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.     

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. This revised version was published online in July 2006 with corrections to the Cover Date.     

Calorimetry in the studies of cement hydration

Calorimetry was applied to an investigation of the early hydration of Portland cement (PC)–calcium aluminate cement (CAC) pastes. The heat evolution measurements were related to the strength tests on small cylindrical samples and standard mortar bars. Different heat-evolution profiles were observed, depending on the calcium aluminate cement/Portland cement ratio. The significant modification of Portland cement heat evolution profile within a few hours after mixing with water was observed generally in pastes containing up to 25% CAC. On the other hand the CAC hydration acceleration effect was also obtained with the 10% and 20% addition of Portland cement. As one could expect the compressive and flexural strength development was more or less changed—reduced in the presence of larger amount of the second component in the mixture, presumably because of the internal cracks generated by expansive calcium sulfoaluminate formation.     

Calorimetry and other methods in the studies of expansive cement hydrating mixtures

In this study, the calorimeter was applied to follow the hydration of special cement mixtures exhibiting expansion or shrinkage compensation. The shrinkage-less and expansive binders were produced by mixing of Portland cement with an expansive additive produced by sintering and composed of calcium sulfoaluminate (yeelimite), calcium sulfate (anhydrite) and lime. The studies were focused on the synthesis of this aluminate??sulfate??lime additive (temperature of burning process as a parameter controlling the relative activity of components) from the materials being the by products and subsequently on the mixture proportions to ensure the hydration process resulting in non-shrinkage or expansion effect. In the experiments the proportions of expansive mixture and cementitious materials were variable. The investigations with aim to find the relationship between the volume changes and composition of initial mixtures in cement pastes and mortars (with sand) were also carried out. The phase composition and microstructure of products were characterized. The expansive additive in the environment of hydrating cement transforms into ettringite and gives an increase of volume when the plastic material transforms to the more rigid matter but before the ultimate hardening takes place. Proper, moderate setting and hardening in strongly modified mixtures is achieved when the calorimetric curve corresponding to the heat evolution on hydration is analogous to that for the basic Portland cement. The rate of heat evolution data are well compatible with the other results related to the other methods of hydration kinetics assessment (e.g. chemical shrinkage) and discussed in terms of the phase composition of hydration products.     

Heat evolution in hydrated cementitious systems admixtured with different set controlling components

Calorimetry has been used in the investigations of cementitious systems with different set controlling admixtures. The kinetics and mechanism of hydration process was thus characterized on two different cement clinkers mixed with calcium sulphate containing materials. These admixtures were collected as a residue in the fluidised bed combustion (FBC) of coals with simultaneous desulphurisation process - so-called bottom ash. Apart from anhydrite/gypsum, they were composed mainly of alumina and silica containing material of disordered structure, originating from the coal contaminations of clay character. Anhydrite/gypsum acts as set controlling admixture. The aluminosilicate component reacts with calcium ions released to the solution from the calcium silicate clinker minerals. It has been found that fluidised bed combustion wastes can be successfully used as set controlling admixture. There is no other harmful effects; those could be easily detectable by calorimetry. However the effect is dependent upon the composition of cement clinker. At low calcium aluminate content a slight acceleration of hydration process can be easily observed, particularly at higher amount of admixture. In the mixtures with high calcium aluminate clinker the heat evolved is slightly reduced in the presence of admixture. The dominating role of aluminate phase in heat evolution process within the first hours of hydration process has been thus proved. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Influence of superplasticizers on the course of Portland cement hydration

A multicell isoperibolic — semiadiabatic calorimeter was used for the measurement of temperature and the determination of the hydration heat evolution at earlier period of cement pastes setting and hardening. The measurements were aimed at the determination of the effect of superplasticizers (SPs) on the course of the Portland cement hydration. Commercial polycarboxylate SP was added to the mixtures and the heat effect was measured. With the increasing content of SP, the hydration temperature increased up to a certain value and then decreased. In case of a sufficient amount of water in the mixture to achieve complete hydration of cement, samples with the highest values of the maximum hydration temperature reached the highest values of the released total heat. If there is not a sufficient amount of water to achieve complete hydration, the samples with the highest values of the maximum hydration temperature reach the lowest values of the released total heat.     

Investigations of cement early hydration in the presence of chemically activated fly ash

The paper describes an attempt of chemical activation of fly ash and claims the usefulness of combination of such investigation methods as calorimetry and infrared absorption for investigations of early periods of cement hydration. The research samples were cement pastes made with an addition of fly ash and admixtures of chemical activators, CaCl₂, Na₂SO₄ and NaOH, whereas a cement paste without fly ash addition and a cement-fly ash paste (both without admixtures) were used as reference samples. In order to investigate early periods of cement pastes hydration, the amount and rate of heat release were registered, and IR spectrums were checked at appointed hydration moments. As a result, it was shown that the combination of calorimetric and IR absorption methods in the investigations of early periods of cement hydration was useful. It was confirmed that the use of chemical activators CaCl₂, Na₂SO₄ and NaOH accelerated the hydration of cement pastes containing fly ash additive in early hours after adding water. The action of activators on hydrating cement system is different for each of investigated compounds.     

Influence of the calcined paper sludge on the development of hydration heat in blended cement mortars

The present study is based on the influence of the addition of a pozzolanic material as a result of the activation of an industrial waste coming from the Spanish paper industry on the heating as well as hydration heat of the cement mortars made with 10 or 20% of active addition. Once the sludge has been calcined at different temperatures (700–800°C) and stays in furnace (2 and 5 h), the calcined products showed high pozzolanic activity. The maximum activity corresponded to the paper sludge calcined at 700°C for 2 h (S1). Besides, it can be proved that there was an increase both of the heating and also of the hydration heat in the first 23–25 h for both additions (10 and 20% of S1) regarding the reference cement mortar. This behaviour would be related to the influence of different effects: filler and pozzolanic during the first hours of reaction, and by the dilution effect for longer hydration times, mainly when 20% of S1 was added.     

Calorimetric determination of the effect of additives on cement hydration process

Possibilities of a multicell isoperibolic-semiadiabatic calorimeter application for the measurement of hydration heat and maximum temperature reached in mixtures of various compositions during their setting and early stages of hardening are presented. Measurements were aimed to determine the impact of selected components?? content on the course of ordinary Portland cement (OPC) hydration. The following components were selected for the determination of the hydration behaviour in mixtures: very finely ground granulated blast furnace slag (GBFS), silica fume (microsilica, SF), finely ground quartz sand (FGQ), and calcined bauxite (CB). A commercial polycarboxylate type superplasticizer was also added to the selected mixtures. All maximum temperatures measured for selected mineral components were lower than that reached for cement. The maximum temperature increased with the decreasing amount of components in the mixture for all components except for silica fume. For all components, except for CB, the values of total released heat were higher than those for pure Portland cement samples.     

Calorimetric studies of cementitious materials — chromium interaction

Cementitious systems based on portland cement are used for immobilization of toxic and hazardous wastes. The addition of waste material may impact the hydration reaction in cement matrix and consequently the setting and hardening process. The progress of reaction can be monitored by heat evolution measurements and the calorimetric results can indicate the declination from standard behaviour.In this study the microcalorimetry was used to evaluate the heat output during the hydration of cements in the presence of different chromium containing salts, viz. CrCl₃, Cr₂(SO₄)₃, Na₂CrO₄ and K₂CrO₄.     

Principal component and cluster analyses as supporting tools for co-crystals detection

The effects of super absorbent polymer (SAP) on the early hydration evolution of Portland cement within 72 h were investigated by isothermal calorimetry, thermal analysis and X-ray diffraction analysis. The results show that the SAP definitely affects the early hydration process of Portland cement, increases the hydration heat evolution rate during the acceleration period and during the main exothermic peak, promotes the earlier appearance of the main exothermic peak, but does not affect the lengths of the initial reaction period and the induction period and the onset of the acceleration period. The SAP can accelerate cement hydration to increase the hydration degree within 72 h. But the dosage variation of SAP has minor influence on the hydration heat evolution and hydration degree. The SAP enhances the formation of Ca(OH)₂ after 12 h to keep higher content than that in the reference paste. The SAP does not affect the maximum content of ettringite, but delays the conversion of ettringite to monosulphate to remain ettringite content higher at later hydration time. Besides, no new phases are found to have formed in cement paste with SAP.     

Hydration of shrinkage-compensating binders with different compositions and water-binder ratios

The calorimetric data of blended shrinkage-compensating binders with different compositions were measured at 25°C at different water-binder ratios using an isothermal calorimeter. The hydration characteristics of shrinkage-compensating binders were evaluated and their influence on the expansive properties of blended shrinkage-compensating binders was determined. Composition and w/b ratio significantly affect the hydration rate and degree of shrinkage-compensating binders, as well as their expansive and mechanical properties. The total heat of hydration of binders decreases with w/c ratios. Its final hydration degree also decreases with w/c ratio. The ternary binders composed with Portland cement, mineral admixture and expansive agent show low hydration heat and rate of heat evolution, but their total heat of hydration increases continuously and surpasses that of binary binder in later period at low w/b ratio. This revised version was published online in August 2006 with corrections to the Cover Date.     

Swelling effects in cross-linked polymers by thermogravimetry

A Brazilian coal power plant generates a waste composed by the fly and bottom ashes produced from coal combustion and by a spent sulfated lime generated after SO₂ capture from combustion gases. This work presents a study of the early stages of the hydration of composites formed by this waste and a type II Portland cement, which will be used for CO₂ capture. The cement substitution degrees in the evaluated composites were 10, 20, 30 and 40%, and the effect of the coal power unit waste on the hydration reaction was analyzed on real time by NCDTA, during the first 40 h of hydration. The results show that the higher is the substitution degree, the higher is the retarding effect on the cement hydration process. Actually, by respective thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis on initial cement mass basis, this effect is caused by double exchange reactions among Ca and Mg components of the waste, during the first 4 h of hydration, which promote a much higher exothermic effect in the NCDTA curve, simultaneously to respective induction periods. The pozzolanic reactions, due to the presence of the waste silica and alumina containing amorphous phases, consume part of the original Ca(OH)₂ content existent in the waste in the case of 30 and 40% substituted pastes, and also from part of the Ca(OH)₂ produced in cement hydration reactions, in the case of the 10 and 20% substituted pastes.     

Isothermal and solution calorimetry to assess the effect of superplasticizers and mineral admixtures on cement hydration

The heat of hydration evolution of eight paste mixtures of various water to binder ratio and containing various pozzolanic (silica fume, fly ash) and latent hydraulic (granulated blast furnace slag) admixtures have been studied by means of isothermal calorimetry during the first 7 days of the hydration process and by means of solution calorimetry for up to 120 days. The results of early heat of hydration values obtained by both methods are comparable in case of the samples without mineral admixtures; the values obtained for samples containing fly ash and granulated blast furnace slag differ though. The results from isothermal calorimetry show an acceleration of the hydration process by the presence of the fine particles of silica fume and retarding action of other mineral admixtures and superplasticizer. The influence of the presence of mineral admixtures on higher heat development (expressed as joules per gram of cement in mixture) becomes apparent after 20 h in case of fly ash without superplasticizer and after 48 h for sample containing fly ash and superplasticizer. In case of samples containing slag and superplasticizer the delay observed was 40 h. The results obtained by solution calorimetry provide a good complement to the ones of isothermal calorimetry, as the solution calorimetry enables to study the contribution of the mineral admixtures to the hydration heat development at later ages of the hydration process, which is otherwise hard to obtain by different methods.     

The effects of solvents on C–S–H as determined by thermal analysis

Un-hydrated Portland cement consists of several anhydrous and reactive phases, that when mixed with water react to form hydrates. The main hydration product of Portland cement is calcium silicate hydrate (C–S–H). It is the main binding phase in a concrete system, hence is important to construction chemists. The concrete engineer measures the compressive strength of concrete after prescribed hydration periods, typically 1, 3, 7, 28 days. It is often convenient to mimic these intervals by stopping the hydration reaction at the same times. Several techniques can be employed to stop this hydration reaction. One of which is solvent-based and involves mixing a polar solvent such as acetone or isopropyl alcohol, with the hydrated cement. This mixing should be vigorous enough to blend the free water, in the partially hydrated cement system, with the polar solvent without altering the cement system’s matrix. The solvent-water mixture has a much lower boiling point and the mixture quickly evaporates out of the system. This achieves two goals. It stops the hydration reaction at the moment of solvent mixing, and it removes free water to prevent further hydration from occurring. This procedure theoretically leaves behind a dry, chemically unaltered, partially hydrated cement paste. In this way, pastes can be analyzed after the prescribed 1, 3, 7 or 28 days of hydration. This paper uses thermogravimetric analysis (TG) results to investigate the assumption that solvents have no thermodynamic or chemical effect on the hydrated cement paste phases.     

Utilization of waste phosphate aluminum slag for cement manufacture

The objective of this study was to investigate the feasibility of application of waste phosphate aluminum slag (PAS) for cement manufacture. To recycle waste PAS and minimize adverse effects on cement hydration induced by phosphate, NH₄OH was used to purify PAS. X-ray diffraction (XRD) analysis was used to determine to confirm the removal of harmful phosphate. The effect of PAS on the hydration product composition, heat release and compressive strength was also investigated. The results demonstrated that NH₄OH was effective in removing harmful AlPO₄ in PAS and 10% NH₄OH was considered as the optimal treatment concentration. In addition, the purification of NH₄OH alleviated the delay in cement hydration caused by AlPO₄ and the heat release curve of purified PAS (PPAS) cement tends to that of OPC. Moreover, the compressive strength of PPAS mortar at 28 days was 49.4 ​MPa, which is 18% higher than the compressive strength of PAS mortar. PAS purified by NH₄OH can be applied to cement manufacturing.     

Influences of fly ash and fluorgypsum on the hydration heat and compressive strength of cement

The hydration heat of pure cement, fly ash single-doped cement, as well as fly ash and fluorgypsum co-doped cement were investigated by means of micro-calorimetry with an eight-channel micro-calorimeter. The results showed that the hydration heat and the hydration rate could be reduced significantly by fly ash and fluorgypsum. However, the reduction was not proportional to the loading of dopant. The exothermic peak of the co-doped cement was appeared earlier than that of the single-doped cement. As the temperature decreased, the hydration heat and the hydration rate of both the doped cement were reduced, and the exothermic peak appeared later. The effect of fly ash and fluorgypsum on the compressive strength of cement was also investigated. The results revealed that the early compressive strength of concrete made up of the co-doped cement was largely higher than that of the single-doped cement. Based on the experiment results obtained in this article, we could conclude that fluorgypsum is a suitable additive for the single-doped cement.     

Simultaneous solidification of two catalyst wastes and their effect on the early stages of cement hydration

Two catalyst wastes (RNi and RAI) from polyol production were considered as hazardous, due to their respective high concentration of nickel and aluminum contents. This article presents the study, done to avoid environmental impacts, of the simultaneous solidification/stabilization of both catalyst wastes with type II Portland cement (CP) by non-conventional differential thermal analysis (NCDTA). This technique allows one to monitor the initial stages of cement hydration to evaluate the accelerating and/or retarding effects on the process due to the presence of the wastes and to identify the steps where the changes occur. Pastes with water/cement ratio equal to 0.5 were prepared, into which different amounts of each waste were added. NCDTA has the same basic principle of Differential Thermal Analysis (DTA), but differs in the fact that there is no external heating or cooling system as in the case of DTA. The thermal effects of the cement paste hydration with and without waste presence were evaluated from the energy released during the process in real time by acquiring the temperature data of the sample and reference using thermistors with 0.03 °C resolution, coupled to an analog–digital interface. In the early stages of cement hydration retarding and accelerating effects occur, respectively due to RNi and RAl presence, with significant thermal effects. During the simultaneous use of the two waste catalysts for their stabilization process by solidification in cement, there is a synergic resulting effect, which allows better hydration operating conditions than when each waste is solidified separately. Thermogravimetric (TG) and derivative thermogravimetric analysis (DTG) of 4 and 24 h pastes allow a quantitative information about the main cement hydrated phases and confirm the same accelerating or retarding effects due to the presence of wastes indicated from respective NCDTA curves.     

Investigation of ageing of alumina cement-based mixtures using thermal analysis and calorimetry

The shelf life of cement and cement-based dry mixtures is often determined by ageing of such materials. The ageing is the result of interactions between cement and other components of cementitious mixtures with moisture as well as with CO₂ from the atmosphere. In this work, the ageing behaviour of calcium aluminate cement and its mixtures with additives of microsilica, fluidized catalytic cracking catalyst waste and ground quartz sand were investigated. The ageing was achieved by storing cement and its mixtures in a climatic chamber for 7 and 14 days at 95% relative humidity and 20 ± 1 °C temperature. Applying thermal analysis, XRD analysis as well as scanning electronic microscopy, it was established that hydration of the cement minerals takes place along with carbonation during the ageing process of cement and its mixtures. The quantities of the products formed during ageing and their crystallinity depend on the nature of additives and the duration of ageing. When applying the method of calorimetric analysis, the influence of ageing on the kinetics of hydration of cement and as well as of its mixtures with the additives used in the work has been established.