1. Why Is Water Retention Critical in Plastering Mortar? Plastering mortar performance depends not only on cement, sand, and other raw materials, but also on its ability to retain water. When applied to substrates such as autoclaved aerated concrete (AAC) blocks, lightweight wall panels, or cast-in-place concrete, the substrate's water absorption characteristics directly influence moisture movement within the mortar. If the substrate absorbs water too quickly, the mortar may lose consistency and workability, while insufficient moisture may interfere with cement hydration. Under unfavorable conditions, these effects can increase the risk of poor interfacial bonding, cracking, and hollow spots. Cellulose ether, particularly hydroxypropyl methylcellulose (HPMC), is an important functional additive for improving water retention and workability in plastering mortar. HPMC increases the viscosity of the aqueous phase and helps retain moisture, slowing water migration and maintaining the ...