Tannery Sludge-slag-based Composite Geopolymer Process Proportioning And Durability Study

Tannery sludge is one of the hazardous solid wastes produced in the production process of tannery industry in China,which contains more Cr,Cu,Ni,Pb,Zn and other heavy metals.Because Cr is easily transformed into highly toxic hexavalent chromium,it will seriously endanger human health and ecological environment if not treated properly.At present,the accumulation of chromium-containing sludge in China has exceeded 4×106 t,and is increasing at a rate of 105 t/a.The high solubility and biological toxicity of Cr(Ⅵ)in tannery sludge is the key and difficult point for disposal.For the safe disposal and resource utilization of tannery sludge,the reduction,stabilization,harmlessness and resourceization of tannery sludge is a major issue that needs to be solved in China,which is of great significance to protect the environment and save resources.In this paper,we used granulated blast furnace slag and tannery sludge as gelling materials to prepare tannery sludge-slag-based composite geopolymer for curing/stabilization of tannery sludge to achieve the purpose of tannery sludge harmlessness/resource utilization.The compressive strength,and total chromium and Cr(Ⅵ)leaching concentrations of the cured bodies were measured,and the pore space,micromorphology,physical phase and chemical bonding vibrations of the composite geopolymers were analyzed by nuclear magnetic resonance(NMR),scanning electron microscopy(SEM-EDS),X-ray diffractometer(XRD)and Fourier infrared spectroscopy(FT-IR)to study the curing/stabilization effect of tannery sludge and the influencing factors.factors,and the related mechanisms were explored.The main findings are as follows:In this paper,we found that the three factors of excitation ratio,alkali excitation modulus and water-cement ratio had significant effects on the compressive strength of tannery sludge-slag-based composite geopolymer,and based on the response surface methodology,we investigated the hydration mechanism of its composite geopolymer and the interaction relationship between its materials and obtained the process formulation with the best mechanical properties,when the excitation ratio was 21.74%,the excitation modulus was 1.02and the water-cement ratio was 0.37.The compressive strength of the composite geopolymer is the highest,and the 28-day compressive strength can reach more than 70 MP.The total chromium and Cr(Ⅵ)precipitation concentration and leaching rate of this material were 1.01mg/L,1.27%and 0.05 mg/L,15.77%,which were much lower than the national hazardous waste emission standard of 5 mg/L and 0.5 mg/L.Compared with 78.6 mg/L and 0.317 mg/L before curing,the leaching concentration was greatly reduced.It was demonstrated that the heavy metal Cr in tannery sludge could be well cured/stabilized in geopolymer and still had a high compressive strength,which could support the utilization of tannery sludge for building materials.The hydration mechanism of tannery sludge-slag-based geopolymer and the solidification mechanism of heavy metal Cr were investigated in the paper.It was found that the hydration products were mainly highly amorphous C-S-H gels,and the Ca/Si ratio was much smaller than that of C-S-H gels of silicate cement by EDS scanning,which was due to the substitution of Si in the Si-O tetrahedra at the bridge oxygen position in the C-S-H gels by Al.The XRD diffraction pattern found that Cr can replace Ca and Si in the C-S-H gel and enter the C-S-H gel structure to produce calcium-chromium garnet(Uvarovite,Ca3Cr2[Si O4]3),and Cr was found to be present in the C-S-H gel in the point scan elemental analysis by EDS.It is beneficial to the further development of tannery sludge building materialization/resource utilization.The paper investigated the durability of tannery sludge-slag-based geopolymer under high temperature deterioration and found that the compressive strength of tannery sludge-slag-based composite geopolymer increased slightly from 50.16 MPa to 63 MPa after 2 h of high temperature deterioration at 200°C,which was presumed by FT-IR to be the condensation reaction caused by the evaporation of free water contained in the specimens.The compressive strength then decreases with the rise of deterioration temperature and remains 45.67 MPa after2 h of high temperature deterioration at 600°C.Ordinary silicate cement basically does not have strength after high temperature calcination at 500°C,indicating that the tannery sludge-slag-based composite geopolymer possesses high temperature resistance far beyond that of ordinary silicate cement.It still has 15.15MPa after 2h of high temperature deterioration at 1000℃,which proves that it has good high temperature resistance.XRD shows that the geopolymer hydration product C-S-H gel disappears after calcination and deterioration at 800℃is replaced by calcium-magnesium yellow feldspar,calcium-aluminum yellow feldspar and magnesium-silica-calcite,and the gel with three-dimensional reticulation structure is changed materially.The NMR results show that the pore size and porosity of geopolymer increase,and the harmless pores and less harmful pores disappear at 800 and 1000°C degradation respectively,and the above are the reasons for the decrease of compressive strength.The paper investigated the durability of tannery sludge-slag-based geopolymer under strong acid and alkali solutions and found that the compressive strength of tannery sludge-slag-based composite geopolymer specimens soaked in strong acid solution decreased with the increase of acidity,and the compressive strength decreased from 50.16 MPa to 25.43 MPa after28 d of soaking in strong acid at p H 1.The FT-IR results showed that the Si-O-Al and Si-O-Si bonds were shifted under the long-term immersion in the strong acid solution,which was analyzed together with the XRD results because the high concentration of H+in the strong acid solution entered the three-dimensional structure of the geopolymer and replaced the C The high concentration of H~+in the strong acid solution entered into the 3D mesh structure of the geopolymer and replaced K~+and Na~+in C-A-S-H for balancing the negative charge,resulting in partial depolymerization of the calcium carbonate and C-A-S-H gel structures,which can also be seen as numerous small cracks in the SEM images.The three-dimensional mesh structure of the encapsulated tannery sludge received damage,which led to the increase of total chromium and Cr(Ⅵ)leaching concentration.The compressive strength of tannery sludge-slag-based composite geopolymer specimens soaked in strong alkaline solution increased with the increase of alkalinity,and XRD and FT-IR showed that the physical phase and chemical bonding did not change after 28 d of soaking.The NMR results showed that the porosity decreased with the increase of alkalinity,and the leaching concentration of total chromium and Cr(Ⅵ)decreased.