| Rheological properties of suspensions have been one of the interest fields of colloidal chemistry. Most researches focused on the rheology of colloidal suspension composed of single type of charged particles, and little on the rheology of mixed colloidal suspensions composed of opposite charged particles. In this paper, the rheological behavior of mixed suspensions composed of Layer double hydroxide compounds (LDHs) particles with positive charges and kaolinite particles with negative charges have been investigated. The works will get better understand on the rheology of suspensions.1. The synthesis and structure properties of samplesUsing the co-precipitation method, Mg-Al-LDHs and Mg-Fe-LDHs were synthesized, and their shape, crystal structure, size distribution and zeta potential were studied with X-ray diffraction (XRD), transmission electron microscope (TEM) and laser particle size analyzer, respectively.TEM micrographs indicated that the LDHs particles were plate-like; XRD pattern showed that the LDHs had Hydrotalcity-like layered structured; The isoelectric point (IEP) of pure kaolinite suspension was about pH=4, and the IEP of LDHs were at a pH value around 11. In addition, the TEM micrographs of LDHs/kaolinite mixed suspensions showed the distinct aggregation behavior between LDHs particles and kaolinite particles.2. The thixotropy of LDHs/kaolinite suspensionsThe thixotropy of suspension is a complicated rheological phenomenon. In our former works, we discovered a new thixotropy behavior, i.e. complex thixotropy by steady-state measurement in LDHs/clay suspensions. But it was found later that the shear rate introduced in steady-state measurement could disturb the structure of suspensions, and then could change the type of thixotropy. Therefore, in this section, the linear oscillatory shear experiment was performedto investigate the thixotropic behavior of LDHs/kaolinite suspensions. In the linear oscillatory experiment the system was subject to a sinusoidal stress. It is supposed to be a non-destructive test, so the change of \tj*\ with / could really represent the recovery process of destroyed structure after cessation of pre-shearing.The results in this section further indicated that the complex thixotropy was a natural phenomenon, not an apparent one. In additional, the thixotropic loop experiment was also adopted to study the thixotropy of LDHs/kaolinite suspensions. Combining the results obtained by the linear oscillatory shear experiment and the thixotropic loop experiment, it was demonstrated for first time that thixotropic loop of complex thixotropy had a crossover point that divided the loop into two sections.The effects of R (R = Wldhs/ ^kaolinite* i.e. the mass ratio of LDHs to kaolinite), pH, electrolytes and polymers on the thixotropy of pure kaolinite suspensions and LDHs/kaolinite with R=0.25 were investigated. The results indicated that the pure kaolinite suspensions showed positive thixotropy and LDHs/kaolinite suspensions with R=0.25 showed complex thixotropy at pH=10.60±0.10. The thixotropic type of LDHs/kaolinite suspensions transformed from positive thixotropy to complex thixotropy with the increase of R. It indicated that the microstructure of the suspension change from steric network structure to weak floe sediments structure.In the range of pH of interest (pH=3~12), the pure kaolinite suspension showed positive thixotropy and didn't be influenced by pH. But the thixotropy type of LDHs/kaolinite suspensions with J?=0.25 transformed from positive thixotropy to complex thixotropy.The structure intensity of pure kaolinite suspension was increased after adding NaCl and CaCl2; with the increase of the content of NaCl, the pure kaolinite suspension showed positive thixotropy, then complex thixotropy and at last positive thixotropy again; CaCb didn't influenced the thixotropy type of pure kaolinite suspension. But the thixotropic type of LDHs/kaolinite suspensions with i?=0.25 transformed from complex thixotropy to positive thixotropy and thencomplex thixotropy again with increase of NaCl content; the thixotropic type of the LDHs/kaolinite suspensions with R=0.25 wasn't influenced by CaCh.The structure intensity of pure kaolinite suspension was decreased by CPAM and HPAM; and the two polymers didn't influence the thixotropy type of pure kaolinite suspensions. For the LDHs/kaolinite suspensions with R=0.25, the thixotropy type transformed from complex thixotropy to positive thixotropy, which indicated that the microstructure of the suspension change from weak folc sediments structure to steric network structure.3. The yield stress of LDHs/kaolinite suspensionsThe effects of R, pH value, electrolytes and polymers on the yield stress of pure kaolinite suspensions and LDHs/kaolinite suspension with iJ=0.25 were investigated by flow curve experiment, creep sweep and stress sweep, respectively. The results showed that the yield stress of pure kaolinite suspension decreased first then increased with the increase of R.In the range of pH of interest, the yield stress of pure kaolinite suspension increased first (pH<7), then decreased (pH>7) with the increase of pH value. For LDHs/kaolinite suspension with R=0.25, the yield stress increased first (pHll) with the increase of pH value.In the content range of NaCl of interest, the yield stress of pure kaolinite and LDHs/kaolinite suspension with i?=0.25 increased with increase of the content of NaCl. On the contrary, the yield stress of the two suspensions was all decreased rapidly with the increase the content of CPAM. The yield stress of pure kaolinite suspension was decreased remarkable only when the content of HPAM was high.4. The rheological fluctuation behavior of LDHs/kaolinite suspensionsThe Theological fluctuation behavior was discovered by steady-steady measurement in LDHs/kaolinite suspensions, i.e. the apparent viscosity of the suspensions was oscillating in time at a constant low shear rate after pre-shearing.The works focused on the effects of R, shear rate, pH, electrolytes and polymers on the Theological fluctuation behavior in order to study the relations between the microstructure of the suspensions and the interaction of the particles.The apparent viscosity of the LDHs/kaolinite suspensions was periodic in time for a constant low shear rate D=ls"' after having been pre-sheared at 1000s'1 for lOmin, the amplitudes of the fluctuation behavior decreased with the increase of R and disappeared eventually.Both the oscillation periods and amplitudes decreased with the increase of shear rate (D), and the fluctuation behavior of the amplitudes almost disappeared at ￡>=5.0s"'.The pH value didn't affect the periods of the fluctuation behavior of LDHs/ kaolinite, but affect the amplitudes. For pure kaolinite suspensions, the amplitudes showed a maximum value at a pH value around 7; and for Mg-Al-LDHs/kaolinite suspensions with R=0.25, the amplitudes showed a maximum value at a pH value around 11.When studied at D = 0.5s"1, the fluctuation behavior of pure kaolinite suspension were hardly affect by NaCl and CaCb- For Mg-Al-LDHs/kaolinite suspensions with R=0.25, the fluctuation behavior were disappeared when the content of electrolytes were higher (C = 0.40mol/L).When studied at D = 0.5s'\ the periods of the fluctuation behavior of pure kaolinite were not affect by CPAM and HPAM, but the amplitudes of the fluctuation behavior increased. On the contrary, the amplitudes of the fluctuation behavior of Mg-Al-LDHs/kaolinite suspensions with R=0.25 decreased.
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