Abstract: The degradation of naphthalene highly effective water reducer (NHWR) in the concrete mixing system of a large-scale hydraulic project situated in dry and hot river valley area of Jinsha River is a critical problem which considerably shortens the concrete setting time. This study investigates the effectiveness of adding alkali to adjust the pH value on NHWR storage quality and the properties of fresh and hardened concrete. A productive experiment on concrete was conducted to examine the consolidation effect at the project site. Results show that a pH level of 9.5 or higher can significantly inhibit the putrefaction of NHWR solution, and remarkably lengthen the setting time of fresh concrete, leading to a decrease in slump and the hourly loss rate of air content. Importantly, there is no detrimental effect on the mechanical property and durability of hardened concrete. Moreover, the concrete mixture produced for the placing area exhibits good workability, and its initial setting time has risen to 13 hours from the previous nearly 7 hours. Consequently, it is now possible to consolidate concrete in three or four placing areas at the same time, which enhances the efficiency and quality of dam concrete placing and produces high comprehensive benefits.

Key words: hydraulic concrete, naphthalene highly effective water reducer, alkali, pH value, setting time