What Effect Does Surface Acid Pickling Achieve on UHPC Panels?

Acid-washing the surface of UHPC panels is an important process for refining finishes. Its purpose is not cleaning, but to selectively etch away the cement paste on the panel surface through controlled chemical corrosion, thereby "exposing" internal components such as aggregates and fibers and creating a unique, richly textured surface. By controlling the type, concentration, temperature, and duration of the acid treatment, Qinglong Group can achieve a wide range of artistic effects—from the fine sandstone texture of "light aggregate exposure" to the rugged rock-strata look of "intense etching." In the stone-imitation UHPC facade of Century Plaza on Nanjing East Road at Shanghai New World, acid washing successfully created the weathered character of natural stone that has endured wind and rain.

Mechanism: Selective Etching and Texture Enhancement

The surface of UHPC is a smooth, dense covering of hardened cement paste. The acid solution (typically diluted hydrochloric acid, hydrofluoric acid, or a mixture of both) chemically reacts with cement hydration products on the surface—such as calcium hydroxide and calcium silicate hydrate—and dissolves them. Quartz sand aggregates and steel fibers are chemically stable and largely immune to the attack. As the surface cement paste is gradually etched away, the sand grains originally embedded inside become progressively exposed, forming an undulating micro-texture. The exposed steel fibers develop a distinctive brown, speckled pattern through rusting. By controlling the depth of etching, the coarseness of the texture can be precisely controlled.

Visual Effects and Textural Layers Achieved

1) Color change: After acid washing, the overall brightness of the panel decreases and the tone shifts toward the natural color of the aggregate (such as the gray-white and yellow of quartz sand), making it appear more rustic and natural. 2) Texture enhancement: The surface changes from mirror-like/soft sheen to matte, and the touch changes from smooth to distinctly gritty. Under light, the undulating texture produces rich diffuse reflection and subtle shadows, giving the facade a more three-dimensional, vivid appearance—uniform in color from a distance, rich in detail up close. 3) Creating a sense of history: Through uneven acid washing or localized deep treatment, the mottled, layered effects formed by long-term weathering and water erosion of natural stone can be simulated, offering strong artistic appeal.

Key Parameters for Process Control

The acid-washing effect is "a science turned art" that requires precise control: 1) Acid formulation: The combination and concentration of different acids determine the etching speed and the fineness of the texture. 2) Duration and temperature: The longer the time and the higher the temperature, the deeper the etching. Optimal parameters must be determined through small-sample testing. 3) Application method: Immersion, spraying, or brushing may be used; different methods produce uniform or mottled effects. 4) Neutralization and cleaning: After acid washing, the panel must be rinsed immediately with plenty of clean water, and residual acid neutralized with an alkaline solution (such as sodium carbonate solution) to prevent later "efflorescence" or continued internal corrosion.

Impact on Performance and Post-Treatment

Light acid washing has a negligible effect on the strength and durability of UHPC, since the etching depth is typically less than 1mm. Deep acid washing slightly reduces surface hardness, but this can be offset by subsequently applying a penetrating protective agent (such as a silicone water repellent). The protective agent not only improves stain resistance but also deepens the aggregate color, making the texture more vivid. The rough surface of acid-washed UHPC panels also favors bonding with mortar, which is an advantage in scenarios requiring later adhesive pasting.

Acid-wash treatment therefore endows UHPC with expressive power beyond its inherent material character. Qinglong Group views this process as an extension of material design: through close collaboration with designers, one-of-a-kind surface artistry can be customized, allowing cold concrete panels to tell stories of time and nature.