Constriction size retention criterion for calcium alginate microcapsules in granular materials

Abstract

Microcapsules have demonstrated to self-heal cementitious materials. However, their prospect in granular materials and other porous cemented materials remains to be tested. An initial challenge to tackle is to ensure the microcapsules retention in open, interconnected porous media. The purpose of this study is to investigate the retention of microcapsules in gravel under the influence of seepage. This scheme largely coincides with the intention of geotechnical filters to prevent the excessive washout of base soils. A mathematical model previously designed for cohesionless soil is used to establish an initial retention criterion based on the constriction size distribution of gravel (the minimum pore opening size distribution formed by various gravel packings) and, the particle size distribution of microcapsules (made of calcium alginate/polydimethylsiloxane) and an equivalent control material (glass beads). The proposed model is verified experimentally by washout tests on microcapsules-gravel and glass beads-gravel mixtures. Optical microscopy is used to assess the integrity of microcapsules after testing. The lower washout of microcapsules compared with cohesionless glass beads could be attributed to their adhesive nature. By incorporating the influence of size change of microcapsules during drying, which is an intrinsic property of the microcapsules, a modified empirical equation is presented for soft and adhesive microcapsules, demarcating the boundary between effective and ineffective retention for microcapsules-gravel mixtures. • Constriction size is used to design microcapsules - granular materials mixtures. • Permeability/washout of glass beads/microcapsules in gravel mixtures are monitored. • Washout of glass beads and microcapsules depend on constriction size of gravel. • Loss of microcapsules is minor due to their adhesion-prone and swelling nature.