Formation and stability of gismondine‐type zeolite in cementitious systems

Abstract Microcrystalline zeolites of the gismondine family are often reported in alkali‐activated and blended cement systems. However, little is known about gismondine's compatibility with other cementitious phases to determine stability in long‐term phase assemblage. Experimental studies were conducted to investigate the compositional field of gismondine stability in the lime‐alumina‐silica‐hydrate systems, with a particular focus on understanding the compatibility of gismondine with other cement phases such as C‐S‐H, ettringite, monosulfate, strätlingite, katoite, gypsum, calcite, portlandite, alkali, silica, and aluminosilicate phases. Results show that gismondine‐Ca forms readily at ~85°C in high aluminosilicate compositions; and persists in the presence of calcite, gypsum, ettringite, katoite solid solution, low Ca tobermorite‐like C‐S‐H, silica and aluminosilicate phases, at 20‐85°C. However, gismondine‐Ca reacts with: (a) monosulfate, producing ettringite‐thaumasite solid solution; (b) portlandite, forming tobermorite‐like C‐A‐S‐H gel and siliceous katoite at >55°C; (c) aqueous NaOH, generating gismondine‐(Na,Ca), a garronite‐like zeolite P solid solution; and (d) strätlingite leading to the conversion of strätlingite to gismondine indicating the metastability of strätlingite with respect to gismondine at 55°C. The outcomes are discussed to provide insights into the long‐term phase assemblage of relevant cement systems such as lime‐calcined clay, alkali‐activated materials, and potentially ancient Roman concrete.