Cave temples, a unique type of cultural relics in China, possess extremely important historical, artistic and scientific value. Cracks are the main cause of instability, seepage,weathering and other forms of damage in these structures. Conventional external curing grouts and reinforcement methods are ineffective in addressing shrinkage cracking and insufficient strength caused by water shortages in deep cracks during the later curing stages. This study prepares montmorillonite (MMT)-modified natural hydraulic lime (NHL) for internal curing through a one-step in situ polymerization process and systematically evaluates the hydration behavior and performance of the modified NHL during curing. Results show that the internal curing effect of NHL is optimal at a 2% MMT content. After 7 days of curing, the autogenous shrinkage of the modified sample is only 56.70% of that of the blank sample, demonstrating excellent water retention. Moreover, the compressive strength of the modified NHL after 28 days of curing increased by 22.03% compared with the blank sample. This improvement is attributed to MMT’s ability to continuously release interlayer-adsorbed water under the sample’s internal humidity gradient during the entire curing process, promoting the hydration reaction of NHL, reducing autogenous shrinkage, and improving the internal curing effect. This study provides an important theoretical and practical foundation for the development of internal curing grouts and reinforcement techniques for cracks in cave temples.