Reaction of Hydraulic Calcium Silicates with Carbon Dioxide and Water

The carbonation of wetted powders of beta-dicalcium silicate (β·2CaO·SiO₂=β-C₂S) and tricalcium silicate (3CaO·SiO₂= C₃S) was studied as a function of reaction conditions. The water/solids ratio is an important parameter and there is an optimum value for each silicate. Relative humidity and the partial pressure of CO₂ also strongly affect the reaction. The rate of carbonation can be conveniently represented by plotting the degree of carbonation against the logarithm of time. C-S-H and calcite are the initial reaction products. Subsequently, carbonation of the C-S-H produces silica gel, whereas aragonite may form if the system is allowed to dry out.     

Early Hydration of Tricalcium Silicate: A Solid Reaction with Induction and Acceleration Periods

The hydration of tricalcium silicate was studied using a sample with a limited particle size distribution. The hydration reaction was analyzed as an example of general solid reactions having induction and acceleration periods. The first product, which may have a structure similar to the reactant, forms on the surfaces of reactant grains. The nuclei of the stable product are produced in the first product layer and act as reaction centers; the main reaction to form a stable product occurs rapidly in the acceleration period. In the hydration of calcium silicates, hydrates produced in this stage are poorly crystalline and metastable and change gradually into a more stable form.     

Hydration of Tricalcium Silicate

The kinetics of paste, bottle, and ball-mill hydration of 3CaO SiO₂ and the effects of additions of electrolytes and alcohols were studied. Paste and bottle hydrations proceed through periods of induction, acceleration, and decay. If 3CaO SiO₂ is hydrated in an excess of H₂O, as in bottle hydration, the reaction rate is lower than that for paste hydration. The ball-mill hydration rate is much the highest and is controlled by the removal of the hydrate layer coating the 3CaO SiO₂ particles. Electrolytes always accelerate and alcohols retard the reaction rate. Experimental results are discussed with reference to modern theories of the 3CaO SiO₂ hydration mechanism.     

Hydration of Tricalcium Germanate–Tricalcium Silicate Solid Solutions

The hydration characteristics of solid solutions of composition 3CaO· x GeO₂·(1 − x )SiO₂ were investigated at 25°C by isothermal calorimetry. The compositions hydrated were for x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0. Full hydration was achieved for compositions in which x < 0.2. Both the rate of hydration and the total heat evolved for complete hydration vary with composition. Hydrate compositions were determined, and these also show a compositional dependence. The hydration product for tricalcium germanate has a Ca/Ge ratio near 1.5. Although the hydration products of the solid solution are far more fibrous, their morphologies are reminiscent of that of calcium silicate hydrate.     

Early Hydration of Tricalcium Silicate

The hydration of tricalcium silicate (C₃S) in the preacceleration stages was studied. The C₃S particles carry a positive charge during the early stages of hydration. Following a rapid hydrolysis of C₃S, calcium ions adsorbed on the Si-rich surface of C₃S particles, greatly reducing their further dissolution, thus initiating the induction period. The [Ca²⁺] and [OH^-] continue to increase at lower rates and, because Ca(OH)₂ crystal growth is inhibited by silicate ions, become supersaturated with respect to Ca(OH)₂. When the supersaturation reaches a value of ∼1.5 to 2.0 times the saturation concentration, nuclei are formed, and rapid growth of Ca(OH)₂ and C-S-H is initiated. These products act as sinks for the ions in solution, thus enhancing the further dissolution of C₃S.     

Structure and Reactivity of Chromium-Doped Tricalcium Silicate

The correlation between the structure and reactivity of Cr-doped tricalcium silicate was studied by ir spectrophotometry and X-ray diffractometry. A very sensitive method of detection of defects (thermostimulated exoelectron emission) allowed differentiation of three types of electron traps; the activation energy for the third trap was 0.8 eV. These crystal defects seem to be associated with sites of high Cr concentration (disordered structure) which may also be responsible for the increased reactivity of the doped phases. The reactivity of the system tricalcium silicate-H₂O ( w/c =0.6) was studied by an electrical conductometric method. Curves of conductivity vs time for tricalcium silicate with varied Cr concentrations indicated significant differences in hydration kinetics.     

Composition and Morphology of Hydrated Tricalcium Silicate

Hardened C₃S paste cured for 1 year at 20°C was examined to confirm the composition and the morphology of hydrated tricalcium silicate. A new technique was used in which the samples for scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were etched in 1% HNO₃-alcohol or in glycerol-alcohol (4:6 by volume). After the surface was etched in 1% HNO₃-alcohol, SEM clearly showed the difference in texture in the outer and inner C-S-H products. The existence of a zonal texture within the inner C-S-H products was shown, in addition to the unreacted core; particles 0.1 to 0.2 μm in diameter were observed. After free CH extraction with glycerol-alcohol, two new types of C-S-H grains could be identified. One type has a smooth surface, which seems to be produced from C₃S grains trapped within the growing CH crystals in the early stage of hydration, the C/S mol ratio for these being >3. The other type is covered with many acicular outer C-S-H gel hydration products and has a C/S mol ratio >3.     

The Influence of Water Activity on the Hydration Rate of Tricalcium Silicate

Tricalcium silicate does not undergo hydration at relative humidities (RH's) below 80%. But, the rate at which its hydration rate decreases as a function of the RH has not yet been elucidated. By invoking correspondence between RH and water activity (a_H, unitless), both of which are related to the chemical potential of water, the reaction evolutions of triclinic tricalcium silicate (i.e., T1‐Ca₃SiO₅ or C₃S) are tracked in water + isopropanol (IPA) mixtures, prepared across a wide range of water activities. Emphasis is placed on quantifying the: (a) rate of hydration as a function of a_H, and (b) the critical (initial, a_H0c or the achieved) water activity at which hydration effectively ceases, i.e., does not progress; here identified to be ≈ 0.70. The hydration of tricalcium silicate is arrested even when the system remains near saturated with a liquid phase, such that small, if any, capillary stresses develop. This suggests that changes in chemical potential induced via a vapor‐phase or liquid‐phase route both induce similar suppressions of C₃S hydration. A phase boundary nucleation and growth (pBNG) model is fit to measured hydration rates from the onset of the acceleration period until well beyond the rate maximum when the water activity is altered. The simulations suggest that for a fixed hydrate nucleation density, any water activity reductions consistently suppress the growth of hydration products. Thermodynamic considerations of how water activity changes may influence reactions/hydrate evolutions are discussed. The outcomes improve our understanding of the chemical factors that influence the rate of Ca₃SiO₅ hydration.     

Effect of Fluorine and Magnesium on the Polymorphism of Tricalcium Silicate: A New Fluorine-Containing Tricalcium Silicate Superlattice

There is evidence both by XRPD and by TEM electron diffraction of the presence of a new tricalcium silicate phase containing fluorine with triclinic cell parameters a = 2.32(7), b = 0.71(3), c = 1.28(3) nm, α= 106.5(3), β= 90(1), γ= 118(1)°. It is a superlattice of the rhombohedral C₃S, whose structure is probably deformed by the fluorine/oxygen substitution plus some calcium vacancies, the deformation being nevertheless smaller than that found in the triclinic polymorph of pure C₃S. Magnesium proves to have an effect additional to that observed for fluorine probably because it fixes the fluorine to the silicate lattice.     

Pore Structure of Calcium Silicate Hydrate in Hydrated Tricalcium Silicate

Adsorption of N₂ and water vapor was studied in completely hydrated tricalcium silicate and in fully hydrated tricalcium silicate from which Ca(OH)₂ had been extracted. Compared with results obtained using N₂, water vapor adsorption led to increased values for small-pore volume, peak shifts to smaller sizes, and decreased values for large-pore volume. Marked hysteresis was observed in the case of water vapor adsorption; the resorption branch apparently represents the true pore structure. Extraction of Ca(OH)₂ from the paste increased the calculated volume of small pores strikingly, suggesting that adsorption is hindered by Ca(OH)₂; this tendency is more obvious in water vapor adsorption. The adsorption measurements indicate the existence of two kinds of pores, i.e. a wider intergel-particle pore and a smaller pore existing within the gel particle. The latter pore was further classified into intercrystallite and intracrystallite pores.     

Kinetics of the Hydration of Tricalcium Silicate

The hydration of tricalcium silicate was followed at a water/C₃S ratio of 0.7 between 5° and 50°C by determining free lime and combined water. Free lime was estimated by the o -cresol method, whereas combined water was calculated from the amount of free water remaining in the paste as determined by extraction with methyl ethyl ketone. The ratio of free lime to combined water was constant throughout the hydration; this ratio indicated that CSH(II) may be represented as 1.68CaO · SiO₂· 2.58H₂O. When maximum supersaturation of the solution with Ca^2+] is attained, the induction period terminates and the reaction proceeds rapidly, probably as the result of propagative surface nucleation-growth of CSH(II). Kinetic equations were derived for these reactions. When the surface of C₃S is entirely covered by CSH(II), the reaction becomes slow and is controlled by diffusion of water. Constants involved in the kinetic equations are evaluated and discussed.     

Polymorphism of Tricalcium Silicate and Its Solid Solutions

In tricalcium silicate and its solid solutions six modifications were established: one rhombohedral ( R ), two monoclinic ( M _I, M _II), and three triclinic ( T _I, T _II, T _III). All these forms are rhombohedral or pseudorhombohedral and extremely similar; the variations in the X-ray diagrams and the transformation enthalpies are minute. Only three of the five transitions were observed by both X rays and DTA: T _I- T _II (600°), T _III– M _I (980°), and M _I– M _II (990°); one was established by DTA ( T _II– T _III, 920°) and one by X rays only ( M _II– R , 1050°C). This scheme of polymorphism is compared critically with results of other authors, and the nature of the various displacive transformations is discussed. X-ray and DTA investigations of solid solutions of Ca₃SiO₅ with Al₂O₃ and ZnO established the phase relations as functions of temperature and composition. With Al₂O₃, two modifications ( T _I and T _II) were stabilized; and with ZnO, five polymorphs ( T _I T _II, M_I, M _II, and R ). With both pure C₃S and the solid solutions, the results are consistent and have a close bearing on the constitution of alites in portland cement clinker.     

硅酸三钙骨水泥浆体的可注射性能研究

本文优化了骨水泥浆体的注射性能研究方法,综合研究硅酸三钙(Ca3SiO5,C3S)骨水泥浆体的液固比(L/P)、凝结时间、固化体力学强度等与可注射性的关系.结果表明高L/P易导致静置后C3S骨水泥浆体注射比的突降,固化体孔隙率增加,强度下降,不利于临床操作和应用;低L/P下,采用较小注射器可以提高C3S骨水泥浆体的注射比,也可避免静置后C3S骨水泥浆体注射比的突降.采用较小的注射器和较小L/P,可以在兼顾骨水泥浆体固化性能和力学性能的基础上,实现C3S骨水泥浆体具有较大的注射比.     

Reaction Calorimetry in Supercritical Carbon Dioxide

Summary: Reaction calorimetry is an efficient tool used to obtain kinetic, thermodynamic and safety data. A reaction calorimeter, RC1e‐HP350, developed in collaboration with Mettler‐Toledo GmbH, allows investigating chemical reactions under supercritical conditions. The main technical difference, compared with a classical liquid system, is that the whole reactor volume is occupied by the media. Heat transfer analysis in supercritical carbon dioxide (scCO₂) by the Wilson plot method shows that the behavior of the internal heat transfer coefficient in scCO₂ is the opposite of the one observed for classical liquid. In scCO₂ the lower the temperature (above the critical point) the better the internal heat transfer coefficient. The evolution of scCO₂ thermodynamical and transport properties near the critical point are responsible for this behavior. The dispersion polymerization of methyl methacrylate in scCO₂, with the polydimethylsiloxane monomethacrylate as stabilizer, is used as a model reaction. A polymerization reaction enthalpy of ?56.9 ± 2.2 kJ · mol^?1 is determined, being in good agreement with previously reported data. The results presented illustrate the accuracy of the heat balance model used and emphasize the potential of reaction calorimetry for the promotion of supercritical fluids technologies.

Technical comparison between liquid and supercritical reaction calorimetry.     

不同SO3掺量下C3S矿物的形成

通过化学分析,应用等温煅烧的方法对不同SO3掺量为(0%,1%,2%,3%,3.5%,5%)的C3S矿物形成过程从动力学角度进行了系统探讨.通过拟合得到了C3S矿物形成的动力学模型,并根据该动力学模型对不同SO3掺量下C3S矿物形成的速率常数进行了计算.结果表明:在1450℃,不同SO3掺量下,C3S矿物形成由三维球形对称扩散动力学模型f(a)=(1-3√(x+y)2控制;随着SO3加入量的增加,C3S矿物形成的速率常数呈非线性变化;从游离CaO的含量和反应速率常数的角度来说,SO3的加入有利于C3S的形成.并且SO3加入量的增加有利于C3S的多相体M1的稳定.     

秸秆炭固定二氧化碳反应的动力学研究

在固定床中,氨水改性秸秆炭对吸附二氧化碳气体的吸附特性进行了测定,分别考察了浸渍时间（1～4 h）、浸渍温度（30～45℃）、浸渍浓度（10%～25%）对氨水改性的秸秆炭吸附二氧化碳性能的影响,结果表明：浸渍氨水浓度越高,吸附性能越好;吸附量随浸渍时间的增加,呈先增大后减小的趋势,存在较强的浸渍时间;浸渍温度升高对吸附性能影响不大。依据吸附反应的基本原理,建立了吸附动力学模型。模型计算得到的穿透曲线,与实验结果吻合较好。     

Compositions of Solutions in Contact with Hydrating Tricalcium Silicate Pastes

The concentration of ionic species in the solution in contact with hydrating tricalcium silicate (C₃S) has been studied as a function of time and in the presence of admixtures. The ionic product for calcium hydroxide (CH) was calculated as a function of time and correlated with calorimetric measurements and crystal growth data. The results support the hypothesis that nucleation of CH crystals controls the induction period and that their subsequent growth controls the second heat peak. The influence of admixtures can be predicted from their influence on CH crystal growth. However, solution data indicate that the system cannot properly be compared with the growth of pure CH; the combined role of C-S-H and CH must be considered. Some implications for C₃S hydration are discussed.     

Hydration and Strength Development in Tricalcium Silicate Pastes Seeded with Afwillite

The formation of afwillite in tricalcium silicate pastes initiated by ding with crystals was studied to determine the effects on development of tensile strength. Afwillite forms in pastes only when fresh surfaces of both tricalcium silicate and afwillite seed are exposed by initial grinding. Higher early strengths reflect increased rates of hydration in seeded pastes, but strengths at later ages are lower than those of untreated pastes. At all ages tensile strength is lower at a given degree of hydration in seeded pastes than in unseeded ones; this behavior is considered to result from changes in pore-size distribution which occur as afwillite forms. Afwillite initially forms concomitantly with calcium silicate hydrate gel but later arises from conversion of the gel.     

二氧化碳与乙醇和异丙醇的二元汽液平衡

应用汽液双循环高压相平衡装置，测定了二氧化碳－乙醇和二氧化碳－异丙醇二元混合物分别在３０３．１５Ｋ，３１０．１５Ｋ和３２９．１５Ｋ，压力范围为１．７８～１０．０４ＭＰａ下的汽液相平衡数据。用ＤＤＬＣ－ＳＲＫ方程关联两个二元体系的汽液相平衡数据，计算值与实验值吻合良好。