Preparation And Characterization Of Oriented Porous Cement Membranes

Silicate cement is the most common type of cement in general use around the world. It is usually made by heating a mixture of clay and limestone, or other similar materials of similar composition. Due to the low cost and widespread availability of the limestone, shale, clay, and other natural materials, silicate cement has been one of the lowest-cost materials widely used in the world. Currently, cement-based composites are the most essential and plentiful construction materials. Besides, silicate cement not only has good rheological properties but also hardens by reacting with water. This kind of hardening process calls hydration reaction. Hydration products make cement-based composites tough and durable. Compared to sinter forging, curing process is much more efficient and environmental-friendly. Though porous cement-based composites have been applied to various fields, such as artificial bone substitute, crude oil adsorption, electromagnetic interference shield, there are few reports on the preparation of porous silicate cement with unidirectional pore channels for separation process.There are many methods to produce inorganic porous materials, such as foaming, isopressing, dissolution, high temperature gasification technique. For the dissolution of pore-forming agent in the solution, particle dissolution method is an effective method for preparing a three-dimensional porous structure, commonly used in-bone tissue engineering structures. High temperature gasification sintering method, achieved by the gasification of pore former at a high temperature, is a common method for the preparation of porous ceramics.Recently, the freezing-cast process has shown to be a promising and versatile method for preparation of the porous materials, as it can produce interconnected pore channels in a tailored manner, e.g. aligned pore channels on a scale of several microns, which will offer favorable mechanical properties and desirable flux. This technique consists of freezing a liquid suspension, which is usually aqueous based in a special mold at low temperatures, followed by demolding and vehicle removal by freeze-dry process to obtain a green body, leading to a unidirectional pore channels in the porous silicate cement bodies. Although freeze-casting has been applied to a wide variety of materials, such as alumina, zirconia, silicon carbide, cordierite and polymeric materials, porous silicate cement with oriented and systematic pore morphology has not been achieved. Since the solidification is often oriented, the porous channels always run from the bottom to the top of the intact sample. The final porosity content can be tuned by varying the solid loading within the slurry, and the freezing temperature affects the size of porosity significantly. To date, water, camphene and tert-butyl alcohol (TBA) have been successfully used as pore forming agents (freezing vehicles). Among them, the unidirectional prismatic pores can be formed by TBA sublimation, which will be appropriate for the filtration process.To the best of our knowledge, there are few reports so far on the use of freezing-cast process to fabricate Porous silicate cement supports. The work presented in this study clarifies the unidirectional porous silicate cement bodies can be achieved using TBA as a freezing vehicle, and the effects of solid content and freezing temperature on porosity, pore morphology and mechanical strength of the porous samples were intensively investigated. The results showed that Cement membrane consists of support layer, transition layer and separation layer. These continuous regions can be formed in one single step without any further densification process. Our macroscale tubular cement membranes can be applied to the pretreatment of seawater or wastewater, and the filtration of blood proteins such as bovine serum albumin (BSA). cement membrane with unidirectional finger-like pore channels (support layer) and dense region (separation layer) was able to filter out 74.8% of of BSA, while maintaining a flux rate between 400 and 500 L·m-2·h-1 at average pressure of 0.2 Mpa. To examine the stability of the cement membranes, we repeated five runs of a filtration test. The selectivity of the membrane was all above 70% and the flux varied between 400 and 500 L·m-2·h-1 no obvious deterioration of either flux or selectivity was observed. These special inorganic membranes with desirable mechanical strength and cost-effective fabrication process may allow them to compete with commercial ceramic-based and polymer-based membranes.