Deterioration of cementitious materials co-hydrating with highly concentrated sodium sulphate is a major durability problem for the use of the materials in low-or intermediatelevel nuclear waste repositories. The sulphate attack is a physicochemical interaction of sulphate ions and cement matrix. Understanding the underlaying mechanisms and influencing factors are important to evaluate the long-term performance of the materials in the nuclear waste repositories. In this study, Portland cement and slag-blended cementitious materials in which the cement is replaced by 42% of slag were hydrated 13% of Na2SO4 by weight. The hydrated samples having diameter between 2.38 mm and 4.75 mm were immersed in water, sodium and magnesium sulphate solution to evaluate its degradation. Solid phases in the samples before and after the exposure were quantified by XRD/Rietveld analysis and the results were compared with thermodynamic equilibrium model. The model prediction agrees with experimental data except U-phase and ettringite. The experimental results show that the U-phase is not stable in exposure solution and it is changed to ettringite, but the model could not predict it. The effect of solution composition, the type of associated cation, and the presence of slag and its replacement ratio on the experimental and simulation results were discussed.

Superabsorbent polymers (SAPs) are used as internal curing agents in cementitious materials, which reduce autogenous shrinkage in concrete as they have a low water-to-cement ratios and improve the freeze–thaw resistance. However, the compressive strength of concrete may also be reduced due to additional voids in the hydrated cement matrix. In this study, we fabricated a delayed absorption type of SAP (I-SAP) composed of cross-linked modified acrylate and studied its absorption characteristics and effect on compressive strength after 28 days. Furthermore, the effect of curing conditions on the strength of concrete and hydrated cement paste with SAP were investigated. The absorption capacity of I-SAP in the synthetic pore solution and deionised water was examined and compared with that of a conventional SAP, and the former was absorbed more by I-SAP. The results revealed that the compressive strength of concrete increased with the addition of I-SAP, particularly with the curing condition of 60% RH. Although the compressive strength of hydrated cement paste with I-SAP reduced in water or sealed curing conditions, no loss of strength in the paste cured at 60% RH was seen. The cement matrix densification due to hydration of belite around the SAP surface is the main mechanism for strength development in concrete cured at sealed and 60% RH. However, the voids formed by SAP control the compressive strength of hydrated paste.

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A trienamine-mediated asymmetric Diels− Alder reaction using a 5-nitro-2,3-dihydro-4-pyridone derivative as a dienophile in the presence of a secondary amine organocatalyst derived from cis-hydroxyproline was discovered. The reaction provides optically active octahydroquinolines through an endo-selective [4 + 2] cyclization pathway. The following stereoselective denitration, isomerization, and/ or hydrogenation generated divergent stereoisomers of decahydroquinolines, which are useful synthons for the total synthesis of Lycopodium alkaloids. Letter pubs.acs.org/OrgLett

The dissolution behavior of Pyrex glass in a model system consisting of 1‐M NaOH with varying amounts of portlandite, representing the glass dissolution in alkaline environment and alkali‐silica reaction (ASR) in cementitious materials, is studied. The Pyrex glass dissolution and the reaction products were characterized using X‐ray diffraction (XRD), 29Si nuclear magnetic resonance (29Si‐NMR), and scanning electron microscopy with energy dispersive X‐ray (SEM/EDX), and the silica and calcium concentrations in the liquid phase were determined using inductively coupled plasma atomic emission spectroscopy (ICP‐AES). The experimental results show that the dissolution of the Pyrex glass continued until it consumed the portlandite and then reached a constant rate, with a linear relationship with the amount of portlandite. The absence of calcium and reduction of silica concentration in the liquid phase with the increase in portlandite indicate the formation of high‐reaction products with portlandite, confirmed by XRD and 29Si‐NMR. The calcium sodium silicate hydrate (C–N–S–H) and sodium silicate hydrate (N–S–H) are the main ASR products; their composition and proportions strongly depend on the reaction time and the amount of portlandite added. A thermodynamic model, which couples geochemical code (PHREEQC) and the experimental silica dissolution rate, was used to predict ASR products and the remaining portlandite. The simulation results predicted the experimental data fairly well for different portlandite additions. The mechanism for Pyrex glass dissolution in the presence of varying portlandite additions is discussed with regard to experimental data and simulation results.