Preparation and Properties of Different Polyether-Type Defoamers for Concrete.

In this study, a series of polyether-type defoamers for concrete which consist of the same alkyl chain (hydrophobic part) but different polyether chains (hydrophilic part) was prepared, and the structure-property relationship of the defoamers was investigated for the first time. Using oleyl alcohol (OA) as the starting agent (alkyl chain), the polyether defoamers with different polyether chains were prepared by changing the amount and sequence of ethylene oxide (EO) and propylene oxide (PO) units. The properties of different defoamers were tested in aqueous solutions, and fresh and hardened mortars; the structure-property relationship of the defoamers was thus studied. The results indicated that the defoaming capacity of the polyether defoamers decreased with an increased EO amount, and the defoamers linked with both EO and PO units (PO before EO) had a stronger defoaming capacity than those linked with EO only. This study is beneficial for the development and applications of novel synthetic polyether-type defoamers for concrete.

The Influence of Alkali-Free Shotcrete Accelerators on Early Age Hydration and Property Development within Cement Systems.

Fluoride-containing alkali-free setting accelerators are a common type of admixture used in tunnel shotcrete but few studies in the literature focus on the effect of their fluoride compounds on the setting and hardening properties of accelerated cement paste under low environment temperatures. Tunnel shotcrete in cold regions or winter construction periods would be obviously influenced by low environment temperatures, especially for its fast setting and quick support applications. The objective of this work is to evaluate the early age hydration behavior of different accelerated cement pastes under 20 °C and 5 °C environment temperatures. In this study, setting time measurement, early age strength development, hydration ion leaching concentration, isothermal calorimetry, X-ray diffraction, and ESEM were performed on cement systems prepared with a non-fluoride alkali-free accelerator (aluminum sulfate solution with over 60% solid content) and a designed fluoride-containing alkali-free setting accelerator (aluminum sulfate and fluoride compound). The results showed that the fluorides obtained in alkali-free accelerators promote C3S dissolution and massive ettringite needles together with monosulfoaluminate (AFm) hydrate formation, thus leading to a quicker setting effect and low sensitivity to low environment temperatures than in non- fluoride groups. However, the rate of mechanical strength development of cement pastes hydrated within 24 h was decreased obviously when fluorine-containing alkali-free accelerator was used. This phenomenon is mainly related to the crystallization of thin-plate shape calcium fluoride (CaF2) formations and promoted conversion of ettringite to monosulfoaluminate hydrate in the accelerating period, thus weakening the denseness of C-S-H gel and inhibiting alite further hydration.

Production of bio-stable fluid sediment from accumulation of cyanobacterial bloom biomass under various water depths.

While fluid sediments normally formed through hydrodynamic erosion and transport was well known, the fluid sediments caused by organic matter accumulation and degradation in eutrophic lakes was rarely investigated. Here, the effects of cyanobacterial bloom biomass (CBB) accumulation and water depth on the occurrence of fluid sediments were studied. Within 30 days of experiments, the variation of sediment height firstly increased to the maximum with rising in water depth, then decreased due to the high hydraulic pressure. While the surface sediments density decreased slightly from 1.35 g cm-3 to around 1.32 g cm-3 without CBB accumulation, and CBB accumulation led to lower density (around 1.02 g cm-3) but higher shear stress of sediments. Through analyzing the extracellular polymeric substances (EPS), it was found that CBB accumulation improved the polysaccharide/protein ratios of sediment. The infrared analysis further indicated that the bound-EPS could protect fluid sediments bio-stabilization. Meanwhile, the enriched Acinetobacter, Pseudomonas in sediments with CBB accumulation might play roles in EPS production, which benefited the bio-stabilization of fluid sediments. Furthermore, the stability of fluid sediments increased with increase in water depth, and the resuspension of biological fluid sediments would occur more likely in the low water depth area. Altogether, this study reported the formation and stability of the biological fluid sediments in eutrophic shallow lakes, and could help provide clues against sediment resuspension in lake ecosystems.

Effects of accumulated cyanobacterial bloom biomass contents on the characteristics of surface fluid sediments in a eutrophic shallow lake.

In eutrophic shallow lakes, cyanobacterial blooms will occur frequently and then accumulate on sediments, leading to the variation in the surface sediment properties. In this study, the influence of accumulated cyanobacterial blooms biomass (CBB) content on surface sediment properties was determined in microcosm experiments through monitoring surface sediment physicochemical and rheological properties. During one-month incubation, it was found that surface sediment volume increased, and the density decreased from 1.36 g cm-3 to 1.13 g cm-3 with increase in accumulated CBB contents. The results of particle size distribution indicated that CBB accumulation in sediments led to sediment flocculation and agglomeration. In the meantime, there were high ratios polysaccharide/protein in extracellular polymeric substances (EPS) with a decrease in bound EPS/colloid EPS under high CBB contents, which enhanced the sediment particle agglomeration and reduced fluid sediment stability. Further, the critical shear stress in rheological test for sediments on day 30 presented an exponential decay (R2 = 0.97) with increase in accumulated CBB contents. And a threshold value at 0.15% accumulated CBB content indicated sediments could be resuspended easier when accumulated CBB content was higher than 0.15%. Altogether, this study showed that the accumulated CBB content had a strong influence on surface fluid sediment properties. The results were important in sediment management since CBB affects sediment suspension for eutrophication shallow lakes.