Fast method to synthesise Ca(OH)2 and Mg(OH)2 nanoparticles and their performance with sodium oleate coatings

• A semiautomated method with localised sonication is used to synthesise nanoparticles. • XRD, TG, and TEM showed pure NPs and complete by-product removal. • Dual coatings with sodium oleate (surfactant) are investigated. • FTIR shows Ca(OH) 2 NPs react with Na oleate to form Ca oleate. • Contact angle tests support the formation of hydrophobic coatings (up to 124°) Ca(OH) 2 nanoparticles (NPs) are often used in coatings to consolidate or reinforce porous materials. However, the synthesis is time-demanding, and hydroxides do not provide water repellence, which is an important concern for outdoor materials. Unlike conventional methods, this paper explores a fast synthetic procedure to generate pure Ca(OH) 2 and Mg(OH) 2 NPs (10 g/L) that were later used in coatings. XRD and TG confirmed the production of pure NPs, and TEM examination revealed evident morphological differences. Additional colour tests, microscopy examination, FTIR and contact angle measurements showed their efficiency as coatings when combined with sodium oleate. Mg(OH) 2 NPs were significantly smaller than Ca(OH) 2 and caused lower colour change but presented worse performance in contact angle tests. By contrast, Ca(OH) 2 NPs formed insoluble calcium oleate coatings in combination with sodium oleate, as confirmed by new IR signals and displacement of the carbonyl band (−C=O) attributed to calcium oleate. In addition, the contact angle reached up to 124° in Ca(OH) 2 NPs / sodium oleate coatings. The results may open new synthetic possibilities for these treatments as well as further applications to extend the hydrophobicity and durability of porous materials and architectural coatings.