Surface and colloidal properties of chalks: A novel approach using surfactants to convert normal chalks into dustless chalks

Two classes of chalks, calcium sulfate (gypsum) based normal chalks and calcium carbonate based dustless chalks are used in teaching institutions globally. The cost of dustless chalks is almost 10 times higher than normal chalks. In many developing countries, chalk manufacturing is a home-based industry and supports the families. However, the fine dust produced by normal chalks has been of great concern for the health of teachers and students. In the present study, we have used XRD, water penetration, contact angle, particle size of dust generated, breaking strength of chalks, abrasion, weight/density, and surface morphology by SEM to delineate the differences in the properties and performance of these two classes of chalks. The results showed that the particle size of dustless chalks produced during writing/erasing process is significantly larger than that of normal chalks. The normal chalks are hydrophilic and absorb considerably more water than dustless chalks. This led to an approach to add a dewetting agent (surfactant) to normal chalk slurry to decrease water penetration in chalk. In addition, this improved mechanical strength and abrasion resistance of the normal chalks due to hydrophobic-hydrophobic interactions between particles. We were able to produce approximately similar quality chalks as the dustless chalks by addition of small concentration of surfactant (0.5 wt% AOT) in gypsum slurry. This approach will assist many small-scale manufacturers for making better quality chalks and diminish occupational health hazard to teachers and students. (C) 2015 Elsevier B.V. All rights reserved.