied at 20 °C, 40 °C, and 60 °C. The results indicate that the elevated temperature greatly stimulates the slag activity and significantly promotes the hydration of HFPC/slag blends. As the temperature increases, the heat curve of the cement pastes with high content slag shows a third or fourth exothermic peak. The hydration of pastes with more than 40% slag is controlled by the nucleation of hydrates in the acceleration stage and by the diffusion of ions in the deceleration stage above 40 °C. In addition, the HFPC pastes show excellent mechanical properties in a high-temperature environment. The HFPC pastes exhibit better sulfate attack resistance performance compared with pure ordinary Portland cement (OPC) ones. Therefore, the excellent performance of HFPC-based materials can be applied in the construction of concrete in mass under the environment of the South Sea in China, to solve the problems of high environmental temperature, large temperature differences, and sulfate attack.

Influences of CO2‐cured cement powders on hydration of cement paste

AbstractCarbonated cement pastes (CCP) can be used as stable carbon storage. In this work, CCP were systematically investigated as supplementary cementitious materials (SCMs)‐like components used in ordinary Portland cement (OPC) paste. The fully hydrated cement pastes were pulverized into powders and then exposed to different CO2 pressure conditions (i.e., 1.0, 2.0 and 2.5 MPa) for 24 h. These CCP powders were then mixed with OPC to form a new binder. Various fresh, hardened and microstructural properties of CCP powders and the mixtures were investigated. Initial results show that the CO2 sequestration level of CCP increased with the growth of CO2 pressure, and albeit a pozzolanic reaction occurred inside the mixture, the relatively higher water demand due to the use of 30% replacement of CCP led to the inferior workability and compressive strength than the control group. Finally, a conceptual model relating to the CCP‐OPC paste was proposed, facilitating the comprehension of the hydration behaviours of CCP‐OPC paste. © 2022 Society of Chemical Industry and John Wiley & Sons, Ltd.

Effect of lithium citrate on hydration of cement paste

This paper systematically investigated the influence of lithium citrate on the properties of cement. Its effect on hydration heat and internal pore structures was examined by isothermal calorimeter and mercury intrusion porosimetry (MIP) analysis, respectively. The evolution of hydration products and changes of the ion concentration in the pore solution were also investigated by thermal gravimetric (TG), X-ray diffraction (XRD)and Inductively Coupled Plasma (ICP). Thermodynamic model was used to further reveal the relationship between lithium citrate and hydration products. The results showed that lithium citrate first retarded the cement hydration, but improved the strength subsequently functioning as an accelerator as early as 7 curing days. This was because upon the addition in the cement, lithium citrate immediately formed a protective outer layer wrapping around cement clinker particles. As hydration further proceeded, the protective layers were gradually broken down, and the released lithium ions then promoted the hydration of C3A to form a lithium aluminate amorphous gel compound, providing new sites for cement hydration and contributing to a higher content of portlandite and calcite, which in turn augmented the strength.

Impact of sulphate source on the hydration of ternary pastes of Portland cement, calcium aluminate cement and calcium sulphate

The present work investigates the hydration and evolution of solid phase assemblage as a function of sulphate source in ternary Portland cement (PC), calcium aluminate cement (CAC) and calcium sulphate (CS¯HX) cement pastes. Two binders are compared, a PC‒rich and a CAC ‒ CS¯Hx ‒rich paste, containing gypsum and anhydrite as sulphate carrier, using a multi-method approach including calorimetry, XRD, TGA, MAS NMR spectroscopy, microscopy and thermodynamic modelling. The overall phase assemblage is very similar for the two-sulphate source in both PC-rich and CAC ‒ CS¯Hx−rich systems. However, the quantitative X-ray analysis, TGA and the 27Al, 29Si NMR show that the amounts of the crystalline hydrates and X-ray amorphous phases are influenced by the type of sulphate. In the long-term hydration, the anhydrite-bearing formulations exhibit the highest amount of ettringite, whereas the gypsum-containing samples develop a higher fraction of AFm phases and X-ray amorphous hydrates. This difference may relate to faster dissolution kinetics of gypsum compared to anhydrite in the studied ternary blends. For the CAC ‒ CS¯Hx −rich pastes, gehlenite from CAC (C2AS) shows hydraulic activity, which primarily results in the precipitation of strätlingite. Higher amounts of strätlingite are identified in the gypsum bearing samples, suggesting that the type of sulphate source impacts the hydration of silicate-bearing phases. Finally, the phase assemblages from thermodynamic modelling (using the GEMS software) are found to be in good agreement with those observed experimentally, although some differences occur as a result of kinetic effects.

Synthesis of polycarboxylic ether superplasticizers based on the high conversion of EPEG in a transition metal oxide heterogeneous catalytic system

The existed polycarboxylic ether superplasticizer (PCEs) that mainly synthesized in a homogeneous catalytic synthesis has the disadvantages of low conversion rate and poor performance stability. In this paper, high performance PCE-1–5 were synthesized via a heterogeneous H2O2-ASA (ascorbic acid) redox system catalyzed with MnO2, Fe2O3, Co2O3, Ni2O3 and ZnO respectively. GPC data show that the molecular weights of PCE-1–5 are higher than that of PCE0 (synthesized via a H2O2-ASA redox system without transition metal oxides). HPLC indicates that the conversion rates of macromonomer for PCE-1–5 are up to 99.81%, which is 27.47% higher than that of PCE0 (72.34%). As results, PCE-1–5 have a good dispersibility, high water reducing rate and are much benefits to the early hydration of cement paste and the strength of concrete. The mechanism research shows that the cement samples doped with PCE-1–5 have a larger zeta potential, stronger complexing ability with Ca2+, and thicker adsorption layer that is as high as 1.03 nm, which makes the spread diameter of cement paste as high as 309 mm. In addition, the obtained microstructure is much denser as is reflected by the nanometer pore size of hardened cement paste (12.26 nm), which makes the 28 d compressive strength of cement test block as high as 60.19 MPa. Generally, the heterogeneous transition metal oxide catalytic redox system provides a good choice for the synthesis of PCEs.

NMR study on the early-age hydration and ion binding of the cement paste prepared with NaCl solutions

Potentially one could use seawater to make concrete, i.e., in non-reinforced applications and in reinforced concrete with non-corrosive fibres. By doing so one could reduce the environmental footprint of concrete as fresh water is getting scarce, but also could reduce the CO2 impact by using fewer resources for making concrete. There have been various studies on the final properties of concrete made with NaCl solutions, mimicking seawater. In this study a specially designed NMR was used to look at the hydration of cement paste with a NaCl solution looking at the pore-structure formation and the dynamic consumption of Na and Cl, i.e., the binding of the ions and incorporation in the hydration products in time. The results confirm that the NMR setup is able to simultaneously and quantitatively measure the H, Na and Cl content during hydration, giving information on both the time dependent pore-structure change as well as the decrease of free Cl− and Na+. Influence factors such as the water-cement ratio and concentration of NaCl on the microstructure development and on the decrease of both Cl− and Na+ were determined, as well as how the Freundlich binding isotherm develops in time. Moreover, it is found that the Na+ and Cl− ion binding can be directly related to microstructure development during the stages of hydration, which can be identified using the T1 relaxation of hydrogen.

Low melting point alloy modified cement paste with enhanced flexural strength, lower hydration temperature, and improved electrical properties

Concrete usually possesses drawbacks of low flexural strength and prone to crack. In this research, Field's metal with a low melting point was introduced into the cement matrix to improve the crack resistance, thereby enhancing the flexural strength and toughness of cement matrix. A comparative study was conducted to elucidate the effects of the Field's metal on the mechanical behavior, microstructure, capillary permeability, hydration, and electrical properties of cement paste. By tuning the fraction of the Field's metal, the flexural strength and toughness showed the highest increase of 41% and 94%, respectively. The enhancement of flexural properties can be attributed to the filling effect of the low melting metal into the voids in the matrix, bridging effect of metal particles and the higher flexural strength as well as the higher fracture toughness of the metal. This approach opens a new research direction for the design and application of an innovative functional building materials.

Effects of fineness and chemical activators on the hydration and physical properties of high-volume fly-ash cement pastes

The effects of the Blaine fineness of fly ash (FA) and chemical activators on the hydration and physical properties of high-volume fly-ash cement pastes were investigated. The initial hydration was assessed via isothermal calorimetry, and the hydrate composition was determined through X-ray diffraction and thermogravimetric analyses. With increasing Blaine fineness, hydration accelerated at early ages. The pozzolanic reaction of FA accelerated with age. The compressive strength increased with increasing Blaine fineness. The chemical activators increased the initial strength but decreased the long-term strength and accelerated Ca(OH)2 consumption. With increasing fineness, the Ca(OH)2 content of the pastes decreased.

Effect of the nanosilica source on the rheology and early-age hydration of calcium sulfoaluminate cement pastes

This work investigates the role of different nanosilica (NS) sources on the rheology and early-age hydration of calcium sulfoaluminate (CSA) cement paste. Specifically, pastes were produced with three NS (two powder and one colloidal suspension) in 1 wt% replacement of cement, with and without superplasticizer. Rotational rheometry, isothermal calorimetry, and in-situ X-ray diffraction (XRD) tests were conducted. The results showed that incorporation NS increased the yield while decreased the viscosity and delayed the shear thickening onset point of CSA pastes, regardless of the NS source. Calorimetry and in-situ XRD revealed that NS incorporation enhanced the CSA cement hydration kinetics while superplasticizer delayed it. Overall, colloidal NS led to the best overall early-age performance, leading to the lowest viscosity and the highest hydration degree over time among the CSA cement pastes investigated.

Cement substitution by sludge-biomass gasification residue: Synergy with silica fume

The increasing volumes of sewage sludge and other wastes of biological origin force the use of downdraft gasifiers for waste utilization; however, thermal processes produce residues that need to be treated in an environmentally acceptable way. The object of the study is to evaluate the possible application of residue, derived from industrial sewage sludge-biomass gasification plants, with silica fume as a partial binder substitute in cement-based materials. At total cement replacement levels of 10 and 12%, the dosages of gasification residue and silica fume varied in the range of 2–6% and 6–10% by cement mass, respectively. The impact of synergy between silica fume and gasification residue on the hydration and microstructure development of cement paste was analysed using SEM-EDS, XRD, TGA and N2 sorption methods. The synergy of wastes was found to change the morphology and assemblage of hydration products and to accelerate the consumption of portlandite. N2 physisorption revealed that the synergy of both substitutes had a significant impact on the distribution of pores in the mesoporosity range up to 30 nm, representing small microcapillary pores. The synergistic impact of the substitutes on selected properties of cement mortars was determined after 7, 28 and 56 days. Compared with the reference, all blended mortars showed 5–40% higher mechanical strength and 25–50% lower water ingress during the entire hydration period. The compressive strength increased with the reduction in the residue-to-silica fume (R/SF) ratio, while the flexural strength and sorptivity were less dependent on the ratio. The results revealed that the total cement substitution level may be increased up to 12%, within 4–6% covered by gasification residue, without losing the mechanical strength and consequently, to reduce the resources of Portland cement and silica fume.

Clinkering design of sulfobelite cements using clay overburden residue from bauxite mining

Sulfobelite (belite-rich calcium sulfoaluminate (BCSA)) cements are promising alternatives to ordinary Portland cement (OPC) due to their lower carbon dioxide generation and lower energy consumption in the clinkering process. Additionally, they are cost effective because of their less-restrictive raw meal composition compared with that of calcium sulfoaluminate (CSA), in which expensive bauxites are used as the alumina source. Diverse mining residues have been proved to be reliable alumina sources for BCSA production, including the clay residues that cover bauxites (Belterra Clay (BTC)) in the Brazilian Amazon region. The clinkering settings of BCSA using a different (less alumina) BTC were examined in this work. The mineralogy of the BCSA clinkers was optimised by a design of experiments and studied using X-ray powder diffraction, scanning electron microscopy and thermal analysis. The raw meal composition, boron doping and firing temperature were investigated to obtain the optimal clinker composition. Hydration of the cement pastes was investigated by calorimetry and the strength development of mortars was tested and compared with that of a commercial OPC. The BCSA mortars produced with gypsum additions developed compressive strength comparable to that that OPC within 28 days. The main hydrated phase was ettringite, with minor amounts of kuzelite, straetlingite and gibbsite.

Hybrid Cements with ZnO Additions: Hydration, Compressive Strength and Microstructure.

The effect of ZnO has already been studied for Portland cement, but the study of its impact on hybrid pastes is scarce. Thus, in this investigation, the influence of ZnO addition on hydration, compressive strength, microstructure, and structure of hybrid pastes is presented. The analyses were made by setting time tests, compressive strength tests, X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis with differential scanning calorimetry, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. The results indicate that the setting time of the cements was delayed up to 39 min with additions of 3 wt% ZnO. Alternatively, the higher values of compressive strength were observed when 0.5 wt% ZnO was added to the cements for all curing days. In addition, no important differences in the microstructure of samples with different additions of ZnO were observed after 28 days of curing. It is expected that the use of ZnO contributes to the delay of the setting time and the increase of the compressive strength without negatively modifying the microstructure of hybrid pastes.

Effect of alkoxysilane on early age hydration in portland cement pastes

Silanes added during mortar or concrete preparation, may modify the fresh state properties, hydration kinetics and mechanical strength of the final product. The effects of silanes on cement hydration have been widely studied in the literature, however, there is some controversy about its effect at short ages. The present study was undertaken to determine the effect of a TEOS-based alkoxysilane (UCA-T), produced by ultrasound-assisted pre-hydrolysis of an oligomeric precursor, on early age cement paste hydration. The nature of the processes modifying the various stages of cement hydration kinetics in the presence of the alkoxysilane was ascertained by analysing paste composition at several ages (defined on the grounds of calorimetric curve results) using XRD, TG-DTG, FTIR and Raman spectroscopy. The calorimetric curve of pastes containing UCA-T exhibited a new early age, pre-induction period exothermal peak, indicative of UCA-T hydrolysis, C3A and C3S dissolution and ettringite and C–S–H gel precipitation. Portlandite, however, did not precipitate but reacts with the Si(OH)4 sourced from UCA-T hydrolysis to generate further C–S–H gel. The induction period following on that new exothermal peak was considerably longer than the period observed in the reference cement, an effect that intensified at higher UCA-T content.

Early strength development and hydration of cement pastes at different temperatures or with superplasticiser characterised by electrical resistivity

The effects of the curing temperature and superplasticiser on the hydration rate and early strength development of cement pastes were investigated using their electrical resistivities. The cement pastes were prepared either at various temperatures or with different dosages of the superplasticiser. A general equation for electrical resistivity development was employed to monitor the paste rate of densification during the hardening period. The cement pastes obtained at a high temperature or optimal dosage of the superplasticiser had a faster densification than those of the pastes obtained at a low temperature or excessive dosage of the superplasticiser because of their higher rates of hydration. The electrical resistivity can be used to evaluate the factors that influence the hydration rate, microstructure, and early physical properties of the cement paste as well as for a better utilisation of admixtures and determination of the optimal dosage for hydration and paste structure formation.

Influence of Dolomite Rock Powder and Iron Tailings Powder on the Electrical Resistivity, Strength and Microstructure of Cement Pastes and Concrete

Dolomite rock powder (the waste stone residue in the production of machine-made sand and stone processing) and iron tailings powder formed by mineral processing industry are solid wastes, which occupy land resources, pollute the environment and release toxic substances without reasonable processing. The dolomite rock powder and iron tailings powder composing a large number of active substances could be advantageous to the cement-based materials. In this study, the electrical resistivity of cement paste and concrete was measured. Meanwhile, the influence of dolomite rock powder and iron tailings powder on the compressive strength of concrete was investigated. The electric flux of concrete was determined to estimate the chloride ion permeability. The scanning electron microscope (SEM) and X-ray diffraction were obtained to investigate the hydration of cement paste. Results showed the electrical resistivity of all specimens presented in this order: specimens with iron tailings < specimens with dolomite rock powder < blank specimens < specimens with ground granulated blast-furnace slag (GGBS) < specimens with fly ash. The correlation between electrical resistivity and curing age of cement paste or concrete has been deduced as a quadratic function. The addition of GGBS could improve the compressive strength of concrete. Meanwhile, when the other three types of mineral admixtures were added, 5% by mass ratio of the total binder materials was the optimum for the compressive strength. The curing ages, the fly ash, the GGBS and 5% dolomite rock powder or 5% iron tailings powder demonstrated a positive effect on the chloride ion impermeability. However, when higher dosages of dolomite rock powder or iron tailings powder were added, the effect was the opposite. Finally, the compactness of the microstructure and the Ca(OH)2 of cement paste could be improved by a small dosage of dolomites or iron tailings (less than 5%).

Interaction of Basalt with Calcium Ion and Their Role in Hydration, Rheology and Strength Development of Cement Paste

Interaction of Basalt with Calcium Ion and Their Role in Hydration, Rheology and Strength Development of Cement Paste

Heat of Hydration, Water Sorption and Microstructural Characteristics of Paste and Mortar Mixtures Produced with Powder Waste Glass

Heat of Hydration, Water Sorption and Microstructural Characteristics of Paste and Mortar Mixtures Produced with Powder Waste Glass