Hydraulic activity of C4A3Cr in presence of C4A3S

This paper deals with synthesis and assessment of the hydraulicity of C₄A₃Cr, analog phase C₄A₃S to , relevant to the phase chemistry and properties of sulfoaluminate cements. C₄A₃Cr, synthesized at 1250 °C is well crystallized phase, latently hydraulic, with hydration accelerated in the presence of C₄A₃S, or CS. Calorimetric curves show reciprocal influence of sulfate and chromate phase in hydration of C₄A₃S-C₄A₃Cr system. The total heat expressed at hydration is nearly the same for all specimens, but the rate of heat evolution depends on the ratio of C₄A₃S/C₄A₃Cr phases. X-ray diffraction pattern and DTA curves showed that, increasing content of C₄A₃Cr in hydrating mixture results in a coexistence of two types of ettringite (chromate and sulfate ettringite) hydrogarnet, gibbsite and monosulfate. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydration of high alumina cement–silica fume composite with addition of Portland cement or sodium polyphosphate under hydrothermal treatment

Various hydrothermal curing regimes were used to investigate the hydration and physical characteristics of two kinds of inorganic binder composites: high alumina cement–silica fume–Portland cement and high alumina cement–silica fume–sodium polyphosphate. Simultaneous thermal analysis (DTA and TG) was used to identify temperature ranges of thermal decomposition of cured samples and to characterize the nature of hydrate products. Two kinds of products are formed. The first ones consist of C₃AH₆, AH₃, calcium carbonate (C–C) as a product of carbonation, and C₃AH_1.5 resulted from the partial decomposition of C₃AH₆ under higher hydrothermal pressure. The second ones are the products formed by acid–base reaction between monocalcium aluminate and sodium polyphosphate to form NaCaPO₄·xH₂O and Al₂O₃·xH₂O that could convert to chemically bonded ceramic binders like hydroxyapatite (Ca₅(PO₄)₃OH) and gibbsite (Al(OH)₃). These two hydroceramic products formed under these conditions act also as binder and could be useful as cement binders for the protection of petroleum, gas, or geothermal wells. Mercury intrusion porosimeter was used for the estimation of the pore structure parameters of the composites. It turned up that longer curing time coupled with higher hydrothermal pressure has improved the pore structure of the first composite, while that of the second has remained unchanged.     

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.     

Effect of additives on the performance of Dyckerhoff cement,Class G,submitted to simulated hydrothermal curing

Stability of Dyckerhoff cement Class G partially substituted (15 mass%) by metakaolin (MK), silica fume (SF) and ground granulated blast-furnace slag (BFS) was investigated after 7 days of curing under standard and two different autoclaving conditions. Mercury intrusion porosimetry, X-ray diffraction analysis and combined thermogravimetric–differential scanning calorimetry were used to evaluate pore structure development, compressive strength and their dependence on the type of additives in relation to the particular phase composition. Hydrothermal curing led to the formation of α-C₂SH and jaffeite, mostly in the case of referential samples and compositions with addition of slowly reacting BFS. Whilst modest hydrothermal curing (0.6 MPa, 165 °C) favoured formation of α-C₂SH, larger amounts of jaffeite were determined after curing at the highest used pressure and temperature (2.0 MPa, 220 °C). Undesired transformation of primary hydration products was prevented especially by addition of highly reactive and very fine SF. Particular composition attained the best pore structure characteristics and compressive strength after curing at 0.6 MPa and 165 °C. Formation of more stable phases with C/S ratio close to 1 was proved by wollastonite formation during DSC analyses. More severe conditions of curing, however, led to the significant deterioration of microstructure and strength of corresponding sample, probably due to the formation of trabzonite, killalaite and zoisite. Considering the values of hydraulic permeability coefficient and compressive strength, replacement of cement by MK improved significantly the properties of cement when compared with the referential as well as with other blended compositions under the mentioned curing conditions.     

Thermal stability of Li2O-SiO2-CaO-P2O5-F glass

Two glasses, the first one with the composition of Li₂O·2SiO₂ and the second one with the addition of CaO, P₂O₅ and CaF₂ in the stoichiometric ratio corresponding to fluoroapatite were prepared and their tendency to crystallize has been studied by non-isothermal DTA analysis. The values of kinetic parameters calculated using the isoconversional integral method have been used to determine the temperature dependencies of both the length of isothermal induction period and the length of overall isothermal crystallization for both glasses. The estimated dependencies indicate that the glass containing CaO, P₂O₅ and CaF₂ has a lower thermal stability.     

Microwave‐assisted synthesis of 2‐(2‐pyridyl)azoles. Study of their corrosion inhibiting properties

An efficient and fast procedure for the synthesis of 2‐(2‐pyridyl)azoles is described using ionic liquids as catalysts under microwave irradiation. The X‐ray crystallographic analyses for three of the four synthesized compounds are presented. Potentiodynamic polarization studies were carried out to analyze the electrochemical behavior of the compounds in corrosive acidic media. Of the four derivatives, one compound was detected to be an effective corrosion inhibitor prototype for oil refinery environments.     

Mechanism and kinetics of glass-ceramics formation in the LiO2-SiO2-CaO-P2O5-CaF2 system

Two glasses based on lithium disilicate (LS₂), with and without fluorapatite (FA), were synthesised in the Li₂O-SiO₂-CaO-P₂O₅-CaF₂ system with P₂O₅: CaO: CaF₂ ratios corresponding to fluorapatite. Glass-ceramics have then been prepared by thermal treatment. The mechanism and kinetics of crystallization as functions of grain size and rate of heating were investigated using thermal analysis methods. The smaller particles crystallize preferentially by surface crystallization, which is replaced by volume crystallization at larger particle sizes. Inclusion of FA in the LS₂ favours crystallization through the surface mechanism. The onset limit for volume crystallization replacing the surface mechanism is at about 0.3 mm for pure LS₂ glass and 0.9 mm for glass containing FA. The calculated activation energies of the glasses (₂99 ± 1 kJ mol-1 for pure LS₂ glass and ₂88 ± 7 kJ mol⁻¹ for glass containing FA according to Kissinger, or 313 ± 1 kJ mol-1 for pure LS₂ glass and 303 ± 8 kJ mol-1 for glass containing FA according to Ozawa) indicate that the tendency of the glasses to crystallize is supported by the FA presence. Bioactivity of all samples has been proved in vitro by the formation of new layers of apatite-like phases after soaking in SBF.      

Use of calorimetry and thermal analysis to assess the heat of supplementary cementitious materials during the hydration of composite cementitious binders

Journal of Thermal Analysis and Calorimetry - The present paper focuses on the suitability, variability and versatility of thermal analysis and calorimetry methods in the study of cement hydration...     

Performance of G-Oil Well cement exposed to elevated hydrothermal curing conditions

G-Oil Well cement was modified by blending it with blast furnace slag and silica fume at various ratios. The hydration was carried out under the hydrothermal conditions (200 °C and 1.2 MPa) up to 7 days. TG and DTG were performed on cured pastes to identify the hydrated products, their quantity and their stability under given hydrothermal curing conditions. The microstructure of samples was observed by a scanning electron microscope. The mechanical compressive strength was determined and the pore structure was analyzed using mercury intrusion porosimeter. It was found out that the compressive strength values of blend G-Oil Well cements markedly increased with increasing blast furnace/silica ratio. The pore structure was consolidated, as demonstrated by the displacement of pore size distribution to the region of micro and nano pores.