Chiang Mai Journal of Science

Study on the Eco-friendly Cleaning Technology for Efflorescence on Structural Surfaces

1. Introduction

Building materials, including cement-based paving bricks, sintered ceramics and natural/arti fi cial stone facing bricks, serve as the predominant choices for material selecti on and constructi on methodologies in municipal public infrastructure, housing development and landscape ameniti es. The applicati on of these constructi on material not only elevate aestheti cs and adornment but also effi ciently inhibit the erosion that induced by external corrosive elements (such as CO2, Cl- and SO4 2-),thereby signifi cantly bolstering the maintenance of structural durability.

However, in the fi eld of engineering, many newly constructed buildings gradually exhibit a specifi c type of surface contaminati on during the initi al period aft er commissioning (approximately 6 to 24 months), typically manifesti ng as a white or yellowish-white crystalline substance that precipitates from within the material[1]. This phenomenon is known in academia and industry as ‘sub-fl orescence’ or ‘ effl orescence’[2]. When these crystals form within a porous medium, they are termed ‘sub-effl orescence’ and when they appear on the surface of a porous medium, they are referred to as ‘effl orescence’ [3]. Effl orescence is a common occurrence that primarily aff ecti ng porous materials such as mortar, concrete, masonry and paving bricks [4]. As presented in Figure 1, the formati on of effl orescence involves a migrati on process of water through the cracks or defects of cementbased material and further triggered a sequence of physicochemical reacti ons: the sodium, potassium and calcium ions and hydroxide ions from the hydrated products were dissolved by penetrated water and then migrates to the surface driven by the capillary tension during the drying process along the capillary pores and cracks or defects. Aft er that, the hydroxide ions were reacted with dissolved carbonates and bicarbonates from the atmospheric environment to form carbonate crystals.The formati on of effl orescence involves a sequence of physicochemical reacti ons, primarily characterized by the interacti on between dissolved alkali (earth) metal ions and carbonate ions. Due to the limited quanti ty of alkali metal ions, calcium carbonate is the predominant component of effl orescence in cementi ti ous materials. These effl orescence products typically exhibit a loose and fl uff y texture, which is generally regarded as an aestheti c issue and can manifest at various stages [5]. Although research indicates that effl orescence typically does not pose a signifi cant threat to the mechanical properti es of building materials or structures, it aff ects the durability and service life of concrete[6][7]. Criti cally, alkali-rich precursors (notably sodium) introduce excepti onally mobile ions. Sodium exhibits markedly weaker gel-binding affi nity and superior pore soluti on mobility relati ve to calcium. Consequently, this facilitates sodium’s capillary migrati on to material surfaces under humid conditi ons. Subsequent reacti on with (bi)carbonate ions yields sodium (bi)carbonate effl orescence. Excessive accumulati on can trigger substanti al crystallographic expansion, thereby potenti ally compromising matrix stability. Thus, effl orescence extends beyond cosmeti c concerns to pose a latent structural risk. Scholars have extensively investi gated factors governing effl orescence. It is found that a higher dosage of alkali contributes to a denser microstructure and higher strength, while more signifi cant effl orescence formati on. Effl orescence also depends on the type of alkali metal ions present in acti vators[8].

Furthermore, management enti ti es grappling with effl orescence frequently necessitate substanti al allocati ons of labor and material resources for cleaning and maintenance[9]. Conventi onal cleaning methodologies involve either the “mechanical method” or the “High-pressure drenching with strong acid”[10]. These approaches are not only labor-intensive and ti me-consuming but also generate large volumes of acidic waste fl uid, exacerbati ng environmental strain on projects[11][12]. In contrast, an eco-friendly cleaning technology should be designed to reduce acidic waste, thereby alleviati ng environmental strain and miti gati ng potenti al health risks associated with oxalate exposure[13][14].Consequently, developing a low-carbon, high-effi ciency cleaning and preventi on technology—based on current constructi on management practi ces and effl orescence causes—holds signifi cant practi cal value for lowering maintenance costs and reducing environmental burdens[15][16].

2. Experiment

*It was categorized based on the service environments classifi cati on outlined in the Code for Design of Concrete Structures (GB 50010-2010).

 

3. References

[1] Pel L., H uinink H. and Kopinga K., Effl orescence pathway diagram: Understanding salt weathering. Constructi on & Building Materials, 2004; 18(5): 309-313. DOI 10.1016/j.conbuildmat.2004.02.003.

[2] C hang J.N., Wang W.C., Wang H.Y. and Hu C.Y., A study of the mechanical properti es and appearance of effl orescence in paving bricks under diff erent curing environments. Constructi on and Building Materials, 2020; 265: 120148. DOI 10.1016/j.conbuildmat.2020.120148.

[3] Veran-Tiss oires S. and Prat M., Evaporati on of a sodium chloride soluti on from a saturated porous medium with effl orescence formati on. Journal of Fluid Mechanics, 2014; 749: 701-749. DOI 10.1017/jfm.2014.247.

[4] Brocken H. and Nijland T.G., White effl orescence on brick masonry and concrete masonry blocks, with special emphasis on sulfate effl orescence on concrete blocks. Constructi on & Building Materials, 2004; 18(5): 315-323. DOI 10.1016/j. conbuildmat.2004.02.004.

[5] Hennetier L., Almeida J.V. and Correia A.M.S., Effl orescence and its quanti fi cati on in ceramic building materials. Briti sh Ceramic Transactions, 2013; 100(2): 72-76. DOI 10.1179/096797801681233.