Prediction Model And Inhibition Methods Of Efflorescence In Cement-based Building External Wall System

Cement-based building external wall system is a new type of multi-functional building exterior wall which has the performances of decoration, loading, insulation etc. It has high productive efficiency, stable quality, low demand of labour and many advantages, which meets the requirements of the building industriallizaion policy and low carbon economy. But cement-based building external wall system easily occurs efflorescence. Efflorescence usually makes lots of white stains appear on the surface of external wall which seriously influence the performance of building decoration. The efflorescence is one of the most important problems which restricts the generalization of cement-based building external wall. At present, there are still many subjects of efflorescence in cementitious material have not been researched. For example, the quantitative method to characterize degree of efflorescence has not been put forward, transport process of efflorescence substance has not been elaborated by numerical models, etc. Based on the mechanism of efflorescence in cement-based building external wall system, several aspects of efflorescence have been researched in this paper besides the method to characterize efflorescence, inhibition methods of efflorescence, physical model and prediction model of efflorescence, etc.Based on the combination of accelerating efflorescence treatment and computer image analysis technique, a quantitative method to characterize efflorescence degree in cementitious material was established. The cementitious material was processed by efflorescence treatment with particular parameters, then the images of surface of cementitious material before efflorescence treatment and after efflorescence treatment were collected. Based on the gray theory, the images of efflorescence were divided into region with efflorescence and region without efflorescence, the relative area of each region can be calculated precisely. According to the principles of'6 meters visualilty', the efflorescence region was classified as 2 efflorescence degree:Severe efflorescence type-I which can be distinguished 6m away and slight efflorescence type-? which can only be noticed inside 6m.The mechanism of efflorescence in cement-based building external wall system was studied by analyzing the chemical composition of efflorescence substance, original efflorescence source, content of efflorescence substance, transport process of efflorescence etc, the physical model of efflorescence in cementitious material was also established. On the basis of the physical model of efflorescence and researches of leaching corrosion in cementitious material, the numerical model of efflorescence based on calcium leaching was established. The key parameters in numerical model of efflorescence were related with the chemical and physical properties of cementitious material. The distribution of calcium ions in liquid phase of cementitious material after efflorescence treatment could be calculated by chemical composition and physical properties of raw material, porosity and content of Ca(OH)2. By analyzing the distribution discipline of calcium ions, the degree of efflorescence on the surface of cementitious material could be predicted.Metakaolin could inhibit the efflorescence of cementitious material effectively. By pozzolanic reaction, metakaolin could decrease the content of efflorescence substance-Ca(OH)2, reduced the porosity which made the efflorescence substance hard to be transported to the surface of material through pore channels. For cement-based building external wall system,10% dosage of metakaolin which replaced the cement in mix proportion could make external wall acquire good performances of both mechanics and anti-efflorescence. By inputing the chemical and physical properties, porosity and content of Ca(OH)2 of cementitious material with metakaolin into numerical model of efflorescence, the distribution of liquid calcium ion could be calculated. The relationship of the distribution of liquid calcium ion from numerical model, the distribution of Ca(OH)2 content in surficial layer of material and the degree of efflorescence on the surface of material from experiments was analyzed, and the prediction method of efflorescence degree of cementitious material with metakaolin and other active mineral admixtures was established.Surface sealing treatment could inhibit the efflorescence of cementitious material effectively as well, but the effect of inhibition was influenced by technological parameters. Spraying cement-based building external wall system 2 times after 2 days curing could make it have the best anti-efflorescence performance and high productive efficiency. Surface sealing treatment did not influence the porosity and content of Ca(OH)2 in cementitious material, it controlled the efflorescence by generating a hydrophobic sealing layer on the surface of cementitious material which hindered water transport through pore structure. Based on the mechanism of efflorescence inhibiting of surface sealing treatment, the numerical model of efflorescence was revised by a correction parameter related to the saturability of water, and a prediction model of efflorescence which can apply to cementitious material with surface sealing treatment was established.