Iodine release behavior from iodine-immobilized cement solid under geological disposal conditions

The iodine release behavior from the iodine-immobilized cement was investigated. From the results of immersion tests using ion-exchanged water (IEW) and calculations, the solubility equilibrium model could describe the iodine release behavior. To assess the performance of cement in an actual environment, it is important to confirm that the solubility equilibrium model is applicable to the geological disposal conditions. From immersion tests using simulated ground water, the release of iodine from the cement occurred in a shorter period of time than in the case of IEW, and reactions with CO₃²⁻ and Cl⁻, which were contained in the simulated ground water, had an influence on the iodine release behavior. As a result of calculations using the solubility equilibrium model, the liquid-solid ratio at which the iodine was completely released was mostly in agreement with the results of immersion tests. The results show that this model can be applied in a wide range of environments.     

人工鱼礁用钢渣混凝土胶凝材料的水化特性

以鞍钢-0.088 mm热闷法钢渣和鞍钢高炉矿渣为胶凝材料的主要组分,以鞍钢0.088~19 mm热闷法钢渣为骨料,制备出了具有较高强度的人工鱼礁用钢渣混凝土。通过X射线衍射分析、场发射扫描电镜分析、差热分析和红外吸收光谱分析对胶凝材料的水化特性进行研究,结果表明：该胶凝材料在水化初期生成大量低碱度水化硅酸钙凝胶和少量钙矾石,水化硅酸钙凝胶是混凝土早期强度的主要来源;而随着水化进程的延续,水化硅酸钙凝胶的继续发展和不断增多、长大的钙矾石对体系空隙的充填则共同使混凝土的后期强度得到进一步的提高。     

Significance of compaction time delay on compaction and strength characteristics of sulfate resistant cement-treated expansive soil

The addition of cement for stabilization of expansive soils is one of the most commonly used methods. As with every calcium-based stabilizer, the time delay between the physical mixing of the stabilizer and compaction plays an important role in achieving the desired results after stabilization. However, a clear insight on the determination of optimum time delay for achieving the maximum desired compaction properties of cement-stabilized soils is yet to be established. Furthermore, the recent studies highlighted the use of sulfate to mitigate the negative effect of compaction time delay. The only drawback with the use of sulfate along with calcium-based stabilizers is the formation of ettringite, which deteriorates the stabilized soil matrix. In view of this, the present study is aimed at using the sulfate resistant cement (SRC) as a stabilizer along with the controlled addition of sulfate solutions to mitigate the negative effect of compaction time delay in stabilizing the expansive soil. To bring out the above effects, three periods of time delays (0 h, 6 h and 24 h) and three sulfate concentrations of 5000 parts per million (ppm), 10,000 ppm and 20,000 ppm were adopted. The experimental results showed that the delay in compaction resulted in the formation of clogs and reduction of strength of SRC-stabilized expansive soil. Upon sulfate addition to SRC-stabilized expansive soil, the formation clogs was not curtailed and resulted in the formation of ettringite clusters. These formations were captured with the help of scanning electron microscope (SEM) images and validated with electron dispersive X-ray spectroscopy (EDAX) analysis. Further, an attempt is also made to explain the mechanism of density and strength reduction with the aid of physico-chemical properties and mercury intrusion porosimetry (MIP) studies.     

Tricalcium aluminate hydration in additivated systems. A crystallographic study by SR-XRPD

Synchrotron radiation X-ray powder diffraction has been used to monitor the evolution of ettringite in C3A-gypsum synthetic mixture and in commercial cement systems during the first hours of the hydration process. The hydration of the paste was achieved using a remote controlled system, in order to collect data as soon as water is added to the system. The use of full-profile Rietveld method during the analysis of the diffractograms collected allowed us to monitor the evolution of phases weight fraction. The rigorous measurement of the lattice parameters and of the diffraction peak shape proved to be very useful to obtain information on the structural evolution of ettringite and on the mean grain size of the crystal phases. Depending on the admixture added to the system, the precipitation of well crystalline ettringite takes some hours. During this “induction” period we observe a significant variation of a and c lattice parameter values for ettringite. In particular a increases from 11.8 Å to 11.24 Å, the value for pure ettringite. The c parameter decreases from 22 Å to 21.48 Å. The lattice parameter variation could be related to small crystallite size effect, but the large variation more likely reflects also crystallographic changes, such as defect re-organization during the nucleation and growth process or also changes in the SO₃ and H₂O content in the ettringite channel. Not surprisingly the amount and the grain dimensions of crystalline ettringite are affected by the chemistry of the system. We observed the same evolution trend during ettringite formation also in shrinkage-compensating commercial cements (composed by mixture of Ca-Al cements, Portland cement and bassanite), in which ettringite is the main hydrous phase present.     

钙矾石形成与稳定及对材料性能影响的综述

综述了钙矾石的物相结构、形成和稳定的条件以及对材料影响的研究结果。     

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.     

Importance of using the natural pozzolans on concrete durability

Natural pozzolans have become important because of their role in concrete durability. This situation has provoked an increase in the use of pozzolanic cement in concrete construction. This paper reports results of different portland-pozzolan cements containing different natural pozzolans, and they were compared with ASTM Types I, II and V cements. The pozzolanic activity and composition of each pozzolan were evaluated. The susceptibility to sulfate attack was studied by measuring the expansion in mortar bars at different ages (according to ASTM C 1012 Method) for 78 weeks. It was found that certain cements containing pozzolans with high activity or low alumina content improve resistance to sulfate attack, although the amount of pozzolan in the cement is important.     

Occurrence of thaumasite in deteriorated concrete

The first case histories of thaumasite in concrete products in the United States were identified by the writer in the 1960s. There included two underground sanitary pipes, a grout in an underground lead/zinc mine, and the bottom of a concrete pavement on grade. In these cases, thaumasite was first identified by optical properties in powder mounts in a petrographic microscope, then further confirmed by X-ray diffraction and characterized in an electron microscope. Morphological similarities and positive distinction between thaumasite and ettringite are considered.     

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