Halim Hammi

In this chapter, the experimental design methodology is applied to optimize the formation conditions of magnesium chloride cement. A factorial design to model and to optimize the operating parameters that govern the formation was used. The studied factors were mass ratio of MgCl2.6H2O/MgO, mixing time and stirring speed. The considered responses were compressive strength and setting time. The optimum operating conditions were quite efficient to have a good compressive strength and suitable setting time. The phases’ compositions of the magnesium oxychloride cement were evaluated by X-ray diffraction, the morphological properties were examined by scanning electron microscopy (SEM) method and their thermal behavior was analyzed by differential thermal analysis/thermogravimetric analysis (DTA/TGA). The raw materials used in the study were magnesium oxide and magnesium chloride hexahydrate obtained from natural brines in the south of Tunisia.

This paper shows that besides the particle size reduction, prolonged high-energy ball milling of portland cement leads to mechanically induced phase transformation from the crystalline to the amorphous state. This research has also investigated the improvement of the mechanical and microstructural properties of cement pastes incorporating mechanically activated cement milled for various durations as partial replacement of the unprocessed cement. Results have shown that compressive strength increases with prolonging mill duration as well as the amorphization degree of the processed cement. The setting times and porosity are significantly reduced, and the degree of hydration is improved by increasing the mill duration and the amorphization degree of the modified cement. The most significant results were observed when 10% by weight of amorphous cement was added to the unprocessed cement. In this experimental condition, the compressive strength was increased by 107% (at 28 curing days), the initial and final setting times were reduced approximately threefold, the porosity was reduced by 52%, and the early degree of hydration was improved by 162%.

The influence of different parameters on magnesium oxychloride cement (MOC) was investigated in this study. The use of experimental design was found to be very beneficial for determining suitable experimental conditions to obtain a better composition of cement. The studied factors used to model and optimise the operating parameters were stirring speed, mixing time, mass ratio of magnesium chloride/magnesium oxide, silica glass powder (SGP) percentage and SGP particle size. The considered responses were compressive strength and setting time. The phase compositions of MOC were evaluated by x-ray diffractograms, the morphological properties were examined by means of scanning electron microscopy and phase decomposition was analysed using differential thermal analysis/thermogravimetric analysis. It was found that the use of SGP can enhance the water resistance of MOC by the formation of forsterite, which can limit the decomposition of phase 5 and prevent the formation of brucite in water.

Nanofibrillated cellulose from eucalyptus pulp, produced by high-pressure homogenization, was used as cement partial replacement for cement paste at a content ranging from 0% to 0.5% by weight of cement. The effect of the content of nanofibrillated cellulose on porosity, thermal properties, compressive strength and degree of cement hydration was investigated. Results have shown an improvement in the compressive strength by more than 50% with 0.3 wt% of added nanofibrillated cellulose. The porosity was reduced by nanofibrillated cellulose addition, and the greatest result was achieved with mixture incorporating 0.3 wt% nanofibrillated cellulose. The coefficient of thermal expansion and the thermal conductivity measurements, relative to nanofibrillated cellulose-reinforced cement pastes, have pointed out the reinforcement effectiveness of nanofibrillated cellulose. The degree of cement hydration has increased with nanofibrillated cellulose content. This trend was confirmed by X-ray diffraction and Fourier Transform Infrared spectroscopy. These analyses have revealed that the presence of nanofibrillated cellulose promoted the hydration of cement, by producing more portlandite and calcium silicate gel, which is likely the main reason accounting for the strong enhancement in the compressive strength.

Extracting valuable salts from reverse osmosis reject brine may be a solution to reduce environmental risk of this effluent if discarded. Isothermal evaporation (25°C) allows recovering many salts like gypsum, halite, and magnesium salts (epsomite, hexahydrite). In this paper, magnesium sulfate crystallization was studied, for this purpose two different quinary diagrams were used to draw the crystallization path. Furthermore, the application of Pitzer model and the use of Phreeqc program were required, respectively, to calculate electrolytes solubility product and to simulate the evaporation steps. Experimentally, X-ray diffraction was adopted to identify the nature of recovered solid phases. The different results of this study are almost similar and confirm the crystallization of epsomite and hexahydrite at the end of evaporation process.