Corrosion of Hardened Paste of Gypsum-Free Portland Cement by Sulfuric Acid

Gypsum-Free Portland Cement (GFPC) is a system of ground Ordinary Portland Cement (OPe) clinker with an anion-active surface-active agent and a hydrolysable alkali metal salt. The basic advantage ofGFPC over OPC is thepossibility ofpreparing workable pastes with very low water-to-cement ratios down to 0.22. The properties of GFPC are different from those ofthe OPC. They display higherstrength, durability, and thermal stability. In this work, the mechanism of acid corrosion of hardened paste of GFPC in sulfuric acid solutions has been studied in details and compared with that of the corresponding OPe system. X-ray energy dispersion measurements (X-ray line analyses) performed on a number of 90-day corroded GFPC and opepaste specimens reveal that at each of the three pH values of 1.2, and 3 of sulphuric acid solutions the hardenedpaste specimens ofGFPC and OPC are corroded by exactly the same mechanism. The only difference between the two types ofcement is based on the corrosion rate. Observations made by scanning electron microscope and energy dispersive analysis by x-ray also imply that the mechanically stronger microstructure of hardened paste of GFPC is more resistant to disintegrating stresses caused by crystallization pressure of gypsum, At relatively higher pH values (pH > 3) and for relatively short exposure time periods (exposure time < 90 days), the corrosion mechanism is simply dissolution and leaching with no gypsum deposit in the corroded layer.

Keywords:

Gypsum-Free Portland Cement , Corrosion , Sulphuric Acid