Air-setting, phosphate-bonded castables rely on a chemical bonding mechanism that activates when exposed to ambient moisture and carbon dioxide. API 936 outlines that these materials should becured in ambient air without being covered, as covering can trap moisture and inhibit the proper development of the chemical bond. Covering or artificially heating during this early curing stage may lead to defects such as cracking or uneven setting.
Theminimum 24-hour air curingperiod ensures sufficient evaporation of excess water and the development of mechanical integrity before any handling, heat exposure, or further processing. Covering these castables may interfere with the air-bonding mechanism and is not recommended.
Reference:
API Std 936, Section 6.3.1: "Air-setting phosphate-bonded refractories shall be air cured, uncovered for at least 24 h." API TR 980, Section on Air-Setting Castables: Emphasizes the importance of allowing unrestricted air exposure during initial set.