A study of the effects of citric acid on the mechanical properties of calcium sulfoaluminate (C4A3S̄) cements in hydraulic pastes and mortars was performed. Clinker made up of C4A3S̄ was obtained from the calcination of a mixture of fly ash, aluminium dross and fluorogypsum at 1100°C. The C4A3S̄ cement was prepared by mixing the clinker with certain percentages of commercial hemihydrate. Hydraulic pastes and mortars were prepared with 0·05% citric acid relative to the total weight of the cement in order to slow down the reactions and also to act as a structural modifier. The initial and final setting times of the pastes were up to 189 and 336 min, respectively. The compressive strength was evaluated in specimens cured at room temperature, reaching, in some mortar formulations, more than 80 MPa at 360 d with high initial development. Hydration reactions were significantly affected by both the citric acid and the increase in temperature, as confirmed by isothermal conduction calorimetry. The main hydration product was ettringite, which promoted the formation of dense and compact microstructures.
This work presents a systematic numerical study of Cherenkov optical radiation generated by a modulated source that moves with uniform velocity on a two-dimensional (2D) photonic crystal (PCr) slab surface. We apply the FDTD technique with emphasis on the dispersion properties of the periodic medium to perform our numerical analysis. The field oscillations generated at the passage of a modulated source in the PCr produce a series of spectral resonances corresponding to the eigenmodes in the spatial frequency domain for the photonic slab. The amplitudes of the field oscillations have maximal values in the group cone closely to the path of the moving charge.
In this investigation, it was studied the effect of curing temperature on the mechanical properties of pastes made with calcium sulfoaluminate cements synthesised from industrial wastes at a relatively low temperature of 1100°C. Fluorgypsum, fly ash, aluminium slag and calcium carbonate were used as raw materials. The main phases of synthesis were calcium sulfoaluminate and gehlenite. Pastes were prepared with different water/cement ratios, 15-25 wt-% hemihydrate and cured at 20 and 40°C. It was found that with higher total amounts of sulphates and increase in temperature, compressive strength is enhanced in the first days of curing and setting times decreased. Ettringite was found to be the main reaction product forming dense structures over time. Calcium sulphates hydration changed the ettringite formation causing fluctuations in the strength development, nevertheless, in some cases the strength surpassed 40 MPa at 28 days. Microstructures and hydration products were studied by SEM and XRD.
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