Initial Curing Temperature Research Articles

A study on strength characteristics of concrete under variable temperature curing conditions in ultra-high geothermal tunnels

The heat conduction of rocks and the ventilation conditions in high geothermal tunnels always result in dynamic changes of curing temperature and humidity of concrete materials, which may further significantly affect the concrete’s performances. According to the field tests on both environment and concrete’s temperature and humidity, a new model for concrete curing was proposed, which can really reflect the curing process of concrete in high geothermal tunnels. Based on the model, concrete specimen tests were carried out in different conditions where the initial curing temperature was set to 40 °C, 50 °C, 60 °C, 70°C and 80 °C, respectively, and the relative humidity (RH) was maintained at 25%, 55% and 95%. The results indicate that the actual high curing temperature of concrete usually decreases with time and tends to become stable 7 days after casting. Though the initial high temperature causes a rapid growth of the concrete strength at the early age, an insufficient strength over the long-term can also be leaded. While RH has a beneficial effect on the increase of initial strength and can effectively reduce the loss of long-term strength caused by the high temperature. All of the temperature, humidity and their coupling effect can influence the strength, and the influence levels follow the order: T&RH < T < RH. Finally, a new prediction model to predict the strength of concrete used in high geothermal tunnels was proposed and verified by the field tests.

Crossover Effect in Cement-Based Materials: A Review

Cement-based materials (CBMs) such as pastes, mortars and concretes are the most frequently used building materials in the present construction industry. Cement hydration, along with the resulting compressive strength in these materials, is dependent on curing temperature, methods and duration. A concrete subjected to an initial higher curing temperature undergoes accelerated hydration by resulting in non-uniform scattering of the hydration products and consequently creating a great porosity at later ages. This phenomenon is called crossover effect (COE). The COE may occur even at early ages between seven to 10 days for Portland cements with various mineral compositions. Compressive strength and other mechanical properties are important for the long life of concrete structures, so any reduction in these properties is of great concern to engineers. This study aims to review existing information on COE phenomenon in CBMs and provide recommendations for future research.

The Effect of Elevated Curing Temperatures on High Ye'elimite Calcium Sulfoaluminate Cement Mortars.

This study investigated the material properties and hydration characteristics of calcium sulfoaluminate cement (CSA) based mortars cured under 3 different initial curing temperatures. Two CSA cements with different M-values were selected. Obtained experimental results of mechanical properties, dimensional stability, and heat release were explained by hydration characteristics from X-ray diffraction, thermal gravimetric analysis, porosimetry, and thermodynamic modeling. Decomposition of ettringite decreased compressive strength but re-formation of ettringite after additional curing at 30 °C helped to recover the strength in CSA cement with a high amount of calcium sulfate. CSA cement with a low amount of calcium sulfate which was designed to predominantly have monosulfate as the main hydration product, showed increased 1-day strength after higher temperature curing but this occurred was at the expense of decreased 28-day strength.

Catalytic and ortho-directing effect of Zn2+, Mg2+, Ba2+, and Ca2+ metal hydroxides on the preparation of phenolic-formaldehyde resin

A one step method was employed to prepare PF (phenol-formaldehyde) resin adhesive from phenol and formaldehyde with Zn2+, Mg2+, Ba2+, and Ca2+ hydroxides as catalysts, and the physicochemical properties of the resulting polymeric material were analyzed. The viscosity of the liquid adhesive decreased at intermediate catalyst concentrations, while the polymerization rate decreased at the highest concentrations; no clear pattern could be observed between catalyst concentration and solid content. Differential scanning calorimetry (DSC) performed on the PF resin catalyzed by Ba2+ showed that the polymer had a curing exothermic peak at around 110 °C, while the initial curing temperatures of PF resins were affected by the type of metal hydroxide used, and were in the order Ca2+ < Mg2+ <control sample. FTIR spectroscopy was used to monitor the degree of ortho/para substitution during the synthesis of PF resin with Ba2+, Ca2+, and Mg2+ hydroxides. The highest ortho/para ratio was achieved with Ba2+ at a concentration of 0.45 wt.%, which also resulted in the most significant increase in the polymerization rate for the formation of PF resin.

Influence of processing temperature on the reaction product and strength gain in alkali-activated fly ash

The roles of temperature and curing periods on product formation and strength gain in alkali activation of low-calcium siliceous fly ash are evaluated. Higher temperature favors the kinetics of amorphous reaction product formation and inclusion of Si in the reaction product, which accompanies the densification of the microstructure. Following initial curing at 60 °C, the reaction product continues to form in the activated system on decreasing the temperature to 25 °C. Initial curing at 25 °C does not lead to formation of reaction product. Reaction product forms in the system for initial curing temperature of 40 °C and higher. The rate of the solid-based reaction, after hardening, is increased by temperature. The total amorphous reaction product content formed in the system depends on the water to solids ratio of the activated mix. The amorphous product composition, however, varies with temperature. With time there is an increase in the Si/Al ratio in the reaction product formed 40 °C and higher. For initial curing at 60 °C, there is no increase in the Si/Al ratio in the reaction product when the temperature is decreased to 25 °C. Reaction product with a higher Si/Al atomic ratio achieves a higher ultimate compressive strength. On increasing the Si/Al ratio in the reaction product from 2.5 to 3.3, the ultimate compressive strength achieved increased from 60 MPa to 74 MPa. Early curing, immediately after mixing, at a higher temperature is critical in achieving a higher Si/Al atomic ratio in the reaction product. Optimum initial curing time at high temperature is based on achieving a high Si/Al atomic ratio in the reaction product.

The properties and formation mechanisms of eco-friendly brick building materials fabricated from low-silicon iron ore tailings

The volume of tailings discharged worldwide from iron ore smelting processes (i.e., iron ore tailings) has increased dramatically in recent years, and the disposal of this waste poses a substantial ecological threat to soil, and surface and sub-surface water. This threat can be alleviated to a large extent by employing iron ore tailings to fabricate brick building materials, while also effectively meeting the large demand for construction materials. The present paper discusses the preparation of eco-friendly bricks using fine-grained low-silica iron ore tailings and a non-cement curing agent system as the primary raw materials, and a triethanolamine hardening accelerator and a stearic acid emulsion waterproofing agent are also employed as minor components. We first evaluate the effect of the waterproofing agent content on the compressive strength and water resistance of the resulting products. In addition, the phase composition, chemical structure, and microstructure of the products are analyzed via X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy to explore the formation mechanism of the brick products. The analyses demonstrate that hydrated calcium silicate gel and ettringite formed in the curing process are strong binders. The water resistance and compressive strength of the product was optimal when the waterproofing agent and curing agent were 0.3 wt% with an initial curing temperature of 60 °C. The compressive strength and saturated compressive strength of the optimally formed brick products after curing for 28 d were 27.2 MPa and 24.3 MPa, respectively. The other physical properties and durability conform to the Chinese JC/T422-2007 standard for unfired bricks made of tailings.

Cure behavior and compatibilization of epoxide hyperbranched polyurethane on silica/benzoxazine blend

ABSTRACTThe compatibility of polymer blends is pivotal for their performance enhancement due to the large surface tension and weak interaction among individual components. In this work, epoxide hyperbranched polyurethanes (EHPU) was used to realize the compatibility between silica (SiO2), generated in situ from tetraethoxysilane, and benzoxazine (BZ). The compatibilization effect and mechanism of EHPU on SiO2/BZ were investigated via the differences of cure characteristics and kinetics between BZ, SiO2/BZ, and EHPU‐SiO2/BZ blend. EHPU‐SiO2/BZ blend exhibited lower initial curing temperature and wider curing window than those of SiO2/BZ and neat BZ. In addition, the co‐curing reaction was existed between EHPU and BZ, which contributed to the reactive compatibilization. More importantly, the growth of SiO2 particles was hindered by the nonreactive compatibilization due to the hydrogen interaction between EHPU and the silanols. Thus, the compatibilization of EHPU on SiO2/BZ blend is a result of the synergetic effect of the hydrogen interaction and the co‐curing. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46879.

Optimization of the fabrication conditions and effects of multi-walled carbon nanotubes on the tensile properties of various glass fibers/unsaturated polyester resin composites

Abstract Conventional glass fiber/unsaturated polyester resin (UPR) composites were fabricated by four layers to verify the optimum fabrication conditions such as the initial curing temperature, fiber changes and vacuum. Before that another optimum condition for stir mixing multi-walled carbon nanotubes (MWCNTs) into UPR was further investigated based on the initial curing temperature. Exothermic temperature measurements, density measurements, thermo-gravimetric analysis and tensile testing were utilized to identify the effects of these factors on the UPR and composite laminates. Consequently, an initial curing temperature range of 25°C–35°C was recommended to start the polymerization. In addition, the fiber combination and applying vacuum were also represented as the other optimum conditions in composite fabrication. The optimum initial curing temperature was applied for modifying UPR with 0.1 wt.% MWCNTs. Then, all of optimum conditions were used to fabricate glass fibers/modified UPR composites. The higher tensile strength and modulus of fiber combination with the adding of MWCNTs were obtained at 25.29 MPa and 1.39 GPa, respectively. The results indicate that simple MWCNT mixing can be used in industry due to the reduction of the fabrication time and better tensile properties.

Evaluation of a Procedure for Determining the Converted Strength of Calcium Aluminate Cement Concrete

Abstract There is renewed interest in North America for the use of calcium aluminate cement (CAC) in infrastructure repair because of its ability to gain strength rapidly even at low temperatures, the ability to customize its fresh workability, and its durability in adverse environments. Conversion of the hydration products of concrete where CAC is the only binder is a well-known phenomenon that is typically accompanied by strength loss, the rate and extent of which is dependent on the temperature history of the concrete, the w/cm, the cement content, and the concrete mixture design. An accelerated method of determining the converted strength of CAC concrete that is convenient for use in the field is presented. This test allows the samples to be cast in the field and left at ambient field temperatures for the first 24 hours; then the specimens are moved to the laboratory and placed in a water bath at 50°C to promote conversion. Robustness testing results are presented for the test method. The effects of the water to cement ratio, the initial (first 24 hours) curing temperature, the length of time before placing the specimens in the 50°C water bath, and the aggregate type are examined. The impact of the low replacement rates (up to 10 %) of finely ground limestone (FGLS) for CAC in concrete mixtures on converted concrete strengths was also studied. Results from testing showed that the majority of mixtures converted 48 hours after submersion in the 50°C water bath; however, differences in the initial curing temperature or aggregate type can delay conversion up to 11 days when specimens are cured following this procedure. Results also indicate that FGLS at replacement rates of 5 % and below in CAC systems may help increase the converted strength observed without reducing initial early strength gain.

Supersulfated binders based on volcanic raw material: Optimization, microstructure and reaction products

The characterization and optimization of supersulfated binders based on a volcanic material was carried out by Taguchi orthogonal array experimental design, investigating the effects of four factors: type of calcium sulfate, % calcium sulfate, %Portland cement and initial curing temperature. The binders increased strength over time. The statistically defined optimum formulation was: 70%volcanic material-10%Anhydrite-20%Portland cement and curing 22 h at 60 °C then at 20 °C; the predicted and confirmed 28-day strength was 20.2–20.6 MPa; optimum conditions are discussed for other ages. The microstructures and chemical composition of hydration products (EDS) and XRD indicated the formation of C-S-H and ettringite as the main hydration products.

Combined Effect of Initial Curing Temperature and Crack Width on Chloride Penetration in Reinforced Concrete Beams

Reinforced concrete (RC) structures are gradually being degraded all over the world, largely due to corrosion of the embedded steel bars caused by an attack of chloride penetration. Initial curing would be regarded as one factor influencing chloride diffusion in concrete in combination with cover cracking that is also of great attention for reinforced structures. In this study, a non-steady state diffusion test of chloride ion involving RC beam specimens with a water-to-cement ratio of 0.5, initial curing temperatures of 5°C or 20°C and three types of crack widths ranging from 0 to 0.2mm was performed. Chloride content at 5°C or was determined. The results show that the higher chloride content was obtained in condition of crack width large than 0.1mm with low initial curing temperature and there are no obvious differences in chloride content when the crack width was not larger than 0.1mm.

Combined Effect of Initial Curing Temperature and Crack Width on Chloride Penetration in Reinforced Concrete Beams

Reinforced concrete (RC) structures are gradually being degraded all over the world, largely due to corrosion of the embedded steel bars caused by an attack of chloride penetration. Initial curing would be regarded as one factor influencing chloride diffusion in concrete in combination with cover cracking that is also of great attention for reinforced structures. In this study, a non-steady state diffusion test of chloride ion involving RC beam specimens with a water-to-cement ratio of 0.5, initial curing temperatures of 5°C or 20°C and three types of crack widths ranging from 0 to 0.2mm was performed. Chloride content at 5°C or was determined. The results show that the higher chloride content was obtained in condition of crack width large than 0.1mm with low initial curing temperature and there are no obvious differences in chloride content when the crack width was not larger than 0.1mm.

Effect of amine-functionalized dispersant on cure and electrical properties of carbon nanotube/epoxy nanocomposites

Abstract The efficient dispersion of carbon nanotubes, CNTs in epoxy resin is a challenging task to reach the final properties of the competent nanocomposite. In this study, the CNT/epoxy nanocomposite was fabricated by solution mixing process using an amine-functionalized dispersant. The dispersion of CNTs in organic solvent was further improved using an ultrasonic horn. According to the data obtained from UV–vis spectroscopy, a minimum ultrasonication time of 10 min is necessary to obtain proper dispersion of CNTs in organic solvent. The cure behavior of epoxy resin containing different CNT loadings was investigated by dynamic DSC thermograms. The higher CNT loading lowers the initial curing temperature and the peak temperature mainly due to high thermal conductivity of CNT. The SEM micrographs of the fractured surfaces showed the effect of dispersant on proper dispersion of CNTs in epoxy. The electrical percolation threshold was measured at about 0.5 wt% of CNT. However, the conductivity results revealed the adverse effect of dispersant on the nanocomposite electrical properties, mainly due to dispersant adsorption and hence screening the CNTs surface charges.

Curing Process and Heat-Resistance of Polyethersulfone Toughened Epoxy Resins

In order to improve the toughness of epoxy resin, hydroxyl-terminated polyethersulfone (PES) with various amounts (0 wt.%, 5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%) were added to bisphenol A epoxy resin (DER331)/ curing agent DETDA (E100) systems, and the influence of PES contents on curing process and heat-resistance was studied. Non-isothermal DSC was used to determine the curing process of uncured DER331/E100/PES systems at heating rate of 2°C/min, 5°C/min, 7°C/min, 10°C/min and 15°C/min separately, and the apparent activation energy was calculated based on Kissinger method. The morphology of the etched cured DER331/E100/PES systems with different PES contents was observed by SEM. The heat-resistance of these systems was investigated by DSC and TGA. The results showed that with the increasing of PES content the curing exothermic peak, the heat of curing reaction, the initial and final curing temperature all decreased at the first and then increased, indicating that when the PES content was low (5 wt.%, 10 wt.%), PES can facilitate the curing process, while, when PES content up to 15 wt.%, PES can prevent or weaken the curing reaction. SEM results indicated that the phase structure changed drastically depending on the PES content. The systems with 5 wt.% and 10 wt.% PES were epoxy-rich phase, with 15 wt.% PES was co-continuity phase, and with 20 wt.% PES showed complete phase inversion (PES rich phase). The glass transition temperature and thermo gravimetric analysis demonstrated that the addition of PES can increase the heat resistance of cured DER331/E100/PES systems.

Room-Temperature Curing and Grain Growth at High Humidity in Conductive Adhesives with Ultra-Low Silver Content

Isotropic conductive adhesives (ICAs) are alternatives to metallic solders as interconnects in solar modules and electronic devices, but normally require silver contents >25 vol.% and elevated curing temperatures to achieve reasonable conductivity. In this work, ICAs are prepared with a silver content of 1.0 vol.% by using polymer spheres coated with nanograined silver thin films as filler particles. In contrast to conventional ICAs, there are no organic lubricants on the silver surfaces to obstruct the formation of metallic contacts, and conductivity is achieved even when the adhesive is cured at room temperature. When exposed to long-term storage at 85°C and 85% relative humidity, the silver films undergo significant grain growth, evidenced by field-emission scanning electron microscopy observation of ion-milled cross-sections and x-ray diffraction. This has a positive effect on the electrical conductivity of the ICA through the widening of metallic contacts and decreased scattering of electrons at grain boundaries, and is explained by an electrochemical Ostwald ripening process. The effects of decoupling heat and humidity is investigated by storage at either 85°C or immersion in water. It is shown that the level of grain growth during the various post-curing treatments is dependent on the initial curing temperature.

Multiple-SO3H functioned ionic liquid as efficient curing agent for phthalonitrile-terminated poly(phthalazinone ether nitrile)

Combined with the advantages of low melting point, high thermal stability and strong acidity, a multiple-SO3H functioned ionic liquid (MIL) was developed successfully as a curing agent to promote the curing reaction of phthalonitrile-terminated poly(phthalazinone ether nitrile) (PPEN-Ph). The curing kinetics over differential scanning calorimetry (DSC) showed that both the initial curing temperature Tp0′ and apparent activation energy Ea’ (based on Kissinger equation) were reduced significantly over MIL (207.9°C and 101.5kJ/mol) compared to the common curing agent ZnCl2 (268.5°C and 201.5kJ/mol). Moreover, under identical curing conditions, the resulting thermosetting resin over former (Td5%=526.1°C) showed better thermal stability than that over latter (Td5%=512.1°C). These results indicated that MIL should be a good candidate as a curing agent for phthalonitrile resins.

Characterisation of pre-industrial hybrid cement and effect of pre-curing temperature

This study aimed to determine the physical-mechanical, mineralogical and microstructural properties of a pre-industrially manufactured hybrid cement (HYC) containing 5% alkaline activator and less than 30% clinker. The effect of the initial curing temperature (25 ± 1 or 85 °C for 20 h) on hydration kinetics and the development of compressive strength were also explored. The hydration products formed were characterised using XRD, SEM/EDX and 27Al and 29Si MAS-NMR. The findings showed that pre-industrial hybrid cement sets when hydrated with water and hardens to a 28-day mechanical strength of 35 MPa. The main reaction product formed was a mix of cementitious gels: C-(A)-S-H and C-A-S-H. Curing at 85 °C for 20 h, shows a behaviour similar to OPC, inhibited ettringite formation and generated more polymerised gels, enhancing 3-day but not 28- or 90-day mechanical strength.

Thermal properties of diacetylene–containing polymer prepared from propargyl-terminated monomer with pyridine

A novel diacetylene–containing main chain-type polymer, poly[2,6-bis(4-propargyl-oxy-benzoyl)pyridine] (P1), was synthesized via oxidative coupling reaction with a new propargyl-terminated monomer, 2,6-bis(4-propargyl-oxy-benzoyl)pyridine (M1), which was incorporated a substituted pyridine into the molecular chain of model monomer, 4,4′-bis(propargyl-oxy)benzophenone (M0). The structures of the new monomer and the resulting polymer were verified by Fourier transform infrared and proton nuclear magnetic resonance spectroscopies. Differential scanning calorimetry was used to investigate the curing behaviors of M1 and P1. The onset curing temperature of M1 was 199.0°C, and the exothermic enthalpy of M1 was 973.4 J g−1, while the initial curing temperature for polymer P1 reduced by 48°C and the exothermic enthalpy was almost half. Thermogravimetric analysis (TGA) showed the excellent thermal stabilities of M1 and its polymer in argon atmosphere (the decomposition temperatures at 5% weight loss were 446°C for M1 and 370°C for P1 and their char yields at 800°C exceeded 71%). The above TGA data were superior to that of the model monomer (M0) and its polymer (P0) under the same conditions.

알칼리 활성화 결합재 활용 모르타르의 황산염 침식 저항성

본 연구는 플라이애시 및 고로슬래그를 활용하여 알칼리 활성화 결합재로 제조된 모르타르 및 페이스트 샘플의 황산염 저항성을 평가하고 황산염 침투에 대한 고저항성 결합재를 제시하는 것이다. 이를 위하여 플라이애시 및 고로슬래그미분말 등의 광물질 혼화재를 결합재로 활용하여 고로슬래그미분말 치환율을 0, 30, 50 및 100%로 제작하였다. 규산나트륨 모듈 <TEX>$Ms[SiO_2/Na_2O]$</TEX>은 1.0, 1.5 및 2.0으로 조정하였으며, 초기 24시간 양생조건을 <TEX>$23^{\circ}C$</TEX> 및 <TEX>$70^{\circ}C$</TEX>로 하고, 10% 황산나트륨 및 10% 황산마그네슘 용액에 각각 침지시키고, 황산염 저항성을 평가하기 위하여 압축강도, 질량변화율, 길이변화율 및 X선 회절분석을 측정하였다. 그 결과 고로슬래그미분말 치환량 및 Ms비가 증가할수록 재령 28일 압축강도 발현이 우수한 결과가 나타났다. 10% 황산나트륨에 침지한 경우에는 모든 시험조건에서 장기적인 강도발현과 질량 및 길이변화율이 작아 황산나트륨 침투에 대한 저항성이 우수한 것으로 나타났으나, 10% 황산마그네슘에 침지한 경우에는 장기적인 강도저하와 질량 및 길이변화가 크게 나타났으며, 그 경향은 고로슬래그미분말 치환량 및 Ms비가 증가할수록 현저하였다. 이것은 황산마그네슘의 경우 규산마그네슘수화물의 생성으로 인한 열화가 지배적으로 작용한 결과로 판단된다. 또한, X선 회절분석 결과 <TEX>$MgSO_4$</TEX> 용액 침지에서의 알칼리 활성화 결합재의 팽창은 Gypsum(<TEX>$CaSO_4{\cdot}2H_2O$</TEX>) 생성 반응에 의한 것으로 확인되었으며, 침지 6개월까지는 Gypsum의 생성이 지속적으로 증가되는 것을 알 수 있다. This paper presents an investigation into the durability alkali-activated materials(AAM) mortar and paste samples manufactured using fly-ash(FA) and ground granulated blast furnace slag(GGBFS) exposed to a sulfate environment with different GGBFS replace ratios(0, 30, 50 and 100%), sodium silicate modules(<TEX>$Ms[SiO_2/Na_2O]$</TEX> 1.0, 1.5 and 2.0) and initial curing temperatures(<TEX>$23^{\circ}C$</TEX> and <TEX>$70^{\circ}C$</TEX>). The tests involved immersions for a period of 6 months into 10% solutions of sodium sulfate and magnesium sulfate. The evolution of compressive strength, weight, length expansion and microstructural observation such as x-ray diffraction were studied. As a results, as higher GGBFS replace ratio or Ms shown higher compressive strengths on 28 days. In case of immersed in 10% sodium sulfate solution, the samples shows increase in long-term strength. However, for samples immersed in magnesium sulfate solutions, the general observation was that the compressive strength decreased after immersion. The most drastic reduction of compressive strength and expansion of weight and length occurred when GGBFS or Ms ratios were higher. Also, the XRD analysis of samples immersed in magnesium sulfate indicated that expansion of AAM caused by gypsum(<TEX>$CaSO_4{\cdot}2H_2O$</TEX>); the gypsum increased up to 6 months continuously.

Experimental verification of GFRP slip-form for in-situ application

This study was conducted to assess the field applicability of the Glass Fiber Reinforced Polymer (GFRP) slip-form proposed in previous studies by testing its performance-related properties in various experiments. We compared the initial curing temperature of concrete, which exerts the greatest impact on reducing the construction duration in the slip-form system, with that when using a steel slip-form. We measured the friction force to compare the cross section formability of the two methods. We then conducted a mock-up test on the basis of the experimental results and assessed its usability and field applicability. The GFRP form maintained a higher curing temperature than the steel form, and the difference in the curing temperature was larger with an increase in the air temperature and smaller with a decrease in the air temperature. The slip-up rate in the slip-forming was confirmed to be affected the most by the initial curing temperature of the concrete, whereas the cross-section formability was affected by the friction force and adhesion of the concrete. The results of the experiments verified that the GFRP slip-form efficiently overcomes the drawback of the steel slip-form system and maximizes the advantages of slip-forming.