Abstract: This study used a fulvic acid (FA) solution with a pH of 6 to simulate the weakly acidic groundwater environment typically caused by humic acid in peat soil. The FA solution soaked soft soil containing humic acid that had been solidified with ordinary Portland cement (OPC). Various methods, including visual observation, microscopic analysis, loss on ignition testing, and osmotic pressure tests, were employed to investigate how this environment influenced the structure and permeability of the humic acid-containing soil-cement. The results indicate that FA undergoes complexation reactions with cement hydration products and polyvalent metal cations released during hydration, leading to the formation of organic metal colloids. This process depletes the hydration products and inhibits further hydration. When the OPC content is low, the quantity of organic metal colloids formed is insufficient, which may cause migration and result in the softening and detachment of the surface soil from the specimens. Additionally, the internal structure of the specimens, influenced by humic acid (HA) and FA, develops granular or blocky interconnected structures with numerous large pores. These changes increase the permeability coefficient (k) of the specimens, with the effect becoming more pronounced with extended immersion. Increasing the OPC content inhibits FA infiltration into the specimen and promotes the deposition of organic metal colloids on the surface, resulting in a dense surface layer. Higher OPC content enhances the bonding of hydration products to soil particles and improves pore filling, increasing the overall integrity. This reduces the number of pores and consequently lowers the k of the specimens. To balance engineering safety with environmental and low-carbon requirements, the OPC content used to solidify humic acid-containing soft soil should be controlled between 23.3% and 30.0%.

Key words: fulvic acid environment, humic acid-containing soil-cement, complexation reaction, organic metal colloid, microstructure, permeability coefficient