The use of thermal analysis to investigate the effects of cellulose ethers on the Portland cement hydration

Thermal analysis (DTA) was used for monitoring the proportions of Ca(OH)₂ formed at the hydration of simple Portland cement (CEM I 42.5 R) samples, and cement samples with 0.5% addition of unmodified hydroxypropyl methyl cellulose (HPMC), respectively, with the addition of starch ether and polyacrylamide modified HPMC. The proportions of Ca(OH)₂ formed after 1, 3, 7, and 28?days of hydration were assessed by the peak areas of the endothermic effect at the temperature range of 493?C503?°C, caused by the Ca(OH)₂ decomposition. The results obtained based on thermal analysis reflect very well the correlation between the Ca(OH)₂ proportions in the samples after different hydration periods and the retarding effect of the hydration processes caused by the cellulose ether's addition. This retarding effect is also evidenced by the setting times of the studied samples and the evolution of their mechanical strengths.     

An alternative thermal method for identification of pozzolanic activity in Ca(OH)2/pozzolan pastes

Pozzolans play an important role in the industry of cement and concrete. They increase the mechanical strength of cement matrices and can be used to decrease the amount of cement in concrete mixtures, thus decreasing the final economic and environmental cost of production; also, as some of them are byproducts of industrial processes (such as silica fume and fly ash) and their use can be seen as a solution for some residues, that otherwise would be disposed as a waste. Pozzolans fixate the Ca(OH)₂ generated during cement’s hydration reactions to form calcium silicate hydrates (C–S–H), calcium aluminate hydrates (C–A–H), or calcium aluminosilicate hydrates (C–A–S–H), depending on the nature of the pozzolan. Traditionally, the pozzolanic activity is identified using the Ca(OH)₂ fixation percentage which is quantified by thermogravimetric (TG) analysis, using the mass loss due to the Ca(OH)₂ dehydroxylation around 500 °C. An alternative method to identify pozzolanic activity at lower temperatures using a standard issue moisture analyzer (MA) is presented in this paper, using the mass loss due to hydrate’s dehydration generated by pozzolans in the pozzolanic reaction. Samples of Ca(OH)₂ blended with different pozzolans were prepared and tested at different hydration ages. Using TG analysis and an MA, a good correlation was found between the total mass loss of the same sample, using the two methods at the same temperature. It was concluded that the MA method can be considered a less expensive and less time-consuming alternative to identify pozzolanic activity of siliceous or aluminosiliceous materials.     

Thermal analysis at the evaluation of concrete damage by high temperatures

Summary Concrete damage by high temperatures includes mass loss, strength and modulus reductions and the formation of cracks and large pores. Thermal treatment reduces the amount of chemically bound water in the hydrate phase. With a rise in temperature, the spatial distribution of Ca(OH)₂ crystals becomes more compact; smaller crystals occur in a unit volume of the cement paste. A rise in temperature affects the pore structure by reducing the specific surface of hydration products. Cement paste becomes more heterogeneous in microstructure and coarser in pore structure. Compressive strength is not only significant parameter showing structural integrity of concrete; permeability influences concrete durability as well. To demonstrate this, permeability coefficients at various high temperatures are presented. The key quantitative insight into the hydrate phase behavior is based on thermal analysis results. Thermogravimetric (TG) mass losses are related to the phase changes represented either by DTA or DTG. Based on these, the tests employing TG mass losses and related DTA and DTG curves answer the question if the hydrate phase is present at individual high-temperature levels and what its quantitative state is. Method of thermal analysis is suitable for the interpretation of concrete behavior when subjected to high-temperature attack. Conclusions are drawn about thermal stability and residual properties of concrete specimens made at the construction site of Mochovce nuclear power plant (Slovakia); and subjected to temperatures up to 800°C. Relations among mechanical properties, permeability, pore median radius and bound water content in concrete are discussed and evaluated.     

Thermoanalytical investigation of calcium chromate

Thermoanalytical methods have been used to study calcium chromate (CaCrO₄) samples. Thermal decomposition temperatures determined by TG in vacuum and by DTA in air could not be successfully used for screening purposes. Other thermogravimetry studies in air indicate that TG can be an effective quality control tool in screening out samples which have an assay value below 97.0% CaCrO₄. More complete analyses using TG in an argon atmosphere gave good results for CaCO₃ and H₂O content as well as for total CaCrO₄. Reliable measurement of Ca(OH)₂ was not achieved. Effluent gas analysis—mass spectrometry was used to identify gaseous products as a function of temperature, in order to verify interpretation of TG curves.Thermogravimetry, differential scanning calorimetry, nuclear magnetic resonance, and mass spectrometry have been used in an effort to explain the unusual weight loss observed between 400 and 600°C for many CaCrCO₄ samples. Ca(OH)₂ decomposition is not the primary cause of this weight loss, as originally suspected, but instead the loss appears to be due to volatization of H₂O trapped in the CaCrO₄ crystal.     

Thermal analysis and microstructure of Portland cement-fly ash-silica fume pastes

Thermal analysis (thermogravimetry and differential thermal analysis) was used with scanning electron microscopy technique to investigate the hydration mechanisms and the microstructure of Portland cement-Fly ash-silica fume mixes. Calcium silicate hydrate (C–S–H), ettringite, gehlenite hydrate (C₂ASH₈), calcium hydroxide (Ca(OH)₂) and calcium carbonate (CaCO₃) phases were detected in all mixes. In the mixes with the use of silica fume addition, there is a decrease in Ca(OH)₂ with increasing silica fume content at 5 and 10% compared to that of the reference Portland-fly ash cement paste and a corresponding increase in calcium silicate hydrate (C–S–H).     

The difference among the effects of high-temperature curing on the early hydration properties of different cementitious systems

The difference among the effects of high-temperature curing on the early hydration properties of the pure cement, the binder containing fly ash, the binder containing GGBS, and the binder containing steel slag was investigated by determining the compressive strength, non-evaporable water content, hydration heat, and Ca(OH)₂ content. Results show that the order of the influence degrees of high-temperature on the early hydration of different binders is the binder containing GGBS > the binder containing steel slag > the binder containing fly ash > the pure cement. In the case of short period of high-temperature curing (only 1 day), the strength growth rate of the concrete containing GGBS is the greatest. Though the influence of increasing high-temperature curing period on the hydration degree of the binder containing fly ash is not the most significant, the strength growth rate of the concrete containing fly ash is the most significant due to the excessive consumption of Ca(OH)₂ by reaction of fly ash. In the case of high-temperature curing, the Ca(OH)₂ content of the paste containing steel slag is much higher than those of the paste containing GGBS and the paste containing fly ash, so though high-temperature curing promotes the hydration of the binder containing steel slag significantly, its influence on the strength growth rate of the concrete containing steel slag is not so significant.     

Conversion of silica-containing additives upon testing of cement compositions for alkali expansion

The peculiar features of the conversion processes proceeding upon the mortar-bar tests [GOST(State Standard) 8269.0] of high-dispersion silica-containing additives (silica fume, metakaolin, and precipitated silica) in the composition of a cement stone and sand-cement mortars at 20 and 80°C were studied. According to the solid-state NMR spectroscopy at 80°C, the additives rapidly loss phase individuality by reacting with Ca(OH)₂ to form calcium silica hydrogel (C-S-H), with Portland cement hydration in the presence of mineral additives proceeding slower than in the initial stone. Compared to Portland cement gel, the C-S-H product formed by the additives is characterized by lower Ca/Si ratio, longer aluminum-silicon-oxygen structural chains, and by higher content of aluminum in them.     

Investigation regarding the effect of viscosity modifying admixtures upon the Portland cement hydration using thermal analysis

The effect of 13 viscosity modifying admixtures (VMA) on the Portland cement hydration was studied in this paper. In this purpose, thermal analyses (DTA and TG) were performed after 1, 7 and 28 days of hydration on cement pastes containing 0.01–0.5 % from the following VMA: diutan gum, welan gum, polygalactomannane ether, natural cellulose fibres, modified polysaccharide, polyacrylamide, high-molecular mass synthetic copolymer, hydroxypropyl starch and a chemically modified starch. It was noticed that the proportion of Ca(OH)₂ from the samples containing polygalactomannane ether and modified polysaccharide was smaller than in the reference sample, which proved their effect of cement hydration delay. For the other VMA, this effect was not detected, on the contrary, the amount of Ca(OH)₂ was higher than in the reference sample.     

CO2 sequestration by high initial strength Portland cement pastes

During the formation of pastes, mortar and concretes have been used to capture CO₂. This work presents a methodology to estimate the carbon dioxide (CO₂) sequestered by high strength and sulfate-resistant Portland cement pastes during their early stages of hydration, by Thermogravimetry and Derivative Thermogravimetry. Water to cement ratio equal to 0.50 and 0.70 were evaluated and the captured CO₂ amount was determined through TG/DTG curve data on initial cement mass basis, obtained during accelerated carbonation from the fluid state and accelerated carbonation after a first hydration process. The experiments were performed in a controlled chamber, maintaining the CO₂ content at 20 vol % and the temperature at 25 °C, at different relative humidity (RH) (60 and 80 %) ambient. The procedure allows one to estimate the amount of CO₂ sequestered by the initial cement mass of a given volume of paste, as well as to evaluate the RH and W/C ratio influence on the amount of hydrated formed products, mainly on the Ca(OH)₂, important for CO₂ fixation.     

Calcium Hydroxide Chlorides: The Ternary System Ca(OH)2‐CaCl2‐H2O at 25, 40, and 60 °C,Phase Stoichiometry and Crystal Structure

The formation of calcium hydroxide chlorides is an important issue in processes of the chemical raw materials industry, in terms of purification of flue gases, and concrete/cement corrosion. Current information on phase compositions given in the literature are contradictory. In this work systematic solubility studies were carried out at 25, 40, and 60 °C in the system Ca(OH)₂‐CaCl₂‐H₂O and the compositions of the ternary solid phases were precisely determined using the Schreinemakers' method. Two ternary phases were identified, the hydrate 3Ca(OH)₂ · CaCl₂ · 12H₂O and the anhydrous calcium hydroxide chloride Ca(OH)₂ · CaCl₂. The crystal structure of 3Ca(OH)₂ · CaCl₂ · 12H₂O was solved by means of single‐crystal X‐ray diffraction at –123 °C (150 K).     

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.     

Calorimetric and thermal analysis studies on the influence of waste aluminosilicate catalyst on the hydration of fly ash–cement paste

Binders containing large amounts of cement substitutes have been a subject of interest for many years because of the possibility to reduce the amount of cement in concrete, and in consequence decrease negative influence of cement production on natural environment. In this work, studies related to hydration of binders where 80 % of cement was substituted by blended pozzolana were carried out. The aim of this work was to investigate activation of fly ash–cement system by addition of spent aluminosilicate catalyst, using calorimetry and thermal analysis as main methods of investigations. It was demonstrated that spent fine-grained fluidised catalytic cracking catalyst acts acceleratingly on early hydration of binder. It seems to be beneficial to use up to 10 mass% of this spent catalyst. Higher amounts may cause changes in the mechanism of early hydration. Because Ca(OH)₂ in such systems is quickly consumed due to pozzolanic reaction it seems beneficial to modify composition of binders by introducing additional amounts of Ca(OH)₂ or cement.     

Sulphate resistance and passivation ability of the mortar made from pozzolan cement with zeolite

Sulphate resistance and passivation ability of the mortars made from pozzolan cement of CEM IV/A (P) type according to European Standard EN 197-1 (zeolite blended cement with 60.82 mass% of PC clinker, 35.09 mass% of zeolite and 4.09 mass% of gypsum abbreviated as ZBC) and ordinary Portland cement (abbreviated as PC) are introduced. Resistance tests were performed in water and 5% sodium sulphate solution (both 20°C) for 720 days. The increased sulphate resistance of pozzolan cement relative to that of PC was found. The key quantitative insight into the hydrate phase behaviour is given by thermal analysis. This is due to pozzolanic reaction of zeolite with PC resulting in reduction of the formed Ca(OH)₂ opposite to the reference PC. Ability of pozzolan cements with 15 to 50 mass% of zeolite to protect steel against corrosion was verified in 20°C/85% RH-wet air within 180-day cure. Steel was not corroded in the mortars made with pozzolan cement containing up to 35 mass% of zeolite. Pozzolan cement of CEM IV/A (P) type containing 35 mass% of zeolite is a suitable cementitious material for concrete structures exposed to sulphate attack. Steel is protected against corrosion by this pozzolan cement in the same measure as the reference PC.     

A Study on the Hydration of Portland Limestone Cement by Means of TG

Subject of this paper is to investigate the hydration process of Portland limestone cement containing 10-35% limestone. Cements, produced by co-grinding of clinker, limestone and gypsum, were hydrated for periods 6 h to 28 d and were studied by means of TG and XRD. The Ca(OH)₂ content of the cements containing limestone is higher than in pure cements, specifically for 10% limestone content and ages more than 1 day. These results are in accordance with the strength development of the studied cements. In earlier ages the Ca(OH)₂ content is slightly lower in the limestone cements and independent of the limestone content. After 1 day curing, the increase of limestone addition causes a relative increase of the non evaporable water. The XRD patterns indicated the presence of carboaluminates in the hydrated limestone cements. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Thermal behaviour of poly(allyl azide)

The paper describes the synthesis of low molecular mass poly(allyl chloride) (PAC) (M _n= 856-3834 g mol^-1) using Lewis acid (ALCL₃, FeCL₃, TiCL₄) and al powder. Branching in PAC was indicated on the basis of elemental analysis and ¹H-NMR spectroscopy. azidation of pac could be carried out at 100°C by using NaN₃ and DMSO as solvent. Curing of poly(allyl azide) (PAA) by cyclic dipolar addition reaction with EGDMA (ethylene glycol dimethacrylate, 5-45 phr) was investigated by differential scanning calorimetry and structure of cured polymer was confirmed by FTIR. A two-step mass loss was exhibited by uncured and cured PAA in nitrogen atmosphere. A mass loss of 20-28% (155-274°C) and 50-61% (330-550°C) was observed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis and thermal properties of cellulose-based polycaprolactones

Cellulose-based polycaprolactone (CAPCL) sheets were prepared from cellulose acetate (CA) and ϵ-caprolactone (CL). Thermal properties of the obtained CAPCL's were studied by differential scanning calorimetry (DSC), thermogravimetry (TG) and TG-Fourier transform infrared spectrometry (TG-FTIR). The glass transition temperatures (T_g 's) of CAPCL decreased with increasing CL/OH ratio, until CL/OH ratio reached 15 and then increased above that ratio. Melting of CAPCL was observed when CL/OH ratio was over 10. The thermal degradation temperatures (T_d 's) of CAPCL increased from ca. 350 °C to 390 °C with increasing CL/OH ratio. The results obtained by TG-FTIR analysis of CAPCL showed that gases with OH, CH, C=O, C-O-C groups evolved by thermal degradation.     

Thermal decomposition of natural iowaite

The thermal decomposition of natural iowaite of formula Mg₆Fe₂(Cl,(CO₃)_0.5)(OH)₁₆·4H₂O was studied by using a combination of thermogravimetry and evolved gas mass spectrometry. Thermal decomposition occurs over a number of mass loss steps at 60°C attributed to dehydration, 266 and 308°C assigned to dehydroxylation of ferric ions, at 551°C attributed to decarbonation and dehydroxylation, and 644, 703 and 761°C attributed to further dehydroxylation. The mass spectrum of carbon dioxide exhibits a maximum at 523°C. The use of TG coupled to MS shows the complexity of the thermal decomposition of iowaite. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal stability of crandallite CaAl<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>(OH)<Subscript>5</Subscript>·(H<Subscript>2</Subscript>O)

Thermogravimetry combined with evolved gas mass spectrometry has been used to characterise the mineral crandallite CaAl₃(PO₄)₂(OH)₅·(H₂O) and to ascertain the thermal stability of this ‘cave’ mineral. X-ray diffraction proves the presence of the mineral and identifies the products of the thermal decomposition. The mineral crandallite is formed through the reaction of calcite with bat guano. Thermal analysis shows that the mineral starts to decompose through dehydration at low temperatures at around 139 °C and the dehydroxylation occurs over the temperature range 200–700 °C with loss of the OH units. The critical temperature for OH loss is around 416 °C and above this temperature the mineral structure is altered. Some minor loss of carbonate impurity occurs at 788 °C. This study shows the mineral is unstable above 139 °C. This temperature is well above the temperature in the caves of 15 °C maximum. A chemical reaction for the synthesis of crandallite is offered and the mechanism for the thermal decomposition is given.     

Thermal studies on Zirconium hydroxide gel formed by aqueous gelation

Zirconium hydroxide gel has been prepared by a novel aqueous gelation process by the controlled hydrolysis of zirconium oxychloride in the presence of sodium acetate. The gel thus formed has been subjected to thermal analysis: TG, DTG, and DSC. Thermal analysis shows that the gel is continuously dehydrated in the temperature range between room temperature and 500?°C. The total mass loss relative to the initial mass is about 44.1%. Thermal analysis shows that the decomposition takes place in three stages. The gel contains absorbed and coordinated water. In the second stage of dehydration, dehydration of the Zr(OH)₄ gel also takes place along with the removal of the coordinated water. The DSC analysis coupled with TG and structural information, indicate that the exothermic processes between 349 and 460?°C can be attributed to the nucleation process of the formation of tetragonal zirconia, with phase transformation at 460?°C.