Calorimetric studies of special cements

The kinetics and even the mechanism of cement reaction with water can be successfully investigated by use of microcalorimetry. In this study this method was applied to follow the hydration of the new family of portland cements containing C₁₂A₇ ^* and C₁₁A₇·CaF₂ addition as well as special cement with C₃A replacement by calcium sulphoaluminate. It has been found that C₁₁A₇·CaF₂ acted as hydration retarder. The heat evolution curves for C₁₂A₇ containing samples without CaF₂ are very similar to those for the reference portland cement samples. XRD and SEM studies confirm the results described above, relating to the retardation of alite hydration. The process is positively modified by the addition of anhydrite. In the presence of calcium sulphoaluminate (4CaO·3Al₂O₃·SO₃) the hydration at early stage occurs with the rapid formation of large amount of the ettringite phase. The calcium fluoride acts as a set retarder. The full compatibility of calorimetry with SEM and XRD results should be underlined. In cement chemistry the following notation is used:C=CaO,A=Al₂O₃,S=SiO₂,H=H₂O etc. for the main oxide constituents of portland cement clinker and hydrates.     

Physical and chemical studies on cement containing sugarcane molasses

Molasses is generally used as a grinding aid in cement and as a water reducer and retarder in concrete. In China, the output primarily consists of sugarcane molasses. In this paper, the effects of sugarcane molasses on the physical performance and hydration chemistry of conventional Portland cement were investigated. The setting times, the normal consistency of cement pastes, the compressive strengths and fluidities of the mortars were respectively determined according to Chinese Standard GB/T 1346, GB/T17671 and GB/T 2419. The effect of molasses on the hydration kinetics of cement was investigated using a calorimeter. The hydration products and pore size distribution of the cement pastes were analysed by X-ray powder diffraction, differential scanning calorimetry and a mercury injection apparatus. The results show that a small amount of sugarcane molasses retards the setting and hardening of cement paste and increases the fluidity of cement mortar, while excess molasses accelerates the setting and hardening. Molasses improves significantly the compressive strength at 3d due to the decrease of porosity. The addition of 1.0 % molasses accelerates the formation of ettringite, prevents the second hydration of aluminate phase and delays the hydration of C₃S.     

Thermodynamics and kinetics of hydrogen absorption–desorption of vanadium synthesized by aluminothermy

This paper studies the addition (0–40% w/w) of natural zeolite (NZ, 84% clinoptilolite) in blended cements made with Portland cement (PC) with low and medium C₃A content. The isothermal calorimetry was used to understand the effect of NZ on the early cement hydration. For low C₃A cement, the addition of NZ produces mainly a dilution effect and then the heat released curve is similar to plain cement with lower intensity. For medium C₃A cement, the curve shows the C₃S peak in advance and a high intensity of third peak attributed to C₃A hydration. The high cation fixed of NZ reduces the ions concentration (especially alkalis) in the mixing water stimulating the PC hydration. The flowability decreases when the NZ replacement level increases. Results of Fratini’s test show that NZ with both PCs used presents slow pozzolanic activity. At early age, XRD and FTIR analyses confirm that hydration products are the same as that of the corresponding PC and the CH is progressively reduced after 28 days and some AFm phases (hemi- and monocarboaluminate) appear depending on the NZ percentage and the PC used. For low replacement levels, the compressive strength is higher than the corresponding PC from 2 to 28 days. For high replacement levels, the early compressive strength is lower than that of corresponding plain PC and the pozzolanic reaction improves the later compressive strength of blended cements.     

Application of conduction calorimetry for study of the reactivity of C2S in the system C2S-C4A3¯S-C¯S-H

A conduction calorimeter was used to determine the effect of the ignition temperature on the hydration reactivity of C₂S in the system C₂S-C₄A₃¯S-C¯S relevant to sulphoaluminate belite cement. The results thus obtained showed that the kinetics and mechanism of hydration of the system C₂S-C₄A₃¯S-C¯S are influenced by the temperature of C₂S synthesis. The hydration reactivity of C₂S increases with increasing temperature of ignition.     

Influence of borax and citric acid on the hydration of calcium sulfoaluminate cement

The hydration of calcium sulfoaluminate cement in the presence of borax and citric acid has been studied using isothermal calorimetry, X-ray diffraction, thermogravimetric analysis, Fourier transfer infrared spectrometry and mercury intrusion porosimetry. The results suggest that the formation of complex between calcium and citrate in the solution adsorbed on the solid surface delayed the C₄A₃ \( \bar{S} \) and anhydrite dissolution and AFt nucleation. At the same time, the presence of borate replaced sulfate anion in ettringite (AFt) to form B-AFt. These reactions have an effect on the setting times and the mechanical strength values. The setting times of the calcium sulfoaluminate cement increased when the mineral admixtures increased in the cement. The effect of borax and citric acid on the strength development at different hydration stages presented different results, namely a decreased strength in an early stage and an increased strength at 3 and 28 days.     

Effects of nano-limestone on hydration properties of tricalcium silicate

In this research, the influences of nano-limestone (NC) on tricalcium silicate (C₃S) hydration, including early hydration rate, setting time, Ca(OH)₂ content, quantification of C–S–H, micrographs of hydration products and compressive strength, were investigated by different methods. Calorimetry results show that the first and second exothermic peak of C₃S with 1 mass% NC is ~29 and ~5% higher than that of control sample and the second peak appears 32 min in advance. Besides, the total hydration heat of C₃S with 1 mass% NC increases by 10.08% compared to control sample at 72 h. The initial and final setting time of C₃S with 1 mass% NC shortens by 23 and 17 min, respectively. In addition, thermal analysis results show that adding NC leads to higher Ca(OH)₂ and C–S–H content at all ages (1, 3 and 28 days). All of the above results imply that the hydration progress of C₃S is accelerated by NC. Correspondingly, the compressive strength of C₃S with 1 mass% NC increases by 9.88% at 28 days. Besides, EDS results reveal that the Ca/Si ratio of C–S–H gel decreases after NC modification. However, excessive content NC (4 mass%) reduces the acceleration effect of NC on C₃S hydration and finally leads to relative low content of Ca(OH)₂ and C–S–H compared with C₃S sample containing 1 mass% NC.     

Determination of activation effect of Ca(OH)<Subscript>2</Subscript> upon the hydration of BFS and related heat by isothermal calorimeter

Isothermal conduction calorimetry, differential thermal analysis (DTA)–thermogravimetric analysis (TG) analysis, and SEM observations have proved the activation effect of Ca(OH)₂ released from the C₃S hydration upon blast furnace slag (BFS). Five sample mixtures of BFS and C₃S and two samples of pure BFS and C₃S were submitted to reaction with water inside the calorimeter at room temperature. The values of hydration heat were recorded up to 7 days. Samples were stored in humidity during 28 days and then were submitted to DTA–TG and SEM analysis. The effect of Ca(OH)₂ upon heat evolution of sample mixtures has been quantified and its influence upon the formation of new hydrates and microstructure of pastes was evidenced.     

The study of 'DSP' binding systems by thermogravimetry and differential thermal analysis

The so-called DSP (Densified Systems containing homogenously arranged Particles) systems represent a high-performance class of inorganic binders. The hydration and hardening processes of some DSP systems, based on calcium silicates (C₃S and C₂S) or Portland cement/clinker with silica fume additions, were assessed, in this paper, using the thermogravimetry (TG) and differential thermal analysis (DTA). These data permit a qualitative and quantitative study of the formed hydrates as well as the estimation of hydration process kinetics. This revised version was published online in August 2006 with corrections to the Cover Date.     

Degree of hydration in cement paste and C<Subscript>3</Subscript>A-sodium carbonate-water systems

This research provides a fundamental understanding of the early stage hydration of Portland cement paste, tricalcium aluminate (C₃A) paste at water to cement ratio of 0.5 and C₃A suspension at water to cement ratio of 5.0 modified by 2 or 4 mass% of sodium carbonate. A high conversion of unreacted clinker minerals to gel-like hydration products in the cement-Na₂CO₃ pastes takes place rapidly between 1^st to 24^th h. Contrary the Ca(OH)₂ formation within the same time interval is retarded in the excess of CO₃²⁻ ions due to intensive rise and growth of CaCO₃ crystals in hydrated cement. Later, the conversion of clinker minerals to the hydrate phase is reduced and higher contents of calcite and vaterite relative to that of Ca(OH)₂ in comparison with those found in the Portland cement paste are observed. As a consequence a decrease in strength and an increase in porosity between hardened Portland cement paste without sodium carbonate and those modified by Na₂CO₃ are observed. C₃A hydrates very quickly with sodium carbonate between 1^st and 24^th h forming hydration products rich in bound water and characterized also by complex salts of (x)C₃A·(y)CO₂·(zH₂O type, whereas C₃A-H₂O system offers C₃AH₆ as the main hydration product. Higher content of the formed calcium aluminate hydrates in C₃A-Na₂CO₃-H₂O system also contributes to early strength increase of Portland cement paste.     

Influence of Sn on the hydration of tricalcium aluminate, Ca3Al2O6

Tricalcium aluminate (Ca₃Al₂O₆, C₃A) containing 0?C5% of Sn was synthesized by solid-state method, and the products were characterized by XRD technique. Differential thermo-analytical technique (DTA) along with X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) were applied to study the hydration behaviour of different C₃A samples with and without the presence of gypsum. Results indicate that C₃A can accommodate small amount of Sn in its structure and remaining amount forms SnO₂. Hydration studies of the synthesized C₃A shows that the additions of 0.5 and 1% Sn increase and 2% Sn decrease the reactivity of C₃A at the initial period (<3?h) of hydration. Increasing additions of Sn also increase the amounts of amorphous phases and hexagonal calcium aluminate hydrates in the cement pastes. The stabilities of these hydration products also increase with increasing content of Sn in C₃A at the experimental conditions. The presence of Sn significantly changes the hydration of C₃A and gypsum solid mixture at the initial period of hydration by enhancing the formation of more amounts of AFt and AFm phases. However, at the later stage of hydration (on or after 3?days), the hydration products in C₃A and gypsum pastes with and without the presence of Sn are almost similar.     

Hydration of Portland cement with alkanolamines by thermal analysis

Four types of alkanolamines (i.e., traditional alkanolamines represented by TEA and TIPA and new alkanolamines represented by DEIPA and EDIPA) were added to Portland cement as chemical additives, and their effects on the cement properties and hydration process were investigated. An isothermal calorimeter was used to track the hydration heat flow of the cement pastes with or without alkanolamines. Thermogravimetric analyses were performed to measure the degree of hydration over the course of 28 days. In addition, X-ray diffraction, MIP analysis and SEM were used as auxiliary tests. The results indicated that alkanolamines improved the compressive strength of the cement mortars. It was found that TEA increased the rate of the second hydration of C₃A, and TIPA accelerated the hydration of C₄AF. DEIPA and EDIPA promoted the hydration of both the aluminum and ferrite phases as well as catalyzed the conversion of AFt to the AFm phase. By contrast, the new alkanolamines represented by DEIPA and EDIPA expressed more superior properties.     

Lessons learned from fires of the wood caused by the spontaneous combustion of coal dust in underground mines

The capture of CO₂ and SO₂ from industrial gas effluents has been done usually by lime-containing products. For this purpose, cement pastes also can be used, due mainly to their calcium hydroxide content formed during hydration. To select the best cement for this purpose, TG and DTG curves of different Portland cement pastes (types I, II, III and G), prepared with a water-to-cement ratio (W/C) equal to 0.5, were analyzed at different ages, at same operating conditions. The curves were transformed into respective cement calcined and initial mass basis, to have a common and same composition reference basis, for a correct quantitative hydration data comparison. This procedure also shows that there is an unavoidable partial drying effect of the pastes before starting their analysis, which randomly decreases the W/C ratio at which were prepared, which indicates that, when results are compared on respective paste initial mass basis, assuming that the ratio W/C has not changed, possible calculation errors may be done. Type I, II and G analyzed cements have shown similar hydration characteristics as a function of time, while the analyzed type III cement has shown a different hydration behavior, mainly due to its highest Al₂O₃ and lowest SO₃ contents, promoting the formation of hydrated calcium aluminates, by the pozzolanic action of the excess of alumina, consuming Ca(OH)₂, which final content at 28 days was the lowest one, among the hydrated cements.     

Effect of addition of nano-magnetite on the hydration characteristics of hardened Portland cement and high slag cement pastes

In this investigation the effect of addition of magnetite nanoparticles on the hydration characteristics of both ordinary Portland cement (OPC) and high slag cement (HSC) pastes was studied. The cement pastes were prepared using a water/solid (W/S) mass ratio of 0.3 with addition of 0.05, 0.1, and 0.3 % of magnetic fluid Fe₃O₄ nanoparticles by mass of cement. An aqueous stable magnetic fluid containing Fe₃O₄ nanoparticles, with a mean diameter in the range of super-paramagnetism, was prepared via co-precipitation method from ferrous and ferric solutions. The admixed magnetite-cement pastes were examined for compressive strength, chemically combined water content, X-ray diffraction analysis, and differential scanning calorimetry. The results of compressive strength revealed that the hardened pastes made from OPC and HSC admixed with different amounts of magnetic fluid have higher compressive strength values than those of the neat cement OPC and HSC cement pastes at almost all ages of hydration. The results of chemically combined water content for the admixed cement pastes showed almost the same general trend and nearly comparable values as those of the neat cement pastes. From the XRD diffractograms obtained for the neat OPC and HSC cement pastes, the main hydration products identified are calcium silicate hydrates, portlandite, and calcium sulfoaluminate hydrates. Addition of magnetic fluid nanoparticles to both of OPC and HSC did not affect the main hydration products of the neat OPC or HSC cement in addition to one main basic difference, namely, the formation of calcium iron hydroxide silicate as a new hydration product with a reasonable hydraulic character.     

Improvement of surface cementitious properties of coarse fly ash by dehydration and rehydration processes

Owing to poor bonding between coarse fly ash particles and hydration products, gap-graded blended cements with fly ash usually show lower compressive strengths than Portland cement. Surface cementitious properties of coarse fly ash were improved by dehydration and rehydration processes in the present study. The results show that during the calcination at 750?°C, C?CS?CH gel is mainly transformed into a new nesosilicate, which is similar to a less crystalline C₂S. The formation of melilite from hydration products is also noticed at 900?°C, however, this will not contribute to rehydration of calcined fly ash. Rehydration of new generated nesosilicate on the surface of coarse fly ash leads to a better bonding between coarse fly ash particles and hydration products. As a result, both early and late mechanical properties of gap-graded blended cements containing 25% cement clinker and 39% calcined coarse fly ash are higher than those of 100% Portland cements.     

Sinterization and hydration of synthesized cement clinker doped with sulfates

This paper aims to evaluate the influence of three kinds of sulfates from the green production of cement on its sintering and hydration. The properties of clinker and hydration were monitored by thermogravimetric and differential thermal analysis (TG–DTA), X-ray diffraction, X-ray fluorescence and isothermal conduction calorimeter. Results indicate that gypsum lowers the decomposition temperature of CaCO₃ and all these Sulfates will enhance the solid-phase reaction but increase melting temperature. Sulfates reduce the content of C₃S, but K₂SO₄ and 2CaSO₄·K₂SO₄ is conducive to the formation of β-C₂S. The hydration induction period is shortened by the sulfates. K₂SO₄ and 2CaSO₄·K₂SO₄ improve the early hydration of clinker, but gypsum may lightly reduce the hydration reactivity of clinker in acceleration period. 2CaSO₄·K₂SO and K₂SO can significantly accelerate the compressive strength development of cement clinker before 3 d; by contrast, gypsum is detrimental for that. The precipitation of hydration products (CH and C–S–H) in clinker with sulfates is more than that of clinker without sulfates at 9 h. K₂SO₄ can accelerate the hydration of clinker without forming ettringite.

     

Influence of hydrothermal curing regimes on the hydration of fiber-reinforced cement composites

Simultaneous thermal analysis (DTA and TG) was applied to two fiber-reinforced cement composites cured in autoclave at 0.3, 1.2, and 2.0 MPa for 24, 72, and 168 h. Mercury intrusion porosimeter (MIP) and scanning electronic microscope (SEM) were used to complete the investigations. Thermal analysis was used to identify temperature ranges of thermal decomposition of cured samples and to characterize the nature of hydrate products. Also, fractured surface and surface microstructure were investigated by means of SEM. Based on the results of thermal analysis, three processes occurred during hydrothermal curing. The first one consists of primary hydration reaction leading to the formation of C–S–H, C₃AH₆, and AH₃. The second one is the process characterized by partial decomposition or interaction of primary products to form secondary products (C₂ASH₈, C₃AH_1.5). The last one is the carbonation of some hydrate products. Hydrothermal pressure can enhance the compressive strength, but can also cause its depletion over time. The MIP study has revealed an existence of bimodal pore size distribution that the characteristic depends on the hydrothermal curing conditions.     

Quantitative study of hydration of C<Subscript>3</Subscript>S and C<Subscript>2</Subscript>S by thermal analysis

This research is part of a European project (namely, CODICE project), main objective of which is modelling, at a multi-scale, the evolution of the mechanical performance of non-degraded and degraded cementitious matrices. For that, a series of experiments were planned with pure synthetic tri-calcium silicate (C₃S) and bi-calcium silicate (C₂S) (main components of the Portland cement clinker) to obtain different calcium–silicate–hydrate (C–S–H) gel structures during their hydration. The characterization of those C–S–H gels and matrices will provide experimental parameters for the validation of the multi-scale modelling scheme proposed. In this article, a quantitative method, based on thermal analyses, has been used for the determination of the chemical composition of the C–S–H gel together with the degree of hydration and quantitative evolution of all the components of the pastes. Besides, the microstructure and type of silicate tetrahedron and mean chain length (MCL) were studied by scanning electron microscopy (SEM) and ²⁹Si magic-angle-spinning (MAS) NMR, respectively. The main results showed that the chemical compositions for the C–S–H gels have a CaO/SiO₂ M ratio almost constant of 1.7 for both C₃S and C₂S compounds. Small differences were found in the gel water content: the H₂O/SiO₂ M ratio ranged from 2.9 ± 0.2 to 2.6 ± 0.2 for the C₃S (decrease) and from 2.4 ± 0.2 to 3.2 ± 0.2 for the C₂S (increase). The MCL values of the C–S–H gels, determined from ²⁹Si MAS NMR, were 3.5 and 4 silicate tetrahedron, for the hydrated C₃S and C₂S, respectively, remaining almost constant at all hydration periods.     

Investigation of early hydration of high aluminate cement-based binder at different ambient temperatures

The effect of spent FCC catalyst on early hydration (up to 48?h) of high aluminate cement (Al₂O₃ >70%) at different ambient temperatures (10, 20, and 30?°C) was investigated. Cement pastes with constant ratio of water/binder?=?0.35 (binder?=?cement?+?addition) and containing 0, 5, 10, and 15% mass of addition as replacement of cement were studied. The hydration kinetics was determined by calorimetric measurements and the structure of hardened binders after 2?days of curing at an appropriate temperature was also investigated using X-ray, SEM, and thermal analysis methods. Due to the fact that hydration of aluminate cements is highly sensitive to temperature conditions as well as certain changes of temperature are inevitable in practice, the evaluation of the impact of the waste catalyst addition in such conditions is justified. On the basis of obtained results, it was stated that the temperature determines the early hydration of high aluminate cement and decides about the influence of waste aluminosilicate. The introduction of the discussed addition has a big impact on the kinetics of cement hydration closely related to the curing temperature. The presence of spent catalyst accelerates the hydration at the temperatures of 20 and 30?°C, but at the temperature of 10?°C this waste aluminosilicate acts as a retarding agent. The effect of the addition on the microstructure of hardened binders after 48?h of hydration is rather insignificant, especially at 20?°C, compared to the influence of the temperature on hydration. At the temperature of 10?°C, a formation of low amount of C₂AH₈ can be observed because of the presence of spent catalyst, while at the temperature of 30?°C the introduction of the mineral addition prevents the hydrogarnet formation.     

Physicochemical Characteristics of Styrene-Butadiene Latex- modified Mortar Composite vis-à-vis Preferential Interactions

The interaction between styrene-butadiene rubber (SBR) film and the ions from C₂S and C₃S hydration of Portland cement mortar composites has been evaluated by Fourier Transform Infrared Spectroscopy (FTIR), and the morphology of the composites characterized with scanning electron microscopy (SEM). The specimen used was cured for 28 days. FTIR spectrum supports the interaction of SBR with cement in the composite. Compressive strength, bulk density and water absorption properties of the cured composites were tested. Addition of SBR latex in Portland cement mortar increases the compressive strength and decreases the water absorption. Bulk density study revels interface formation in the composite. The role of the interface in relation to compressive strength of the composite has been discussed. A simple Matrix System model is shown to account composition dependence of bulk density.     

Heat of hydration of high reactive pozzolans in blended cements: Isothermal conduction calorimetry

A study was carried out comparing silica fume (SF) and dealuminated kaolin (DK) as pozzolanic materials in blended cements. Ten, 20 or 30 wt% of SF or DK were substituted for Portland cement. The kinetics of hydration up to 45 h were studied using isothermal conduction calorimetry. Blends containing pozzolanic materials usually have decreased heats of hydration compared to pure cement during the period of C₃S hydration, i.e. during the main hydration peak. Depending on the chemical composition and the activity of the pozzolan, the reaction taking place with the lime typically contributes to the heat output after the main hydration peak.The pozzolanic activity of DK is the principal factor and heat evolution increases with respect to pure PC mortar, during the first 15 h. The presence of hydrated silica (silanol groups) in DK increases the pozzolanic activity especially before and during induction period. The acidic silanol sites are capable of a fast acid-base reaction with the alkalis and with any Ca(OH)₂ present in cement during the induction period.