Presence of higher Hemihydrate content in cements in OPC causes false set, however in blended cements like PPC and PSC invariably false set is not observed in physical testing of the cements, as the fly ash and slag components ten to retard the cements; however such cements at application sites show problems of slump loss. Many a times at sites, extra water is added in an attempt to compensate for the slump loss which affects early strengths and hardening of concrete. Under humid climatic conditions or in humid storage conditions, such cements with higher Bassanite contents shows formation of soft / Hard lumps in cement bags generates complaints / rejection of cement in market.
Gypsum dehydration is a regular phenomenon in cement plants mainly due to use of hot clinker, temperature rise during grinding converting gypsum to Bassanite and conversion back to gypsum after absorption of moisture. Various measures taken at cement plants to avoid gypsum dehydration are; controlling the clinker temperature below 1100C or providing internal water spray (mist) during cement grinding to avoid overshoot of temperature. Inspite of taking all these measures it has been observed that the soft / hard lump formation is observed the hemi-hydrate or Bassanite content in cements are >40%.
The Cement plants having XRD can easily test and evaluate the hemihydrates in cements during grinding, however in cement plants when XRD is not available it is very difficult to determine / monitor and control the Bassanite content.
The paper discusses a simple, rapid and effective Marsh cone test developed at Author’s lab to study dehydration of gypsum; the test is based on the principle of flowability with time of cement slurry at optimized w/c ratio. The flowability through marsh cone correlates with the Bassanite content in cements.
The paper concludes that this test could be effectively used to determine and monitor the Bassanite (hemi-hydrate) contents. The test would assist in optimisation of grinding parameters at which the % Bassanite can be effectively controlled at desired levels through periodic monitoring by the Marsh cone flowability test, during cement grinding.