Study of stabilized phases in high alumina cement mortars Part I Hydration at elevated temperatures followed by carbonation

The use of temperatures higher than 30°C during the hydration of 1:3 high alumina cement mortar leads to the formation of the cubic hydrate C₃AH₆, hence avoiding the conversion of CAH₁₀ to C₃AH₆. The subsequent carbonation of C₃AH₆ with CO₂ and thermal treatment leads to the formation of stable carbonated phases. The optimum hydration ages are the minimum necessary times to get C₃AH₆: 3 hours at 80°C, 6 hours at 60°C. The product is in a state we call “semiplastic” in which the next structural transformation - carbonation - occurs more easily than when the specimens have a rigid structure. The effect of a super-water-reducing admixture has also been studied.     

The hydration of calcium dialuminate and its mixtures containing slag

In a paste CA₂ hydrates very slowly at 20°C but the reaction is more rapid at 40°C. At both temperatures the products of hydration comprise C₂AH₈, gibbsite and alumina gel after 7 days. For mixtures of CA₂ + CA the initial hydration at 20°C is dominated by CA. In the presence of ground granulated blastfurnace slag CA₂ or a mixture of CA₂ + CA will form strätlingite on hydration at 40°C within 3 days. Slag addition reduces the amount of heat evolved during the first 24 hours of hydration at both 20° and 40°C.     

Expansion associated with ettringite formation at different temperatures

Expansion of hydrated mixtures made with C₃A, CaSO₄ · 2H₂O, Ca(OH)₂ and SiO₂, at 22, 30, 40, 50, 60°C, was studied to verify if expansion is associated with colloidal ettringite formation or with the solid state conversion of C₄AH₁₃ to monosulfate hydrate in presence of calcium hydroxide. From the results of our investigation it can be drawn the conclusion that mortars expansion is in connection with colloidal ettringite formation and the monosulfate hydrate is formed only when the greatest expansion is ended. The increase in hydration temperature seem to be favorable to the formation of colloidal ettringite.     

Impact of prolonged warm (85°C) moist cure on Portland cement paste

A commercial Portland cement paste was fabricated as 200-g cylinders to a water/cement weight ratio of 0.50. After 30 days cure at 20°C, cylinders were additionally cured at 20°C and 85°C, both ±2°C, in sealed, vapour-saturated systems for 8.4 years. Thereafter, cylinders were allowed to stand, still in sealed state, at 20° for 1.5 to 2.0 years. The 20°C cure mineralogy and microstructure is essentially normal: only a little unhydrated clinker persists and the matrix consists of relatively coarse, blocky Ca(OH)₂ crystals embedded in a groundmass of C-S-H together with some AFt (ettringite). However, prolonged 85°C cure significantly alters the microstructure and mineralogy. Clinker hydration progressed only slowly between 28 days and 8.4 years, with the result that 30% cement clinker persists. Subsequent prolonged storage at 20°C has apparently not allowed hydration to restart. Ca(OH)₂ is present in approximately unchanged amounts, comparing the two cures, provided allowance is made for the presence of unhydrated clinker. Paste porosity is, however, significantly increased at 85°C relative to 20° cure. The 85°C mineralogy consists of four solid hydrate phases: Ca(OH)₂, C-S-H gel, with a Ca/Si mole ratio close to 1.52, katoite (a siliceous hydrogarnet) and a hydrotalcite-like phase. The amounts of these phases are determined. The compositions of the C-S-H gel and hydrogarnet have been estimated by transmission electron microscopy and microprobe analysis. The amount and composition of the mineral phases can be recalculated to yield a bulk composition of the cement that agrees with a batch analysis.     

Solubility properties of ternary and quaternary compounds in the CaO--- Al2O3--- SO3---H2O system

Solubility measurements are reported for four hydrated cement phases. A technique employing repeated dispersion and filtration is described which enabled the solubility (activity) products of ettringite (C₆A ₃H₃₂) and hydrogarnet (C₃AH₆) to be calculated with confidence. Free energies of formation are also given. These phases dissolve congruently, whereas monosulphate (C₄A H₂) and tetra-calcium aluminate hydrate (C₄AH₁₃) exhibit incongruent dissolution, and can therefore not be described by a conventional solubility product. The results represent additional data for inclusion in thermodynamic databases currently being applied to the modelling of cement systems.     

Hydration of mixtures of C12A7 and granulated blastfurnace slag

Mixtures of C₁₂A₇ and ground granulated blastfurnace slag develop high early strength and produce a fast setting cement. At 50°C the strength of the 1:1 mixture does not show any reduction at least up to 60 days although the ‘conversion’ of the initial calcium aluminate hydrates to the ‘cubic’ C₃AH₆ takes place. The hydration chemistry of these cementive mixtures has been studied by conduction calorimetry and X-ray diffraction. It appears that the formation of the compound C₂ASH₈ may be related to the good strength properties of these cements at higher than ambient temperatures.     

The formation of calcium sulfoaluminate hydrate compounds: Part I

The formation of ettringite (3CaO · Al₂O₃ · 3CaSO₄ · 32H₂O) and monosulfate (3CaO · Al₂O₃ · CaSO₄ · 12H₂O) from tricalcium aluminate (3CaO · Al₂O₃) and gypsum (CaSO₄ · 2H₂O) in sodium hydroxide (NaOH) solutions was investigated by isothermal calorimetry and X-ray diffraction analyses. Tricalcium aluminate/gypsum mixtures with a molar aluminate-to-sulfate ratio of 1:3 were hydrated at constant temperatures from 40 to 80°C in deionized water and 200 and 500 mM of NaOH solutions. Ettringite was the only crystalline phase ultimately formed between 40 and 80°C, regardless of whether hydration was carried out in deionized water or sodium hydroxide solutions. The rates of ettringite formation were retarded in sodium hydroxide solutions at all temperatures when compared to hydration in deionized water. The apparent activation energy for the conversion of the tricalcium aluminate/gypsum mixture to ettringite was observed to depend on the concentration of sodium hydroxide.     

Hydrothermal synthesis of Ba(Ti, Sn)O3 fine powders and dielectric properties of the corresponding ceramics

Fine powders of submicron-sized crystallites of BaTiO₃ were prepared at 85–130°C by the hydrothermal method, starting from TiO₂.ξH₂O gel and Ba(OH)₂ solution. The products obtained below 110°C incorporated considerable amounts of H₂O and OH⁻ in the lattice. As-prepared BaTiO₃ is cubic and converts to the tetragonal phase after heat treatment at 1200°C, accompanied by the loss of residual OH⁻ ions. Hydrothermal reaction of SnO₂.ξH₂O gel with Ba(OH)₂ at 150–260°C gives rise to the hydrated phase, BaSn(OH)₆.3H₂O, due to the amphoteric nature of SnO₂.ξH₂O which stabilises Sn(OH)₆²⁻ anions in basic media. On heating in air or releasing the pressure in situ at 260°C, BaSn(OH)₆.3H₂O converts to BaSnO₃ through an intermediate, BaSnO(OH)₄. Solid solutions of Ba(Ti,Sn)O₃ are directly formed from (TiO₂ + SnO₂)..ξH₂O gel up to 35 mol% SnO₂. At higher Sn contents, the hydrothermal products are mixtures of BaSn(OH)₆.3H₂O and BaTiO₃, which on annealing at 1000°C result in monophasic Ba(Ti,Sn)O₃. The sintering characteristics and the dielectric properties of the ceramics prepared out of these fine powders are presented. The dielectric properties of fine-grained Ba(Ti,Sn)O₃ ceramics are explained on the basis of the prevailing diffuse phase transition behaviour.     

Influence of CdO on the early hydration of 3CaO.Al2O3

The influence of CdO on the hydration of 3CaO.Al₂O₃ at 25 °C was studied by X-ray diffraction, scanning electron microscopy, thermal gravimetry, and calorimetry for hydration times less than one day. The CdO had a small effect on the thermal characteristics of hydration. Slightly less heat was liberated when CdO was added to the 3CaO.Al₂O₃. The presence of CdO increased the formation of the cubic hydrated phase. The cadmium was retained in the matrix as Cd(OH)₂.     

Hydration reactions in pastes C3S+C3A+CaSO4.2aq+H2O at 25°C.I

A characteristic retardation of the hydration of C₃A is found in pastes C₃S+C₃A+CaSO₄.2aq+H₂O of weight ratios 1:3:z:4 at certain values of z, when sulphate concentration becomes insufficient for monosulphate formation. This retardation is ascribed to precipitation of amorphous Al(OH)₃, when C₃A dissolves in a limited amount of the aqueous phase shielded from the rest by C₄AH₁₃ or C₄AH₁₉. Evidence for the conversion of C₃AH₆ into C₄AH_n in supersaturated Ca(OH)₂ solution is found.     

HYDROTHERMAL REACTION OF FLY ASH/HYDRATED LIME: CHARACTERIZATION OF THE REACTION PRODUCTS

Class F coal fly ash was slurried with hydrated lime at 90°C in 1/3, 5/3, 9/3, and 15/3 weight ratios and for 3, 5, 7, and 9 hours of hydration, in a process to prepare sorbents for SO₂ removal. The amounts of aluminum, silicon, and calcium in the product of the pozzolanic reaction were determined in order to study the evolution of product composition with the initial raw materials ratio and hydration time and to relate this composition to the desulfurization capability of the material. Al, Si, and Ca were present in the solid product for any raw materials ratio and hydration time, showing that calcium silicates, calcium aluminates, and/or calcium aluminum silicates were obtained simultaneously. The products formed show a nearly constant molar ratio of Al₂O₃/SiO₂ independent of the experimental conditions tested and similar to the Al₂O₃/SiO₂ ratio in the fly ash. The SiO₂/CaO molar ratio in the products decreased as the initial fly ash/Ca(OH)₂ ratio decreased, being approximately constant for each ratio with respect to hydration time after 5 hours of hydration. The maximum moles of CaO, SiO₂, and Al₂O₃ per gram of sorbent in the reaction product were found for any hydration time for the 5/3 sorbents, meaning that at this initial ratio the pozzolanic reaction takes place at the highest rate. The capacity of the sorbent for SO₂ removal depends not only on the amount of products produced by the pozzolanic reaction but also on the specific surface area of the sorbent.     

不同温度下矿渣对铝酸盐水泥早期水化行为的影响

通过凝结时间、抗压强度、电阻率、浆体内部温度测试和水化产物分析,研究了20 ℃、35 ℃和50 ℃下矿渣(GGBFS)对铝酸盐水泥(CAC)早期水化行为的影响。结果表明,掺入矿渣会逐渐减小CAC 72 h的化学收缩,降低化学收缩速率峰值。20 ℃时,电阻率变化曲线出现了明显的晶相转变期,化学收缩曲线存在明显的诱导期; 35 ℃时,凝结时间延长,掺入矿渣抑制了电阻率的发展;50 ℃时,电阻率在接近24 h时显著降低,凝结时间显著缩短,掺入矿渣缓解了24 h电阻率的减小。矿渣-铝酸盐水泥体系的水化产物和抗压强度受养护温度的影响较大。20 ℃时,掺入40%(质量分数)矿渣减少了CAH₁₀的生成量,降低了硬化浆体的强度;35 ℃和50 ℃时,1 d水化产物主要为C₂AH₈和少量C₃AH₆,掺入矿渣延缓了强度的倒缩。在28 d龄期时,不同养护温度下掺入矿渣均能促进C₂ASH₈的生成。     

Preparation of alumina microshells by the emulsion evaporation technique

Spherical hollow alumina powders (alumina microshells) were prepared from evaporation of water-in-oil (w/o) type emulsions by employing an aqueous Al(NO₃)₃ solution as water, mineral oil as the organic phase and a non-ionic surfactant, Arlacel 83, as the emulsifier. It was found that 65% mineral oil, 30% aqueous Al(NO₃)₃ and 5% Arlacel 83 composition produced stable, w/o type emulsions by mechanical stirring at 20°C. The aluminum ion concentration was varied between 0·25 and 2·0 to investigate its effect on the emulsion droplet size. Alumina microshells obtained from the evaporation of w/o emulsions were characterized with respect to size and distribution. The influence of aluminum ion concentration on these properties was also studied.     

Synthesis and characterization of calcium aluminate nanoceramics for new applications

Two types of nanocalcium aluminate powders containing 70 and 60 wt.% Al₂O₃ were prepared by thermal decomposition and investigated in terms of mineralogical composition, hydration, mechanical properties and microstructure. The results revealed that S1 is composed mainly of the CA and CA₂ phases, while S2 composed of CA and C₁₂A₇ phases after heat-treatment at 1000 °C. The maximum crystallite sizes for S2 and S1 were 44 and 52 nm and the minimum ones 12 and 19, respectively. Both samples still have small crystallite size after heat-treatment at 1000 °C. The present phases, i.e. CA, CA₂ and C₁₂A₇ affect the properties of hydrated and sintered ceramic bodies. S2 hydrated sample achieved higher strength (58.5 MPa) than S1 (52.4 MPa). The higher strength of S2 is ascribed to the presence of CA and C₁₂A₇ as a major component, since it reacts rapidly with water. In S1, the poor hydration of CA₂ at the early stage of hydration lowers strength after 7 days hydration as compared with S2. Cold crushing strength (CCS) data of the sintered ceramics bodies exhibit high strength of both samples after firing at 1550 and 1450 °C for S1 and S2, respectively. This is due to the formation of a ceramic bond.     

Investigations on the structure of fully hydrated portland cement and tricalcium silicate pastes. II. Total porosity and pore size distribution

On a series of OPC pastes hydrated at 20 °C and 90 °C and Ca₃SiO₅ pastes hydrated at 20 °C the total porosity and pore size distribution were measured. Data obtained by mercury porosimetry and nitrogen adsorption did not agree well as the volumina of pores filled with Hg and N₂ differed. Regardless on the method employed the found pore size distribution depended on the starting water-solid ratio; as equal porosity, it was different in different hydrated materials.     

碱性和无碱液体速凝剂促凝早强作用机理对比研究

碱性和无碱速凝剂掺入水泥后的水化机理不同,导致应用性能存在明显差异。本文通过测试凝结时间和砂浆抗压强度等宏观性能对比了两种速凝剂的应用性能,并通过水化放热分析、XRD定量分析、热重分析和SEM微观形貌观察等微观方法综合分析了两者的早期水化历程。结果表明:碱性速凝剂加入水泥后,[Al(OH)₄]^-加快了水泥中石膏的消耗速度,水化初期生成大量钙矾石(AFt),促进了硅酸三钙(C₃S)矿物的水化,缩短了水泥浆体的凝结时间并提高了砂浆的早期抗压强度,但石膏的加速消耗也使得单硫型水化硫铝酸钙(AFm)和水化铝酸钙(C-A-H)等水化产物提前生成,影响了水泥基材料的后期抗压强度发展;无碱速凝剂加入水泥后,[Al(OH)₄]^-和SO^2-₄在液相中生成了大量AFt,促进了铝酸三钙(C₃A)和C₃S矿物的水化,影响了氢氧化钙(CH)的结晶析出。值得注意的是,SO^2-₄不仅促进了C₃A生成AFt的过程,也延缓了水泥中石膏的消耗及AFm和C-A-H等产物的生成,因此无碱速凝剂的加入除了明显提高早期抗压强度外,后期28 d抗压强度也不受影响。     

Thermodynamic investigation of the CaO---Al2O3---CaSO4---H2O system at 50°C and 85°C

The CaO---Al₂O₃---CaSO₄---H₂O system has been investigated at 50°C and 85°C, by calculations of the equilibrium solubility surfaces of AH₃, C₃AH₆, CH, ettringite, monosulphoaluminate and gypsum. The results indicate that monosulphoaluminate becomes a stable phase at elevated temperature. Since ettringite also remains stable at 85°C, new invariant points are created, for which calculated aqueous phase compositions are given.     

The hydration of 11CaO · 7Al2O3 · CaF2 AT 20°C

Hydration at 20°C of C₁₁A₇ · CaF₂ which exists in regulated set cement was investigated by means of conduction calorimetry, differential thermal analysis, scanning electromicroscopic observation, X-ray diffraction and chemical analysis. The process of hydration and rate of hydration were determined and influence of the various calcium sulfates, calcium hydroxide and carboxylic acid upon the hydration of C₁₁A₇ · CaF₂ was discussed. Mechanism of the strength development was also deduced in relation to the structure of hardened paste.     

Promotion of hydrogen permeation on metal-dispersed alumina membranes and its application to a membrane reactor for methane steam reforming

A mesoporous membrane for selective separation of hydrogen was prepared usingthe sol-gel method. Some metal salts such as RuCl₃, Pd(NH₃)₄Cl₂, RhCl₃,, and H ₂PtCl₆, were added to the boehmite sol and coated on a porous alumina substrate before firing at 500°C. It was foundthat the permeability of hydrogen and the separation factor for a hydrogen-nitrogen gaseous mixture of these metaldispersed membranes exceeded the limitations of the Knudsen diffusion mechanism. Although the gas permeation through a neat alumina membrane is governed by the Knudsen diffusion, the metals dispersed in alumina membranes were effective in promoting hydrogen permeation. These metaldispersed alumina membranes were also used in a membrane reactor for methane steam reforming at low temperature. In the temperature range of 300 to 500°C, the membrane reactor attained a methane conversion twice as high as the equilibrium value of the packed bed catalytic reactor system as a result of the selective removal of hydrogen from the reaction system.     

A study on the hydration products of a non-expansive sulfoaluminate cement

In the context of a project aiming at the preparation of an energy-saving cement within the CaO---SiO₂---AlO₃---SO₃ system having non-expansive properties, clinker was prepared by sintering a mix of industrial raw materials at 1280 °C in a laboratory oven.

The hydration products of the cement cured to 90 days were studied by Scanning Electron Microscopy, X-Ray Diffraction and Thermogravimetric Analysis, and found similar to these of a typical sulfoaluminate cement. The low rate of hydration of the belite phase was attributed to its microcrystalline form.