The rheological properties of three Na-activated, trioctahedral Mg-bentonites (hectorite clay from the CMS Source Clay Project repository, saponite clay from Spain and stevensite clay from Rhassoul, Morocco) and a sepiolite clay from Greece were examined after dynamic ageing at temperatures up to 230°C. The 5% w/v suspensions were prepared by dispersing the clay mineral samples in distilled water. The suspensions underwent dynamic, thermal ageing for 16 h before determination of the viscosity, filtration loss, filter cake thickness and pH and the concentration of dissolved Na+ and Mg2+. Thermal ageing contributed to the dispersion of clay particles, with a direct effect on plastic and apparent viscosity, introducing pseudoplastic behaviour. With the exception of the stevensite clay at 230°C that displayed limited dissolution at 230°C and partial conversion to kerolite, the clays were stable at high temperatures. The Na-activation of all clays except for stevensite was not adversely affected by thermal ageing. Thermal ageing of stevensite at 230°C facilitated Na exchange and yielded suspension with high viscosity and low filtrate loss. Only the suspensions of hectorite and those of stevensite aged at 230°C met with American Petroleum Institute specifications. The thermal behaviour and rheological properties of the clays might be interpreted according to the intrinsic properties of the clay minerals, such as layer charge and charge distribution.

The aim of this study was to help drive the Tunisian construction industry towards a more sustainable approach given the existence of abundant local raw material deposits that could be exploited for the production of low-CO2 binders. Various clay sediments from the Kebili region (southern Tunisia) were characterized by chemical analysis, X-ray diffraction, thermal analysis and geotechnical tests to determine their suitability for the preparation of geopolymer binders. The clays consist of illite and kaolinite with other accessory minerals. To test the possibility of using these materials as precursors for the production of low-CO2 and low-cost geopolymers, the raw samples were calcined and activated by addition of solid sodium silicate. Compressive strength tests performed on four alkali-activated clays show that promising mechanical performance may be achieved, with mechanical strength values as high as 25 MPa after 7 days, depending on the clay composition. The mechanical strength is related to the SiO2:Al2O3 and Al2O3:(NaO2 + K2O) ratios. Careful selection of the raw materials is, therefore, an essential step in the exploitation of clay deposits to be used for the production of ecological materials such as geopolymers.

The aim of this research was to determine the temperature of kaolin calcination in order to obtain an intermediate product (metakaolin) for the synthesis of geopolymers with potential application as self-supporting zeolitic membranes. The products obtained were analysed with X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The structural analysis of the metakaolins obtained suggested that the optimal temperature for the proposed application is 700°C. After alkali activation of metakaolin, it is possible to obtain zeolite A and hydroxysodalite. The factors analysed, determining the type and quantity of crystalline phases, were activation temperature and concentration of sodium hydroxide solution (activator). The largest amounts of zeolites were obtained by alkali activation with 9 mol/dm3 NaOH solution at 70°C.

An experimental investigation was conducted on the alkali activation of a kaolinitic clay using an alkaline mixture composed of hydrated lime (Ca(OH)2) and sodium carbonate (Na2CO3) solution. The Ca(OH)2/Na2CO3 alkaline mixture was developed to overcome the high cost and chemical aggression associated with classical alkaline solutions. The mineralogical composition and microstructure of the alkaline mixture and alkali-activation products were studied using X-ray diffraction, attenuated total reflectance-Fourier transform infrared spectroscopy, thermogravimetric analysis and energy-dispersive X-ray-scanning electron microscopy techniques. Pirssonite (Na2Ca(CO3)2·2H2O), calcite (CaCO3), and sodium hydroxide (NaOH) formed as a result of mixing of Ca(OH)2 with Na2CO3. The alkali activation of natural clay with the Ca(OH)2/Na2CO3 alkaline mixture produced a binding agent identified as hydroxysodalite phase (Na8Al6Si6O24(OH)2·4H2O) when pure kaolinite was used, and cancrinite carbonate (Na6Ca1.5[Al6Si6O24](CO3)1.5·1.8H2O) when kaolinitic clay with high a calcite content was used. The mechanical strength of the binder developed was evaluated on cylindrical specimens containing granite waste as a filler material under dry and soaked conditions. The classical NaOH activator was used for comparison. For specimens produced using a Ca(OH)2/Na2CO3 mixture as the alkaline activator, the recorded strength valuewas 21 MPa which was 35% less than that achieved by the classical NaOH solution. Durability tests on samples soaked in water for 24 h showed a reduction in strength from 34 to 22 MPa for specimens prepared with NaOH solution, and from 21 to 11 MPa for the specimens prepared with a Ca(OH)2/Na2CO3 alkaline mixture.