Thermodynamic Predictions Research Articles

Precipitation behavior of the G-phase strengthened 7Ni maraging steels

In this study, G-phase strengthened 7Ni maraging alloys were studied using a combination of thermodynamic prediction based on the TCFE-7 database and advanced experimental techniques, including micro-hardness testing, electron backscatter diffraction (EBSD), in-situ X-ray diffraction (XRD), transmission electron microscopy (TEM), and atom probe tomography (APT). The martensite reversion and phase stability of overcooled austenite were precisely determined for a series of Fe–7Ni–2Si-based alloys, validating the effectiveness of thermodynamic predictions in martensite transformation.Based on these theoretical prediction, aging hardness measurements and microstructural observations further revealed that Ni16X6Si7-G (X = Ti, Nb, Ta) precipitates are effective strengthening phases in 7Ni maraging steel, with the exception of Ni16X6Si7 (X = Mn, Zr) due to their significantly different thermal stabilities. Experimental results showed that the Ni16X6Si7-G (X = Ti, Nb, Ta) precipitates remained stable and densely distributed within the martensitic matrix after aging at 500 °C, resulting in high aging hardness values ranging from 350 to 550 HV. Among the studied alloys, the 1Ti alloy strengthened by the Ni16Ti6Si7-G phase exhibited the finest particle radius (estimated at 1.4 nm) and the highest number density (estimated at 1.9 × 1024/m3). Additionally, it is worth noting that the Ni16Zr6Si7-G phase was believed to form through eutectic reaction with α-Fe during solidification and the Ni16Mn6Si7-G phase was only stable at temperatures below 460 °C and was not detected experimentally.These findings enhance our comprehension of G-phase precipitation and strengthening in 7Ni maraging steel and underscore the potential for utilizing thermodynamic calculations and advanced experimental techniques to guide the design and optimization of high-strength alloys.

(Invited) Using the Model Bacterium Geobacter Sulfurreducens to Understand Microbial Electrochemical Conversion of Nitrogen Gas into Ammonium

There is a growing need to decrease reliance on the Haber-Bosch process. To convert nitrogen gas (N2) into ammonia (NH3) for fertilizers, this temperature- and pressure-intensive process consumes roughly 2–3% of global energy and releases over 740 million tons of CO2 each year. Many microorganisms naturally convert or “fix” N2 into ammonium (NH4 +) at room temperature and pressure using an enzyme called the nitrogenase. Attempts to increase NH4 + yields from microorganisms through genetic engineering are seeing signs of success, but we lack methods to increase NH4 + generation rates and overcome one of the largest challenges: O2 gas. Increasing O2 gas concentrations typically increases metabolic rates of aerobic microorganisms; however, in the case of microorganisms that fix N2 (called diazotrophs), O2 can shut down the nitrogenase and stop cell growth. To overcome these challenges, we are using electricity to drive N2 fixation in exoelectrogenic bacteria. These bacteria have the unique ability to naturally transfer electrons to anode electrodes in microbial electrochemical technologies (METs). Increasing the voltage applied to METs increases the metabolic rates of exoelectrogens. The anode chamber is kept anaerobic, which eliminates O2-driven inhibition of the nitrogenase. We previously showed that the N2 fixation rates of a mixed microbial community in a MET increased more than three times when the applied whole-cell potential increased from 0.7 V to 1.0 V. By adding a chemical inhibitor that prevented the incorporation of NH4 + into larger biomolecules, NH4 + was excreted by the cells and into the medium. Based on the acetylene reduction assay (a proxy for N2 fixation), we estimated that we recovered about 10% of the theoretical NH4 +generated by the cells. If close to 100% of the theoretical NH4 + could be excreted from the cells and recovered, we predicted an energy demand of around 3 MJ/mol-NH4 +, which is close to the range of value reported for the Haber-Bosch process. To develop METs that generate NH4 + at competitive production rates and energy demands, we are now focusing on the model exoelectrogenic diazotroph Geobacter sulfurreducens. Using a single organism with a fully sequenced genome allows us to understand how the N2 fixation process responds to electrochemical variables. Towards these efforts, we conducted a transcriptomic analysis of G. sulfurreducens at two fixed anode potentials: +0.15 V and −0.15 V. This approach revealed which genes are turned on and off in response to these potentials. The presence of NH4 + decreased the expression of genes associated with N2 fixation which was expected due to the known sensitivity of nitrogenases to NH4 +. On the other hand, the two anode potentials had a dramatic and unexpected impact on the expression levels of N2 fixation genes. At −0.15 V, nitrogenase genes were significantly up-regulated, as were genes associated with NH4 + uptake and transport, such as glutamine and glutamate synthetases. Considering that −0.15 V provides less energy to the cells relative to +0.15 V (based on thermodynamic predictions), the results suggest that the cells responded to this highly energy-constrained environment by increasing expression of N2 fixation pathways. Based on our new knowledge of how G. sulfurredcuens regulates N2 fixation and NH4 + production, we have developed new strains of G. sulfurreducens that can excrete NH4 + during N2 fixation. The new engineering toolkit that we have created in this bacterium can now be leveraged to maximize NH4 + production in METs.

Thermal-Hydraulic Modeling of Oil-Immersed Motor Pump

The integrated design of the motor and axial piston pump eliminates the coupling structure, resulting in a compact and lightweight motor-pump structure. The challenge of motors overheating has always been a major concern. To address this issue, the hydraulic oil throughout the motor pump is utilized for cooling the high-speed motor, effectively improving the power density and heat dissipation capability of the hydraulic power unit. This integrated design approach has successfully resolved the significant issue of overheating motors, leading to enhanced performance of the hydraulic power unit. To address this concern, the entire motor pump’s oil is utilized to cool the high-speed motor. Consequently, the thermodynamic prediction of high-speed motor pumps has become increasingly important. In this study, the impact of motor heat generation on hydrodynamics is analyzed, and the heat transfer of the motor pump is investigated using the control volume method. Furthermore, thermodynamic models of hysteresis loss, eddy current loss, alternating current loss, churning loss, and throttling loss are established for the oil-immersed motor pump. The change in oil viscosity is also considered. The instantaneous temperature change rule of the oil within the oil-immersed motor pump is derived. Additionally, the influence of various working conditions such as pressure and speed on the temperature of the motor pump’s key node is examined. The experimental results indicate the accuracy of the thermodynamic calculation, and the significant effect of motor loss on the leakage temperature.

A Machinability Assessment of the Novel Application of Field-Assisted Sintering Technology to Diffusion Bond (FAST-DB) and Functionally Grade Dissimilar Nickel-Based Superalloys

This work presents an alternative processing route to the conventional powder HIP—forge route for Nickel-based superalloys. Demonstrating how the field-assisted sintering technology (FAST) process can be exploited to successfully diffusion bond or functionally grade two or more Nickel-based superalloys from powder feedstock. The robustness of the process has been further demonstrated by the successful bonding of one alloy in powder form to another in the solid form. Chemical and microstructural analysis of the diffusion bond between the alloys is characterised, in both cases, with a short diffusion zone—in agreement with thermodynamic model predictions. A gradual transition in microhardness across the bond region was measured in all samples. A machinability assessment was also carried out through a simple face turning operation. Analysis of the cutting forces and machined surface shows signs of a directionality when machining across the bond region between two alloys, indicating that care must be taken when machining multi-alloy FAST-DB components.

In-situ redox conditions in hydrothermal diamond-anvil cell experiments using various metal gaskets

Hydrothermal diamond-anvil cells (HDACs) have been widely used in various research fields since they were introduced in 1993. Unlike temperature (T) and pressure (P), the redox states of the samples in HDAC experiments were poorly controlled or measured; they were usually estimated by observing the reduction/disappearance of redox-sensitive mineral(s) (e.g., Mo metal) and corresponding formation and growth of such mineral(s) (e.g., MoO2) without in-situ redox measurement. In this study, we demonstrated the generation of H2 during experiments using various metal gaskets, including inert (Re) and chemically more reactive metals (Ni, Co, Mo, and W), in the pure H2O system under various P-T conditions. A technique for quantitative Raman spectroscopic analysis of H2 in HDAC is designed. Based on this technique, the oxygen fugacity (fO2) values of Mo-MoO2 buffer at the experimental P-T conditions were then calculated from these measured H2 pressures. We demonstrated that steady-state redox control can be maintained at a fixed T between 400 and 500 °C in HDAC experiments using a Mo gasket for durations of up to two hours and for those using a Re gasket with an additional Mo rod in the sample between 400 and 800 °C for durations of up to 90 min, regardless of the considerable changes in sample pressure. This demonstration was established by the balance between the rates of H2 generation and H2 loss from the sample chamber. The loss of H2 enhanced the consumption of H2O and therefore reduced the sample pressures during the experiments.We believe that hydrogen contents in every HDAC experiment, which attempt to control the redox state, need in situ measurement, then estimate the real redox state and compare it to thermodynamic equilibrium prediction. To enhance in situ redox control and measurements, modifications of HDAC are required for improving the quantitative Raman spectroscopic analysis technique in both sensitivity and accuracy.

Thermodynamic prediction of ferroelectric mechanical to electrical energy harvesting capabilities utilizing phase dependent electromechanical coupling factors

Prediction of the mechanical to electrical energy harvesting capabilities in [011]C cut PIN-PMN-PT is presented as a function of thermodynamic analysis of hysteresis in the stress induced ferroelectric phase transformation, and phase dependency of the electromechanical coupling factors. Energy conversion rates vary from 0.528 for low transformation hysteresis to 0.432 for high transformation hysteresis. While these predicted values are lower than the experimentally determined value from literature (“approximately 60%”), differences are explained through limitations of the mechanical loading mechanism. This brings the predicted values into agreement with literature. Ferroelectric rhombohedral phase behavior is found to dominate energy harvesting capabilities.

Thermodynamic Insights into the Oxidation Mechanisms of CrMnFeCoNi High-Entropy Alloy Using In Situ X-ray Diffraction.

This paper utilizes in situ X-ray diffraction (XRD) to investigate the high-temperature oxidation behaviour of CrMnFeCoNi high-entropy alloy (HEA). We found that (1) Mn is the major oxide-forming element in both vacuum and air environments, leading to the formation of non-protective oxides that deplete the bulk alloy of Mn; (2) no oxides like Cr2O3, Fe2O3, or Fe3O4 were observed during the high-temperature oxidation behaviour of CrMnFeCoNi, which contradicts some previous studies on the isothermal oxidation of CrMnFeCoNi HEA. We also analysed and compared the experimental results with thermodynamic calculations by using ThermoCalc version 2022b software following the CALPHAD method. ThermoCalc predicted spinel oxide in a vacuum environment, along with halite oxides observed in experimental results; also, in an atmospheric environment, it predicted only spinel, indicating the need for further investigation into factors to validate the thermodynamic predictions. Our study shows that the in situ HTXRD technique is a powerful tool to accurately identify time-temperature-dependent phase formation/transformation for studying oxidation behaviours and understanding oxidation mechanisms in HEAs.

Thermodynamic prediction of precipitations behaviour in HAZ of a gas metal arc welded S690QL with varying Ti and Nb content

For a significant increase in the strength of high-strength fine-grained structural steels with a nominal yield strength ≥690 MPa, the addition of microalloying elements such as Nb and Ti is required. The standard specifications for the chemical composition of these steels (e.g., in EN 10025-6) often only give the manufacturer limit contents to achieve the defined properties. The effect of the alloying elements in the heat affected zone (HAZ) is sometimes completely contrary. This makes it difficult to adequately predict the batch dependency regarding weldability and the load-bearing behaviour of the welded joint. Three different micro-alloyed steels of the grade S690QL were produced on a laboratory scale, focusing on different Nb and Ti contents. To investigate the tempering effect, these were gas metal arc welded in three layers. In addition to metallographic investigations of individual HAZ areas, thermodynamic phase calculations were carried out using Thermo-Calc, following variations in the chemical composition. This provides an understanding of phase transformation, precipitation growth, and dissolution during welding as a function of temperature and cooling conditions. The results show a divergent metallurgical behaviour in the HAZ of the three different micro-alloyed steels. Thereby, the Ti micro-alloyed grade showed a strong softening of the HAZ in contrast to the Nb micro-alloyed grade. This can be attributed to a contrary precipitation behaviour during welding.

Thermodynamic Justification of Obtaining Ferroalloy from a Mixture of Chrysotile-Asbestos Waste and Carbonaceous Rock

The results of research on the thermodynamic prediction of obtaining ferroalloys with the reduction and distillation of magnesium from a mixture of chrysotile-asbestos waste produced by Kostanay Minerals JSC and carbonaceous rock are presented. The research was carried out by thermodynamic modelling methods using the HSC-6.0 Chemistry software package of the Finnish metallurgical company Outokumpu based on the principle of the Gibbs minimum energy and a rotatable second-order plan (the Box-Hunter plan). Based on the conducted studies on the distribution of Si, Al, Mg between the condensed and gas phases, it was found that the formation of ferrosilicoaluminum of the FS45A10 brand with a content of 7.5-10.1% Al and 42.5-44% Si occurs in the presence of 25-25.9% iron at 1959-2000°C; ferrosilicon of the FS45 brand with a content of 41.0-44.4% Si and < 2.5% Al is formed in the temperature range of 1800-1877°C in the presence of 25.4-35% iron. In addition to branded ferrosilicon and ferrosilicoaluminum, it is possible to obtain Fe-Si-Al ligatures containing 2.5-10.1% Al and 41.0-44.7% Si from a mixture of chrysotile-asbestos production wastes and carbonaceous rock. In the presence of 25

Thermodynamic prediction of biogas production and combustion: The spontaneity and energy conversion efficiency from photosynthesis to combustion

Due to inappropriate energy consumption and subpar waste management, the growing population leads to an increase in energy demand and environmental damage. The creation of biogas enhances the management of organic waste and provides renewable energy. The production of biogas, which is a mixture of several gases, primarily methane and carbon dioxide, occurs during the four phases of aerobic digestion (AD). The four stages are hydrolysis, acidogenesis, acetogenesis, and methanogensis. Biogas comprises 50–70% Methane, 25–40% carbon dioxide and remaining traces of gases. In this paper, these biogas production processes are not thoroughly studied utilizing thermodynamic principles. Using thermodynamic criteria, this article analyses and assesses why the aerobic digesting process is spontaneous. Entropy, enthalpy, and Gibbs free energy of reactions during the various stages of the formation of biogas are examined. At Standard Temperature and Pressure Conditions (STP), the combined four phases of biogas formation are exothermic and spontaneous based on the analysis of Entropy, enthalpy, and Gibbs free energy of reactions from balanced equations at standard conditions of temperature and pressure. A unit mole of glucose is used to compare the energy released during photosynthesis with the energy released during biogas burning. Three moles of methane can be produced from decomposition of glucose. The combustion of three moles of methane yields 2406.876 kJ energy and if the same amount of moles of biogas (60% methane and 40% carbon dioxide) burns the energy yield is reduced to 1604.584 kJ. From the energy captured by photosynthesis i.e., 2808.1 kJ/mol 57.141% of the energy is recovered in the biogas combustion process. This efficiency reaches to 85.711% if pure methane is burned. As the separation of methane from biogas, mixture in actual plants is not economically feasible the combustion of methane gas the ideal limit of the efficiency.

Effects of elevated temperature exposure on the residual stress state and microstructure of PVD Cr coatings on SiC investigated via in situ X-ray diffraction and transmission electron microscopy

Pure Cr coatings are being considered as corrosion-resistant layers for SiC-SiCf composite fuel cladding in light water reactors. The residual stress state and interdiffusion behavior of Cr coatings deposited on SiC substrates using bipolar high-power impulse magnetron sputtering (B-HiPIMS) and direct current impulse magnetron sputtering (DCMS) were studied using in situ X-ray diffraction experiments at 300 °C, 700 °C, and 1200 °C. After elevated temperature exposure, evolution of the coating microstructure and coating/substrate interdiffusion was characterized by transmission electron microscopy. The compressive residual stress of the dense B-HiPIMS coating at room temperature was gradually relieved at 300 °C, while the stress transitioned to a tensile state at 700 °C. Grain growth of the Cr coating and a Cr–SiC interdiffusion layer consisting of newly formed Cr–C and Cr–Si phases were observed in the B-HiPIMS coating after exposure at 700 °C. The phases in the interdiffusion layer were identified to be Cr7C3, Cr3Si, and Cr5Si3Cx experimentally and by thermodynamic predictions. For the DCMS coating, however, changes in residual stress and microstructure at the elevated temperatures were limited by the inherent porosity and inhomogeneous structure of the as-deposited coating. At 1200 °C, both coating types interacted rapidly with the underlying SiC substrate in the early stages of the experiment. The results from this study suggest that the characteristics of the Cr coatings do not significantly change at the reactor operating temperature of 300 °C, while changes in microstructure and stress state are expected in off-normal conditions of 700 °C and 1200 °C. The results from this study suggest the B-HiPIMS coating is best-suited as a protective coating for SiC fuel cladding.

Tailored ASD destabilization - Balancing shelf life stability and dissolution performance with hydroxypropyl cellulose

Amorphous solid dispersion (ASD) formulations are preferred enabling formulations for poorly water soluble active pharmaceutical ingredients (API) as they reliably enhance the dissolution behavior and solubility. Balancing a high stability against unwanted transformations such as crystallization and amorphous phase separation during storage on the one hand and optimizing the dissolution behavior of the formulation (high supersaturation and maintenance for long time) on the other hand are essential during formulation development. This study assessed the potential of ternary ASDs (one API and two polymers) containing the polymers hydroxypropyl cellulose together with poly(vinylpyrrolidone-co-vinyl acetate) (PVP VA64) or hydroxypropyl cellulose acetate succinate to stabilize the amorphously embedded APIs fenofibrate and simvastatin during storage and to enhance the dissolution performance. Thermodynamic predictions using the PC-SAFT model revealed for each combination of polymers the optimal polymer ratio, maximum API load that is thermodynamically stable as well as miscibility of the two polymers. The stability predictions were validated by three months enduring stability tests, followed by a characterization of the dissolution behavior. The thermodynamically most stable ASDs were found to be the ASDs with deteriorated dissolution performance. Within the investigated polymer combinations, physical stability and dissolution performance opposed each other.

GAASP: Genetic Algorithm-Based Atomistic Sampling Protocol for High-Entropy Materials

ABSTRACT High-entropy materials are composed of multiple elements on comparatively simpler lattices. Due to the multi-component nature of such materials, atomic-scale sampling is computationally expensive due to the combinatorial complexity. This study proposes a genetic algorithm-based methodology for sampling such complex chemically disordered materials. Genetic Algorithm-based Atomistic Sampling Protocol (GAASP) variants can generate low as well as high-energy structures. GAASP low-energy variant in conjugation with metropolis criteria avoids premature convergence as well as ensures detailed balance condition. GAASP can be employed to generate low-energy structures for thermodynamic predictions, and diverse structures can be generated for machine-learning applications.

Intrinsic-group-contribution PC-SAFT and its application in performance analysis of high-temperature organic Rankine cycle with siloxanes and alkanes

Thermodynamic property prediction of working fluids is of great importance on the performance analysis of high-temperature organic Rankine cycles (ORCs) for utilizing low-grade heat sources. In this study, an intrinsic-group-contribution perturbed-chain statistical associating fluid theory (iGC-PC-SAFT) is proposed for the high-temperature ORC performance analysis with 8 siloxanes and 10 alkanes. By adjusting the initial values and corresponding value ranges of PC-SAFT parameters repeatedly according to the changes in molecular structures, the PC-SAFT parameters were fitted to determine the initial iGC-PC-SAFT parameters. Afterwards, the final iGC-PC-SAFT parameters for each working fluid were obtained based on a global optimization algorithm. Furthermore, the iGC-PC-SAFT equation of state (EoS) was used to calculate the thermodynamic properties of working fluids. Compared with the results from Coolprop database, maximum relative deviations for the saturated liquid density and the compressibility factor in the superheated region are 0.95% for MM, and 0.84% for MD2M, respectively. Besides, the average absolute relative deviations of ORC performance parameters calculated by the iGC-PC-SAFT EoS at different input pressures of turbine, including the thermal efficiency, turbine power output, pump power consumption, heat rejection, mass flow, and exergy efficiency are less than 2.04% for siloxanes (except for MM) and 6.09% for alkanes, respectively.

A modelling approach to prepare synthetic urine for struvite precipitation studies.

Nutrient recovery from wastewater is an effective strategy to prevent eutrophication and provide value for the treatment process. Human urine is a small but highly nutrient-rich stream in the total flux of domestic wastewater from which struvite (MgNH4PO4.6H2O) could be recovered and used as a fertiliser. Consequently, synthetic urine was used in most struvite precipitation studies, due to biohazard issues in real human urine. A modelling approach was developed to formulate synthetic urine recipes based on elemental urine composition, using matrix solving strategy to select and quantify chemical salts for synthetic urine preparation. The model also included mass balance, chemical speciation, and equilibrium dissociation expression for solution thermodynamics predictions in the formulated urine. In this study, synthetic solutions of fresh and stored urine were assessed with this model using Engineering Equation Solver (EES) software to calculate the quantity of salts, pH, ionic strength, and struvite saturation index. Simulation results in EES were successfully verified using PHREEQC simulations, while model validation comprised the examination of urine composition with their reported recipes.

GC-NORM-based thermodynamic framework for evaluations of organic reactions involving carbon dioxide utilization

Carbon emissions are one of the greatest challenges facing humanity. Carbon capture, utilization and storage offer solutions to achieving carbon neutrality, where computer-aided synthesis planning techniques enable the utilization of carbon dioxide through the design and evaluation of synthetic routes with the necessary thermodynamic properties. Nevertheless, the known thermodynamic property models have some limitations that make the planning of synthetic routes challenging. Therefore, the goal of this work is to provide a GC-NORM-based framework for the prediction of nine thermodynamic properties based on a comprehensive experimental dataset. The framework combines the group contribution method and norm indexes-based method for thermodynamic property predictions. These two methods complement each other, and both have acceptable regression results with R2 greater than 0.9 for most properties. Finally, the applications of the proposed framework are highlighted by organic reaction analyses for carbon dioxide utilizations involving the predictions of reaction heat and reaction equilibrium constant.

Flue Dust Reactions and Sticking Mechanisms in a Copper Flash Smelting Furnace Waste Heat Boiler: A Sampling Study

The formation of accretion layers within the waste heat boiler is a serious operational concern as it can potentially increase boiler downtime and, hence, limit continuous production. In previous Computational Fluid Dynamics studies, the accretion formation was predicted for an industrial-scale waste heat boiler, using a dust stickiness sub-routine of the model. In this study, a dust sampling campaign was used to validate this stickiness sub-routine. Furthermore, various sticking and reaction mechanisms of flue dust were investigated and compared to thermodynamic predictions. While the results suggest that the sub-routine is valid, the comparison of thermodynamics and species in the samples showed that the chemical reactions of the flue dust did not reach the thermodynamic equilibrium.Graphical

Role of Aluminum rejection from isothermal ω precipitates on the formation of α precipitates in the metastable β-titanium alloy Ti-10V-2Fe-3Al

The formation of isothermal ω phase precipitates and its influence on subsequent fine-scale α precipitation is investigated in a metastable β-titanium alloy, Ti-10V-2Fe-3Al. Atom-probe tomography and high-resolution transmission electron microscopy reveal that the rejection of Al, a potent α stabilizer, from the growing isothermal ω precipitates at 330°C, aids in the formation of α precipitates. Additionally, the presence of α/ω and α/β interfaces conclusively establish that these α precipitates form at the β/ω interface. Interestingly, the local Al pile-up at this interface results in a substantially higher than equilibrium Al content within the α precipitates at the early stages of formation. This can be rationalized based on a novel three-phase β+ω+α metastable equilibrium at a lower annealing temperature (330°C, below the ω solvus). Subsequent annealing at a higher temperature (600°C, above the ω solvus), dissolves the ω precipitates and re-establishes the two-phase β+α equilibrium in concurrence with solution thermodynamic predictions.

Cu-infiltrated (Nb,W)C solid solution carbides

Binderless NbC with 0–25 wt% WC powder mixtures were pressureless sintered in vacuum at 1330, 1390 or 1450 °C to generate open porosity (Nb,W)C skeletons. The formation of residual minor amounts of W2C and WC was discussed as a function of WC addition and compared with thermodynamic predictions. The structures with 15–25 wt% WC addition with 16–35% open porosity were used as a skeleton for Cu infiltration at 1390 °C in H2. The microstructure of the as-sintered homogeneous (Nb,W)C grains in the skeleton changed into a core-rim structured grains after Cu infiltration due to the partial dissolution of Nb into the liquid Cu phase. The fracture toughness, hardness, Young's modulus, electrical conductivity and flexural strength of the as-sintered porous and near fully dense Cu-infiltrated materials were assessed and discussed.

Increasing the Reliability of Formation Factor-Based Transport Property Prediction for High Performance Concrete Mixtures Through Innovative Matching Pore Solution Curing

Recent specifications for concrete bulk resistivity tests (ASTM C 1876 and AASHTO TP 119) recommend a submerged standard (simulated) pore solution (SPS, solution conductivity = 78.74 mS/cm) for curing concrete specimens. The rationale for the bucket test is that immersing concrete specimens in a soak solution similar to their pore solution would eliminate the need to determine pore solution resistivity for calculating the formation factor (FF) of concrete mixtures. However, the thermodynamic modeling predictions of 91-day pore solution concentration (PSC) for eight high performance concrete (HPC) mixtures evaluated in the current study showed SPS to be a close representation only for reference ordinary Portland cement (OPC) and binary silica fume mixtures. In contrast, the average PSC of Class F and Class C fly ash mixtures was approximately 12% to 20% lower and 20% to 40% higher, respectively, than the SPS. Accordingly, an innovative matching pore solution (MPS) curing approach was developed in which mixtures are grouped based on the influence of supplementary cementitious materials (SCMs), that is, their type and replacement levels of the long-term PSC of concrete mixtures and, thereby, cured in a simulated solution matching the average PSC of a particular group. Based on experimental work in the current study, HPC mixtures under MPS demonstrated a lower coefficient of variation (COV) and more comparable (<10% difference) bulk resistivity (BR) and surface resistivity (SR) measurements compared with SPS. Moreover, the MPS improved the reliability in FF determination and FF-based transport property prediction for HPC mixtures, as verified by lower mean absolute error and improved R2 between FF-predicted diffusion coefficients and experimental measurements.