Green and effective route to convert chromium-free leather waste into protein retarding material for building gypsum

Leather manufacturing generates a large amount of leather solid waste, among which the effective treatment of leather waste scraps rich in protein resources has always been a challenge for the tanning industry. In this paper, a green and convenient method was devised for the first time to prepare protein retarding material (PRM) for gypsum from chrome-free tanned leather waste. Aldehyde tanned leather scraps as the main chrome-free leather waste were directly hydrolyzed through high-pressure hydrothermal treatment without any chemicals to obtain PRM. Compared with chemical treatment (acid or alkaline hydrolysis), high-pressure hydrothermal method avoided the decrease of protein product purity due to without the help of acid and alkali. The results showed that PRM obtained from leather waste had a significant effect on prolonging the initial setting time and delaying the release of hydration heat of gypsum. With the dosage or molecular weight of PRM increased, this retarding effect on gypsum was more remarkable. SEM, XRD, and FTIR analysis indicated that the presence of PRMs inhibited the hydration process of gypsum hemihydrate to gypsum dihydrate. Through polypeptide-calcium chelation, PRMs could bind free Ca2+ ions dissolved from gypsum to delay them from participating in the hydration reaction. The larger molecular weight or the more dosage of PRM, the stronger ability to bind calcium ions in gypsum. Chemical structure characterization and molecular dynamics simulation of PRM-Ca chelate further proved that PRMs mainly through carboxyl groups on molecular chain combined with Ca2+ in gypsum system to inhibit hydration reaction, thereby fulfilling the retardation demand. This study indicated that PRM from leather scraps can be a promising candidate for gypsum retardation, which provides a path for the efficient utilization of leather waste.