含钙矾石和水镁石双膨胀源水泥硬化浆体及其混凝土的体积稳定性分析

本文建立了在水泥硬化浆体中来自钙矾石和水镁石结晶膨胀作功方式的模型,计算了结晶膨胀能全部被用作体外膨胀功时所产生的膨胀率,由此分析了现有含钙矾石膨胀源或水镁石膨胀源水泥硬化浆体体积的稳定性,并研究得到含钙矾石和水镁石双膨胀水泥或胶体硬化浆体及其混凝土的长龄期膨胀及其体积稳定性,适用于补偿水泥混凝土的早期收缩和后期冷缩与干缩。     

The influence of alkali-free and alkaline shotcrete accelerators within cement systems: Influence of the temperature on the sulfate attack mechanisms and damage

The resistance to sulfate attack of mixtures accelerated with alkali-free and alkaline accelerators was found to be mainly influenced by the Al³⁺ and SO₄²⁻ added via the admixtures. Microstructural observations showed decalcification and disintegration of the CSH gel, which acted as an additional Ca²⁺ supplier as compared to the CH for ettringite formation. The CSH decalcification was mainly observed with a homogeneous distribution of the alkali-free admixture. The disintegration of the CSH gel increased the porosity and allowed more sulfate solution to penetrate into the specimens. This process promoted the swelling of the specimens and directly contributed to the expansion, explaining the lack of a direct relationship between the ettringite formation and the expansion. Moreover, the CSH gel disintegration, typical for MgSO₄ attack, also occurred with Na₂SO₄ solutions and depending on the aluminate-sulfate distribution and the extent of the CSH gel disintegration, different damage types were detected. At higher temperatures (65 °C) the damage was mainly controlled by the growth, the rearrangement and the thermal stability of ettringite.     

Ettringite formation: A crucial step in cement superplasticizer compatibility

The rheology of cementitious system containing superplasticizer is the consequence of a physical process due to the electrostatic repulsion between particles, but also of a chemical process linked to the nature of the phases that are formed. Ettringite crystallization play as a key role in this matter and the nature of the sulfate phase added to control cement setting is as important as its dosage. Alkali sulfates, which provide only SO₄²⁻ ions, do not promote the formation of ettringite for which the presence of large amounts of Ca²⁺ is necessary. The adsorption of superplasticizer molecules on hydrated cement grains slows down the dissolution rates of the constituents and modifies the nature of the compounds formed. It could result in a modification of the ettringite morphology.     

Evaluation of the treatment of chromite ore processing residue by ferrous sulfate and asphalt

The effectiveness of the treatment of chromite ore processing residue (COPR) with ferrous sulfate and encapsulation into asphalt were explored separately and in combination. The asphalt treatment was conducted by mixing COPR or ferrous sulfate pretreated COPR with varying amounts of asphalt. To assess the efficacy of the treatment, the leachability of toxicity characteristic leaching procedure (TCLP) total chromium (Cr) from all treated samples was determined for curing periods up to 16 months. X-ray absorption near edge structure (XANES) analyses were also performed to evaluate the Cr(6+) concentration in the selected samples. The combination treatment of ferrous sulfate and the encapsulation of the treated COPR into asphalt reduced the TCLP total Cr concentration to lower than the regulatory limit of 5mg/L for Cr contaminated soils, after 16 months. However, the Cr concentrations were still higher than the universal treatment standards (UTS) of 0.6 mg/L for hazardous waste. On the other hand, treatment with ferrous sulfate alone or the encapsulation of the COPR in asphalt failed to meet the TCLP total Cr concentration of 5mg/L, after 16 months. XANES analyses results showed that more than 75% Cr(6+) reduction was achieved upon pretreatment with ferrous sulfate.     

Use of sulfoaluminate cement and bottom ash in the solidification/stabilization of galvanic sludge

Solidification/stabilization (S/S) process can improve the physical characteristics of wastes, reduce their leaching and limit the solubility of their heavy metals. The identification of binders able to assume the fixation of contaminants is essential for the success of the technique. In this study, calcium sulfoaluminate cement was added to another waste, bottom ash, in order to treat galvanic sludge. The properties of the resultant solid matrix (MS) were determined: setting time, compressive strength and products of hydration. Solid matrix composed of 77% waste and only 23% cement presented initial setting time lower than 4 h and 28 day-strength of 6 MPa. SEM investigations showed that contaminants present in the galvanic sludge (Cr) were encapsulated in the hydrated phases and particles of bottom ash.     

The impact of cement parameters on Delayed Ettringite Formation

Delayed Ettringite Formation (DEF) in concrete is likely to lead to swelling and cracking in structures which have undergone early age heating to a temperature of over 65 °C. Application of a method that accelerates this process has made it possible to study the impact of cement properties on DEF. For two temperatures reached by the concrete (75 °C or 85 °C), the study considers a domain defined by the sulphate content [2.6-3.6%], the alkali content [0.5-1%] and the Blaine specific area [3330-4635 cm²/g] of the cement. The impact of these parameters and the interactions between them on swelling are discussed. Monitoring of the dynamic elastic modulus of the concretes shows that this property may be reduced by DEF, but that it may increase again once the swelling process has ceased, probably due to the gradual filling of voids by ettringite formed under conditions of limited supersaturation.     

Temperature dependence, 0 to 40 °C, of the mineralogy of Portland cement paste in the presence of calcium carbonate

Thermodynamic calculations disclose that significant changes of the AFm and AFt phases and amount of Ca(OH)₂ occur between 0 and 40 °C; the changes are affected by added calcite. Hydrogarnet, C₃AH₆, is destabilised at low carbonate contents and/or low temperatures < 8 °C and is unlikely to form in calcite-saturated Portland cement compositions cured at < 40 °C. The AFm phase actually consists of several structurally-related compositions which form incomplete solid solutions. The AFt phase is close to its ideal stoichiometry at 25 °C but at low temperatures, < 20 °C, extensive solid solutions occur with CO₃-ettringite. A nomenclature scheme is proposed and AFm-AFt phase relations are presented in isothermal sections at 5, 25 and 40 °C. The AFt and AFm phase relations are depicted in terms of competition between OH, CO₃ and SO₄ for anion sites. Diagrams are presented showing how changing temperatures affect the volume of the solid phases with implications for space filling by the paste. Specimen calculations are related to regimes likely to occur in commercial cements and suggestions are made for testing thermal impacts on cement properties by defining four regimes. It is concluded that calculation provides a rapid and effective tool for exploring the response of cement systems to changing composition and temperature and to optimise cement performance.     

粉煤灰对UEA混凝土补偿收缩性能的影响

本文论述了粉煤灰及U型膨胀剂 (UEA)分别对水泥基复合材料的作用 ;通过实验系统地分析论证 :在不同养护温度下 ,粉煤灰和UEA共同内掺使用对膨胀性能的影响 ;得出了粉煤灰对UEA混凝土补偿收缩性能有抑制作用的结论 ;指出在利用UEA混凝土无缝设计施工或大体积泵送时 ,必须通过实验确定各组分的掺量 ,并采取湿热养护措施