Interaction between α-calcium sulfate hemihydrate and superplasticizer from the point of adsorption characteristics, hydration and hardening process

Superplasticizers (SPs), namely sulfonated melamine formaldehyde (SMF) and polycarboxylate (PC), were independently admixed with α-calcium sulfate hemihydrate based plaster to improve the material's performance. SMF and PC gave, respectively, 38% and 25% increases in the 2 h bending strength at the optimum dosages of 0.5 wt.% and 0.3 wt.%, which are determined essentially by the maximum water-reducing efficiency. The peak shift of binding energy of Ca2p_3/2 detected by X-ray photoelectron spectroscopy (XPS) suggests that SPs are chemically adsorbed on gypsum surface. A careful examination of the strength development of set plaster allowed the hydration and hardening process to be divided roughly into five stages. SMF accelerates early hydration, while PC decelerates it. Both SPs allowed similar maximum water reductions, giving a more compact structure and a decrease in total pore volume and average pore diameter, and thus leading to higher strengths in the hardened plasters with SPs.     

Study of the retarding mechanism of linear sodium polyphosphates on α-calcium sulfate hemihydrate

Linear sodium polyphosphates possessing different average chain lengths were characterized and studied as retarder for α-calcium sulfate hemihydrate. Their effect on the hydration kinetics of aqueous α-CaSO₄·0.5H₂O pastes was quantified by heat flow calorimetry, mini slump test and determination of time-dependent degree of hydration. Already at very low dosages (0.025% bwob) polyphosphates strongly retard hydration of hemihydrate. The retarding effect decreases with increasing chain length. Interaction between polyphosphates and calcium ions and Ca^2 + concentration dependent formation of insoluble precipitates were studied via pH and turbidity measurements. Additionally, the influence of polyphosphates on individual hydration steps (binder dissolution and gypsum precipitation) was determined. It was found that the working mechanism of polyphosphates mainly relies on a strong decrease of binder dissolution rate while delayed gypsum precipitation from the solution and inhibition of dihydrate crystal growth by adsorption of polyphosphate play a minor role.     

Simulation study on the relationship between a high resolution αs-plot and the pore size distribution for activated carbon

The effects of the micropore structure of activated carbons on the high resolution α_s-plot for nitrogen adsorption isotherms were examined with the grand canonical Monte Carlo simulation. Adsorption isotherms of nitrogen were simulated in graphitic slit pores at 77 K as a function of the slit width (w). As no pore effect was observed below P/P₀=0.6 for w=3.5 nm, α_s-plots for the simulated adsorption isotherms were constructed using the standard isotherm simulated for w=3.5 nm. The simulated α_s-plots had filling and cooperative swings which were experimentally shown in the previous works, and the shape of the simulated α_s-plot varied with the micropore structure. As the subtracting pore effect (SPE) method for the specific surface area (SSA) determination using the α_s-plot was proposed in the previous experimental works, the theoretical ground for the SPE method was discussed. The best evaluation method of SSA using the α_s-plot was shown, almost agreeing with the SPE method. This simulation study showed clearly that the SPE method is available for pore systems of w≥0.7 nm, whereas even the SPE method underestimates the SSA of the pores of w≤0.6 nm. The observed swings of the α_s-plot were simulated using the different micropore size distribution. The bimodal micropore size distribution lead to both of filling and cooperative swings, while a single pore size distribution <0.9 nm gave only the filling swing. Thus, four representative types of the α_s-plot for activated carbons were proposed and it was shown how to understand the micropore size distribution through the α_s-plot.     

Open hole compressive strength of composite laminates and sandwich panels: comparison between Budiansky–Fleck–Soutis model and experiments

Abstract

In this study, the compressive behaviour of carbon fibre reinforced plastic quasi-isotropic laminates and sandwich panels with carbon fibre reinforced plastic face sheets and syntactic foam core has been investigated. Experimentally determined open hole strengths have been compared with theoretical predictions obtained by applying a linear cohesive zone model. The unnotched compressive strength has been experimentally determined, and the in-plane fracture toughness has been analytically predicted as input parameters of the model. Buckling phenomena occurred on some specimens, and they have been taken into account. Evaluation of macroscopic failure modes in compression tests on unnotched specimens led to a better understanding on the advantages of the analytical model and on the possibility of applying the model to sandwich structures. The experimental results were in good agreement with the analytical prediction by the Budiansky–Soutis–Fleck cohesive zone model, and the difference between theoretical and experimental open hole strengths of Syncore sandwich panels was <9%.     

Effect of coal particle size distribution,volume fraction and rank on the rheology of coal–water slurries

The aim of this study was to understanding of the effect of coal particle size distributions on rheology of coal–water slurries (CWS). Experiments have been carried out on the coal samples that were different in rank. Besides two different Turkish lignites (Soma and Istanbul–Agacli), a bituminous coal from Siberia (Russia) has been used. In addition to the determination of the chemical and physical properties of the coal samples, their zeta potentials were also measured. The pulps of different solids percentage composed of coal particles with d₅₀ sizes of 19, 35 and 50 μm were used to determine the effect of volume fraction on the viscosity of the slurry.     

Effect of particle size and shape of diluent on the reaction process of combustion synthesis of α-Si3N4 powder

α-Si₃N₄ powder was prepared by combustion synthesis using different particle sizes and shapes of Si₃N₄ diluent. The effects of different diluents on combustion temperature, phase composition, and microstructure of the product were investigated. The role of diluents in combustion synthesis is discussed. When no ammonium salt was added, because of the higher reaction temperature, the phase transformation of the fine particle diluent with the best barrier effect was also enhanced, and the α content of the product was the lowest. When the ammonium salt is added, the liquid phase Si content decreases at high temperatures, the lower reaction temperature and the Si₃N₄ generated before Si melting make the barrier effect of the diluent fully play. Finally, Si₃N₄ powder with 86% α content was synthesized by combustion with 2 μm Si₃N₄ diluent.     

An adsorption strength model for the electrochemical codeposition of α-Al2O3 particles and a Fe–P alloy

An adsorption strength model is developed to describe the mechanism of codeposition of -alumina and Fe–P alloy. Based on hypotheses concerning the effective adsorption and the distribution of the adsorption strength of particles on the cathode, a general expression relating the content of embedded particles to the suspension concentration and the current density is deduced. This relationship is in good agreement with the experimental results. The variation of the content of particles in deposits with current density is an overall balance of two opposing effects which leads to a maximum in the particle content against current density curve.     

On the effects of particle size and preform porosity on the mechanical properties of reaction-bonded boron carbide infiltrated with Al-Si alloy at 950 °C

The infiltration of boron carbide preforms with Al alloys at relatively low temperature prevents the formation of the undesired Al₄C₃ phase. In the present study the effect of boron carbide powder particle size on the mechanical properties and phase composition of composites infiltrated with Al-20%Si alloys at 950 °C was investigated. According to XRD analysis, the infiltrated composites contain Al₈C₇B₄, AlB₂ and AlB₁₂, as well as non-reacted Al and Si that originated from solidification of Al-Si alloy. The presence of small amounts of SiC was noted in specimens fabricated from fine boron carbide powder. No evidence for the formation of non-desired aluminum carbide phase was obtained. Infiltration of ceramic preforms with virtually the same green density generated composites with an elastic modulus and bending strength that continuously decreased from 270 GPa and 405 MPa to 195 GPa and 345 MPa for powder with 90% particles close to 3 μm and powder with 90% particles close to 180 μm, respectively. These results ambiguously confirm that boron carbide particle size strongly affects mechanical properties of reaction-bonded composites infiltrated with Al-Si alloy at 950 °C and reflect the amount of newly formed ceramic phases appearing during infiltration and the presence of defects at the metal-ceramic interface.     

Reply to the discussion by Q. Zeng and S.L. Xu of the paper “Numerical simulation of moisture transport in concrete based on a pore size distribution model”

This paper presents a reply to the discussion of the paper “Numerical simulation of moisture transport in concrete based on a pore size distribution model” by Q. Zeng and S.L. Xu.