Abstract
Direct casting is a processing method that can produce large parts of complex geometry. It uses dispersions of ceramic particles in water and a hydraulic binder for consolidation. Studies on castable structures have reported the generation of a significant fraction of pores due to the presence of water. This effect occurs because drying preserves part of the interparticle pores originally occupied by water. The fraction of pores obtained in these cases can reach levels higher than 50%, affecting the properties of the structure. However, the porogenic behavior of water in these materials was not investigated in depth. In this work, aqueous suspensions of calcined alumina and hydratable alumina (hydraulic binder) of different water contents (40–60 vol.%) were prepared in a paddler mixer. For compositions of lower (5–35 vol.%) or higher (66–80 vol.%) water amounts, casting and curing steps were assisted by external pressure and a rotating device, respectively, to produce homogeneous microstructures. The total porosity after drying was similar to the initial volumetric content of water in the compositions when side effects, such as sedimentation and air entrapment, were prevented. Below water content of 25 vol.%, particles packing flaws became the main pore generator and the hydroxylation reaction of the binder no longer occurred effectively.
Highlights
In the process known as direct casting, ceramic particles are mixed with water, additives, and a binder to produce a stable suspension [1,2,3,4,5]
The presence of water generates strong capillarity forces amongst large and fine particles (Figure 1(b)) [17, 18, 20, 21]. Such forces are known as Wall effect and improve the packing efficiency of the system. irdly, water is a major component of most binding systems for castable suspensions (Figure 1(c)) [22,23,24,25]. e reactions amongst water and hydraulic binders during mixing and curing steps produce hydrated compounds that restrain the matrix particle movement
Structures of low homogeneity are produced. To overcome these aspects and study the impact of a wide variation in the water content in physical properties of castable systems, the present study used two strategies: (1) to produce suspensions of very high solid loads (60–95 vol.%), water was added to the powders as a spray and the consolidation was assisted by isostatic pressing and (2) to prevent particle sedimentation, the initial setting of very low solid-load suspensions (20–55 vol.%) was performed in a rotating device that slightly agitates the liquid inside the molds before hardening. e samples prepared by these methods were compared to others produced in a conventional paddler mixer and static casting conditions
Summary
In the process known as direct casting, ceramic particles are mixed with water, additives, and a binder to produce a stable suspension [1,2,3,4,5]. Stable suspensions containing high solid loads can be produced using particles of optimized size distribution, proper dispersants to prevent agglomeration, and suitable mixing of high shear and turbulence [16,17,18,19,20] Under favorable conditions, such suspensions can present low viscosity and excellent workability, which are requirements to facilitate the casting step and prevent the undesired entrapment of air bubbles into the structure. Aqueous suspensions of calcined alumina or silica (dense matrixes), several inorganic porogenic agents (hydrotalcite [8], Al(OH)3 [9, 10], Mg(OH)2 [27], and CaCO3 [28]) and hydratable alumina were produced and directly cast using hydraulic binders (hydratable alumina, mainly) In these systems, the total porosity levels attained after curing and drying (50– 120°C, 45–60%) were close to the volumetric amount of water initially added during mixing (40–55%). To overcome these aspects and study the impact of a wide variation in the water content in physical properties of castable systems, the present study used two strategies: (1) to produce suspensions of very high solid loads (60–95 vol.%), water was added to the powders as a spray and the consolidation was assisted by isostatic pressing and (2) to prevent particle sedimentation, the initial setting of very low solid-load suspensions (20–55 vol.%) was performed in a rotating device that slightly agitates the liquid inside the molds before hardening. e samples prepared by these methods were compared to others produced in a conventional paddler mixer and static casting conditions (solid load varying from 20 up to 75 vol.%)
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