news
What are the reasons for the presence of numerous pores,roug
The excessive presence of pores,rough surfaces,and pinholes on the surface of fiber cement board is mainly due to the combination of three core reasons:the presence of air in the slurry,insufficient exhaust during molding,and abnormal interfaces and curing conditions.
1.Raw materials and formulation issues
Excessive water-cement ratio and insufficient sand content:The slurry is too thin and has poor cohesion,allowing air bubbles to remain,and the aggregates sink,forming pores.
Improper selection of admixtures:Excessive or conflicting addition of air-entraining agents and water-reducing agents,resulting in poor stability of air bubbles and their concentration on the surface.
Uneven dispersion of fibers and high content of mud/impurities:The fibers aggregate and encapsulate air,and impurities hinder the escape of air bubbles.
Insufficient slurry mixing:The air is not dispersed,resulting in local aggregation of air bubbles.
2.Molding and equipment process issues
Insufficient pressure/vibration:Insufficient pressure in the press,uneven pressure application,or insufficient vibration frequency/time,preventing the escape of air bubbles.
Defects in casting/roller pressing:Uneven dehydration of the screen wheels,air entrapped in the felt belt,and air being trapped in the mold during molding.
Problems with molds/backing plates:Rough surfaces,oil stains/old slurry,uneven application of release agents,and retention of interface pores.
Large drop in material discharge:Excessive free fall of the slurry,resulting in segregation and air entrainment.
3.Curing and environmental issues
Improper steam curing/curing:Rapid temperature rise,unstable pressure,rapid expansion of water vapor inside the mold,resulting in pinholes;Insufficient curing,rapid loss of surface moisture,causing rough surfaces.
Sudden changes in environmental temperature and humidity:Premature formation of a film on the surface under high temperature and high humidity,preventing the escape of internal gases.
