Heat Evolution Curves Research Articles

The investigation of micro-crack generation in cementitious mine seals via temperature and strain analysis

Mine seals are employed in underground coal mines to effectively isolate abandoned areas from active workings. The seals are engineered to prevent air movement and endure dynamic pressure events resulting from methane explosions. Mine seals are made from cementitious materials and undergo a curing process that involves an exothermic reaction known as hydration. The presence of moisture (hydration) may result in the mixture contracting, which could subsequently lead to cracking during the early phases of the curing process. Cracks in the seal can influence the performance and structural stability. This study examines the formation of micro-cracks in cementitious mine seals throughout the curing process by evaluating temperature and strain data collected from thermocouples and strain gauges that were installed during the pouring phase. The samples for the mine seal were prepared using two distinct mixtures: traditional concrete and pumpable products sourced from conventional suppliers. Heat evolution curves were produced for each sampling location corresponding to the three samples. The samples of Mixture A and B, prepared according to specifications, exhibited the anticipated heat evolution curves, with the maximum temperature occurring at the center of the samples. In contrast to the other mixtures, the samples of mixture C, which were prepared outside of the specified parameters, exhibited the highest temperature at the bottom of the sample. The strain measurements showed a correlation with temperature readings, indicating that mixtures A and B exhibited the highest strain at the center of the sample, while mixture C demonstrated the greatest strain at the bottom of the sample. The examination of the gathered data reveals a significant likelihood of micro-cracks forming in each of the samples.

Effect of using calcined eggshells as a cementitious material on early performance

This study focuses on the viability of using calcined eggshells (CES) as a partial replacement of cement by analysing early age performance. Eggshells found on the landfill sites are contaminated with smelly organic components. One of the recommended ways for extracting pure stable calcite from waste eggshells is to remove the volatile component and organic components by calcining them. To get the calcined eggshells, raw eggshells were calcined at 400°C, 600°C, and 800°C. Each temperature was held constant for the duration of one hour, two hours, and three hours, respectively. These calcined eggshells were replaced with cement at the rate of 5%, 10%, and 15% in addition to the mixes containing eggshells and limestone with the same replacement level. For early age performance, setting times were estimated using the Vicat method, the heat evolution was monitored using isothermal calorimetry during the first 24 h and the compressive strength after 24 h were measured. Due to the disturbance of sulphate (SO3) balance, a certain reduction was observed in the setting time. The calcined eggshell proved to be a better accelerator as compared to both eggshells and limestone. In the heat evolution curve, the secondary shoulder peak appears in mixes containing high-temperature calcined eggshells which indicates the formation of the AFm phase. The cumulative heat release associated with calcined eggshells is higher as compared with eggshells and raw eggshells when replaced with cement. Moreover, the compressive strength was found higher in the calcined eggshells with undecomposed calcite as compared to the calcined eggshells with decomposed calcite. Also, the reverse trend was observed in heat of hydration versus strength correlation in decomposed and undecomposed calcite condition.

Formation and hydration of eco-friendly cement using industrial wastes as raw materials

In this work, the optimal conditions of the synthesis of eco-friendly cement by using industrial wastes as well as the peculiarities of its early stage hydration were investigated. The eco-friendly cement was synthesized within the 1000–1250 °C temperature range when the targeted composition was 60% of belite, 20% of ye’elimite, and 20% of brownmillerite. It was determined that the optimal sintering temperature for eco-friendly cement is 1100 °C because the primary compounds were fully reacted, and hydraulic active compounds were dominant in the products. Microcalorimetry analysis was performed for the investigation of early stage hydration. The best results of hydration were obtained with the eco-friendly cement which was produced by using mixtures with silica gel waste: three exothermic reactions were observed in the heat evolution curve, while the cumulative heat was equal to 264 J/g after 72 h. Additionally, the sequence of compounds formation during the first day of hydration was analyzed. It was determined that the composition of the initial mixture impacts the hydration rate of synthetic eco-friendly cement; however, it did not affect the mineralogical composition of the hydration products. These results were confirmed by XRD, STA, and SEM analysis.

The effect of fly ash fineness on heat of hydration, microstructure, flow and compressive strength of blended cement pastes

In this paper, an experimental study on the effect of fly ash fineness on the heat of hydration, microstructure, flow and compressive strength of blended cement pastes was carried out and evaluated against control cement paste. Fly ashes with different fineness: classified fly ash, run-of-station fly ash and grounded run-of-station fly ash; with a median particle size of 17.4, 11.3 and 5.7 μm, respectively, from the same power station source in Australia were used to partially replace Portland cement at 20% and 40% by weight of cement using a fixed water-to-binder ratio of 0.40. Results of this study showed that the cumulative heat of hydration of blended cement paste decreased as fly ash content in blended cement paste was increased. For a given cement replacement level, blended cement paste containing finer fly ash released more heat of hydration when compared to coarser fly ash. Moreover, increasing the fineness of fly ash resulted in a higher consumption of calcium hydroxide at 7 and 28 days reflecting pozzolanic reactivity and, thus, a denser microstructure than blended pastes containing coarser fly ash as revealed by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and compressive strength results. In addition, the incorporation of fly ash in the blended pastes led to the introduction of an additional hydration peak in the heat evolution curve possibly due to the late activation of fly ash by calcium hydroxide renewing the C3A reaction and converting ettringite to monosulfate. The flow of the freshly blended cement pastes was also found to improve slightly with increasing fineness of the fly ash. In addition, the hardened blended cement pastes containing 20% ground run-of-station fly ash showed comparable compressive strength with the control cement pastes at both 7 and 28 days mainly due to the higher fineness of the ground run-of-station fly ash and increased reactivity compared to coarser grade fly ash.

Hydration Kinetics of Composite Cementitious Materials Containing Copper Tailing Powder and Graphene Oxide.

The hydration heat evolution curves of composite cementitious materials containing copper tailing powder (CT) and graphene oxide (GO) with different contents are measured and analyzed in this paper. The hydration rate and total hydration heat of the composite cementitious materials decrease with the increase of CT dosage, but improve with the increase of CT fineness and GO dosage. The hydration process of the cementitious systems undergoes three periods, namely nucleation and crystal growth (NG), phase boundary reaction (I), and diffusion (D), which can be simulated well using the Krstulovic–Dabic model. The hydration rates of the three controlling processes of the composite cementitious system decrease with the increase of CT content, but improve slightly with the increase of CT fineness. GO enhances the controlling effect of the NG process of the cementitious systems with or without CT, thus promotes the early hydration as a whole.

Composition and metabolism of fecal microbiota from normal and overweight children are differentially affected by melibiose, raffinose and raffinose-derived fructans

The aim of the study was to investigate the metabolism of non-digestible oligo- and polysaccharides by fecal microbiota, using isothermal microcalorimetry. The five tested substrates were raffinose, melibiose, a mixture of oligo- and polysaccharides produced from raffinose by levansucrase, levan synthesized from raffinose, and levan from timothy grass. Two inocula were comprised of pooled fecal samples from overweight or normal-weight children, from healthy adult volunteers and a pure culture of Bacteroides thetaiotaomicron as a reference bacterium for colon microbiota. The growth was analyzed based on the heat evolution curves, and the production of organic acids and gases. Taxonomic profiles of the microbiota were assessed by 16S rDNA sequencing.Raffinose and melibiose promoted the growth of bifidobacteria in all fecal pools. Several pool-specific substrate-related responses to raffinose and melibiose were revealed. Lactate-producing bacteria (Streptococcus and Enterococcus) became enriched in the pool of overweight children resulting in lactic acid as the major fermentation product on short saccharides. Acetic and butyric acids were prevalent at fermentation in the normal-weight pool coinciding with the enrichment of Catenibacterium. In the adult pool, the specific promotion of Bacteroides and Lachnospiraceae by levans was disclosed. In the fecal pool of normal-weight children, levans stimulated the growth of Senegalimassilia and Lachnoclostridium and this particular pool also showed the highest maximum heat production rate at levan fermentation. Levans and raffinose-derived oligosaccharides, but not raffinose and melibiose were completely fermented by a pure culture of Bacteroides thetaiotaomicron.The main conclusion from the study is that fecal microbiota of normal and overweight children have different compositions and they respond in specific manners to non-digestible oligo- and polysaccharides: raffinose, melibiose, raffinose-derived oligosaccharides and levans. The potential of the tested saccharides to support a healthy balance of colon microbiota requires further studies.

Influence of Calcium Sulfate Varieties on Properties of Calcium Aluminate Cement-Based Self-Leveling Mortars

The effect of calcium sulfates varieties on the properties of calcium aluminate cement-based self-leveling mortar have been investigated, and the hydration kinetics, hydrated products and microstructures are characterized by isothermal calorimeter, X-ray diffraction and mercury intrusion porosimetry, respectively. The results show that the technological properties of mortars are significantly affected by calcium sulfate varieties and content. The setting times are shortened drastically with the addition of calcium sulfates. Mortars with hemihydrate show higher early strength and less drying shrinkage. In contrast, using anhydrite in mortars cause lower strength and higher drying shrinkage at early age but larger growth of strength in the late. The increasing calcium sulfates content may result in the delay of main hydration peak in the heat evolution curve. For formulation with hemihydrate, the appearance of main hydration peaks are advanced compared with formulation without addition of calcium sulfates. Moreover, mortar microstructures are optimized by addition of β-hemihydrate, the proportion of large pores are lower than that of mortars with anhydrite.

Application of calorimetry as a main tool in evaluation of the effect of carbonate additives on cement hydration

Calorimetry was applied to follow the hydration in the Portland cement–dolomite–limestone mixtures. In the experiments the limestone additive of various fineness (standard component of various common cements), as well as the dolomite additive (not a standard component) were used. The rate of hydration versus time for common cements reflects the proper setting and early hardening during the first days after mixing with water (two or three peaks and the induction period between them). The aim of measurements presented in this work was to show the course of heat evolution curve and the heat evolved values, equivalent to the acceleration/retardation of hydration, in case of the pastes produced from Portland cement and the carbonate additives mixed in variable proportions, as well as to verify the results by other methods. The rate of heat evolution accompanying cement paste hydration, total heat evolved, conductivity of hydrating suspension and rheological (flow) properties versus time are modified by the fine grained carbonate additives. This is due to the hypothetical nucleating effect of limestone and dolomite.

Calorimetric study of magnesium potassium phosphate cement

The thermal behavior of magnesium potassium phosphate cement (MKPC) during the early reaction period has been monitored by isothermal conduction calorimetry at 298 K. It is found that the typical heat evolution curve of the MKPC hydration has one endothermic valley and two distinct exothermic peaks. It is believed that the endothermic valley corresponds to the dissolution of potassium dihydrogen phosphate and the exothermic peaks are related to the dissolution of magnesia and the formation of magnesium potassium phosphate hexahydrate, respectively. The influences of the water to powder ratio, the magnesium to phosphate molar ratio and the setting retarder content on the reaction can be reflected in a change of shape and intensity of the peaks on the heat flow curve. The variation trend is consistent with the change of the setting time of MKPC pastes.

Heat evolution in hydrating expansive cement systems

Calorimetry was applied to follow the hydration of special cement mixtures exhibiting expansion or shrinkage compensation. The standard, common cements show generally less or more visible shrinkage on setting and hardening but mixed with and expansive agent, usually of aluminate and sulfate nature, they can exhibit the increase of volume. The calcium aluminate cement CAC 40 was ground together with special sulfate–lime sinter to produce an expansive additive to Portland cement (CEM I 42.5R). The expansive additive in the environment of hydrating cement transforms into ettringite at “right time” to give expansion before the final setting and hardening takes place. In the experiments the proportions of components of expansive mixture and basic cement were variable. The rate of hydration versus time for common cements is commonly known and reflects the moderate setting and early hardening during the first days after mixing with water (two peaks and the induction period between them). The aim of measurements presented in this study was to show the course of heat evolution curve and the heat evolved values, equivalent to the acceleration/retardation of hydration, in case of the paste with the expansive mixture, as well as the pastes produced from Portland cement and the components of expansive additives added in variable proportions. It was possible to see how the calorimetric curve and consequently the hydration process itself declines from the controlled setting/hardening. These measurements were supplied by the examples of phase composition studies by XRD.

Investigation into relations among technological properties, hydration kinetics and early age hydration of self-leveling underlayments

Technological properties such as flow value, setting times, compressive strength and early age dimensional stability as linear shrinkage and expansion have been studied for two types of self-leveling underlayments (SLU) in which the kind of calcium sulfates was varied. The influence on hydration kinetics has been measured by isothermal heat flow calorimetry. The results obtained for the technological properties change significantly when different kinds of calcium sulfates are used. The basic trend for the results changes when the composition changes from a calcium aluminate cement system to a Portland cement system. Additionally, there was an interesting relationship to final dimensional stability and shape of heat evolution curve. Moreover the time to reach plateau of dimensional stability was related to the development of compressive strength. In the meanwhile, using hemihydrate in Portland cement systems caused low compressive strength and significant expansion. On the other hand, in the results of XRD measurements, the first genesis of Ettringite corresponded to the first shrinkage of SLU. Interesting results related with technological properties and hydration kinetics or the results of XRD other than these above results were obtained.

Characterization and Performance Prediction of Cement-Based Materials Using a Simple Isothermal Calorimeter

In the present study, early-age heat evolution of over 120 mortar mixes was monitored for 24 hours after mixing using a simple isothermal calorimetry device. This paper describes the processes of calorimetric characterization and performance prediction of these mortar materials. The features of the heat evaluation curves obtained from the calorimetry tests and the effects of the mortar materials, mix proportions, and curing conditions on the heat evaluation curves were studied. A derivative method was employed for exemplifying the physical changes in the tested materials from the heat evolution curves, and the results were compared with set times from ASTM C403 tests. The study indicates that the simple calorimetry technique can be used to flag changes in cementitious materials, pre-screen concrete materials and/or mix design, identify incompatibility of cementitious materials, and forecast setting time and early-age strength.

Calorimetric Evaluations of Bacillus subtilis Vegetative and Spore Cells Colonial Growth and Suppressive Effects of Sucrose Monopalmitate

The colonial growth of Bacillus subtilis vegetative and spore cells was evaluated by determining the time course of the growth heat changes (growth thermogram) with a microbial calorimeter. The actual heat evolution curve (f (t) curve) obtained from the thermogram was in good agreement with the viable cell number changes. From the logarithmic f (t) curve, the exponential growth phase was estimated to be the period of between 31 and 35 h after inoculation and the growth rate constant (mu') was 0.36 h(-1). The f (t) curve was also obtained from the thermogram of the spore cells. The starting time of growth of the spore cells was 9 h later than that of the vegetative cells. This period corresponded to the germination period. The growth profile after germination was almost similar to that of the vegetative cells. The suppressive activities of sucrose monopalmitate (SMP) against the vegetative and spore cells were evaluated by the f (t) curves. Our results show that SMP inhibited metabolic inductions during the lag phase of the vegetative cells and the germination of the spore cells. Microbial calorimetry is a method suitable for non-destructive growth measurements of microbial colonies on the surface of a solid medium.

Heat evolution in alkali activated synthetic slag–metakaolin mixtures

In this study the calorimeter was applied to follow the early hydration of mixtures produced using three different synthetic slag vitreous materials, differing with alumina to silica ratio and mixed or not with synthetic two different metakaolin additions (kaolin heated with sodium containing admixture or without admixture). These mixtures were processed with sodium/potassium hydroxide solutions and placed inside the chamber of calorimeter. The kinetics of hydration process was thus characterized and the hydraulic properties of slag–metakaolin mixtures were very well assessed. Substantial heat evolution was found in the presence of activators, in many cases exceeding 100 J g–1 for 24 h process; heat evolved on hydration with water only was very poor, below 20 J g–1 after 24 h. The rate of heat evolution vs. time plots showed specific shape, more or less similar to the typical heat evolution curves reported for cements. Finally, some conclusions were drawn and the amount/concentration of activators was selected for further studies.

Specific features of the thermodynamics of activation in the LaNi5-H2 and CeNi5-H2 systems

Using heat conducting Tian—Calvet calorimetry and volumetric measurements, the first hydrogen absorption—desorption cycles in the LaNi5-H2 and CeNi5-H2 systems were studied. The pressure—composition isotherms were plotted, the equilibrium pressures of hydrogen along the absorption and desorption branches and in the region of hysteresis for different activation steps were determined, and the enthalpies of phase transitions α → β and β → α were calculated. The profiles of the heat evolution curves were analyzed. It was concluded that the mechanism of the reactions studied changes upon activation.

Influence of alkali on restrained shrinkage behavior of cement-based materials

Experiments have been conducted to study effects of high alkalinity on restrained shrinkage behavior and cracking sensitivity of cement-based materials at early ages. The restrained shrinkage test has been conducted with an ellipse ring setup and the initial cracking time was monitored with a continuous conductive strip. Alkali content and alkali type as well as the shrinkage–hydration relationship have been studied. The experimental results have shown that the cracking sensitivity of a cement-based material is increased with an increase in alkali content. The influence of the excess alkali on the cracking sensitivity is more obvious for cement paste with a low water-to-cement ratio (w/c) than that with a high w/c. The hydration processes and microstructure development of cement paste have been investigated using heat of hydration measurement and electrical resistivity measurement. The superimposed resistivity curve and heat evolution curve provide more comprehensive understanding on factors influencing shrinkage development.

Evaluation of growth activity of microbes in tea field soil using microbial calorimetry

Calorimetry was used to study the heat evolution associated with microbial growth in tea field soils (growth thermograms) during the microbial degradation of added organic matter. From the actual heat evolution curves (f(t) curves) derived from the growth thermograms under various conditions of pH and water content of the tea field soils, the apparent growth rate constant (μ′) of soil microbes was determined as an index of their growth activity. In addition, μ′ was compared between tea field soils treated with organic versus compound fertilizer. From the results, the following conclusions were made: (i) In acidic tea field soil (pH less than about 6.0), the growth activity of soil microbes declined with decreasing pH, and μ′ was twofold higher at pH 6.0 than at pH 4.3. (ii) In tea field soils (Andosol) with a water content less than about 50%maximum water holding capacity (MWHC), the growth activity declined with decreasing water content, and μ′ at 52.5%MWHC was double that at 35.2%MWHC. (iii) The growth activity of microbes in tea field soil treated with organic fertilizer was approximately 1.2-fold higher than that treated with compound fertilizer.

Evaluation of Growth Activity of Microbes in Tea Field Soil Using Microbial Calorimetry

Calorimetry was used to study the heat evolution associated with microbial growth in tea field soils (growth thermograms) during the microbial degradation of added organic matter. From the actual heat evolution curves (f(t) curves) derived from the growth thermograms under various conditions of pH and water content of the tea field soils, the apparent growth rate constant (μ′) of soil microbes was determined as an index of their growth activity. In addition, μ′ was compared between tea field soils treated with organic versus compound fertilizer. From the results, the following conclusions were made: (i) In acidic tea field soil (pH less than about 6.0), the growth activity of soil microbes declined with decreasing pH, and μ′ was twofold higher at pH 6.0 than at pH 4.3. (ii) In tea field soils (Andosol) with a water content less than about 50%maximum water holding capacity (MWHC), the growth activity declined with decreasing water content, and μ′ at 52.5%MWHC was double that at 35.2%MWHC. (iii) The growth activity of microbes in tea field soil treated with organic fertilizer was approximately 1.2-fold higher than that treated with compound fertilizer.

Heat Evolution in Hydrated Cementitious Systems Admixtured with Fly Ash

In this study a calorimeter was applied to investigate the hydration of cements with fly ash (pulverised fuel ash – PFA) admixture. Four cements were used to produce the binders containing from 5 to 60% fly ash. The process of hydration in cementitious systems with fly ashes is slower than in reference pastes without admixtures. However, the calorimetric calculations and the shape of heat evolution curves seem to indicate a complex interaction between the components of cement and ash resulting in the increasing total heat evolved values per unit of cement. At higher fly ash content the accelerating effect of alkalis and alumina should be taken into account and discussed in terms of the composition of initial cement. The modifications of hydration kinetics and mechanism in this case is very well visualised by means of calorimetry.

The effect of temperature on the early hydration of Portland cement and blended cements

The effect of temperature, in the range 10–60°C, on the early hydration of two Portland cements from Mexico and three blended cements has been investigated by means of isothermal conduction calorimetry. The blended cements were based on the two Portland cements with partial replacement by ground granulated blast furnace slag, GGBFS (60%), pulverized fuel ash, PFA (30%) and blast and a Mexican volcanic ash (22%). An increase in the temperature of hydration brought about initial acceleration of all five cement systems, as indicated by the increased magnitude of the maximum of peak II and shifts to earlier times. There was a clear contribution from the GGBFS to the heat evolution curves of the blended cement incorporating GGBFS, which increased with increasing temperature. By way of contrast, there was no enhancement of the cement hydration in the early stages of hydration in the case of the PFA blended cement, and the appearance of peak II was slightly retarded. The incorporation of volcanic ash had little effect on the initial hydration, but gave an additional peak after peak II, similar to that observed in the GGBFS system. At this stage, the total heat evolved was greater than that in the corresponding neat cement, especially at 20°C. It has also been demonstrated that alkali activation of GGBFS and volcanic ash was readily brought about by NaOH at 40°C. The enhanced activity observed in the Portland cement–volcanic ash system is attributed to activation of the ash by the portlandite liberated during the early hydration of alite.