Synthesised water repellent granular solids

Abstract

Water repellent granular solids can be synthesised and used as a geomaterial to inhibit water infiltration. Compared to conventional ground improvement practices, they allow for a smoother integration in the natural or built environment. The research aimed at revealing the influence of chemistry and particle characteristics of granular solids (including a natural sand) on water repellency. The aim was implemented via the following objectives: 1. develop and optimise contact angle (CA) measurements as a means of quantifying water repellency on granular solids 2. determine the relation between water repellency and chemical concentration and investigate the surfaces of the synthesised granular solids 3. establish the relation between water repellency and particle characteristics of granular solids and 4. devise an approach to enhance water repellency of granular solids by powder-coating.

The sessile drop method was used to address the first objective. To reduce uncertainty associated with the measurement of CAs, a new technique called Semi-automated was devised that considered all the factors that influence the measurement of CAs. Comparison of standard deviations of CAs with existing techniques showed that the Semi-automated technique improved CA measurements on water repellent granular solids by 33%.

For the second objective, dimethyldichlorosilane (DMDCS) was used to synthesise Leighton Buzzard sand (LBS) and its effect on water repellency was compared to eight chemical products. The critical concentration of DMDCS was the lowest and consequently allows for minimum use of resources to achieve maximum CA. Changes in surface roughness over time were observed with all chemical products following synthesis; with DMDCS, an overall decrease was noted. Thus, synthesis of water repellent granular solids by chemical means is a dynamic process.

The third objective was met by using glass beads, LBS and crushed glass of different particle sizes. They were first synthesised with DMDCS beyond their critical concentrations to isolate chemistry. An increase in CAs was recorded as particle size decreased and when particles became more angular. These results were attributed to differences in void fraction and due to pinning effects on sharp edges. Relatively larger void fractions were measured with the finer particle sizes. As particle size decreased, particle shape became more predominant in dictating wettability. Therefore, by using specific particle characteristics of granular solids, their wettability can be controlled.

For the last objective, following a stepwise powder-coating procedure on different particle sizes of a sand in two mixing ratios, CAs were shown to increase. The maximum increase was recorded with the coarsest fraction. CAs were independent of mixing ratios. Surface roughness of the sand particles increased without any change in their particle shape. Coating powders on granular solids was shown to be a novel and effective approach to increase CAs.

This research has provided alternative routes to gauge and enhance water repellency in granular solids. In addition to chemistry, particle characteristics at both nano and micro-scales were shown to influence CAs. These findings pave the way to engineer and identify water repellent granular solids to be used as geomaterials in engineering applications.