Study on the Modification and Mechanism of Mechanical Properties of Hemihydrate Phosphogypsum by Different Alkalis

Abstract

The alkaline environment plays a significant role in the performance of hemihydrate phosphogypsum (HPG). Nevertheless, the impact and mechanism of the alkaline environment within the HPG mineral admixture composite system remain unclear. This research examines the effects of three distinct alkalis KOH, NaOH, and Ca(OH)2 on the mechanical properties, hydration products, and microstructure of the ternary system composed of HPG, fly ash (FA), and ground granulated blast-furnace slag (GGBS). Findings indicate that alkali types not only influence the formation of hydration products but also alter the crystal morphology of gypsum. KOH's effect on AFt exhibits a lag phenomenon, whereas NaOH and Ca(OH)2 demonstrate consistent patterns where low concentrations facilitate AFt formation and high concentrations inhibit it. The effects of KOH and NaOH on gypsum crystallization are primarily determined by the cation adsorption on the sunlit face of gypsum crystals, resulting in the formation of M2(SO4)2 complex salts (where M denotes K, Na, or Ca). In contrast, Ca(OH)2 primarily modifies gypsum crystal morphology by selective impurity absorption.

Keywords: Mechanical Properties, Hemihydrate phosphogypsum, Alkali types, Gypsum crystallization