Inducing Factors And Mechanism On Efflorescence Of Alkali Activated Slagfly Ash Mortars

Efflorescence of alkali activated slag-fly ash mortar（ASFM）refers to the acid-base neutralization reaction between alkali cations in material matrix（mainly related to the type of activator）and acidic gas such as carbon dioxide in the environment under certain temperature and humidity conditions.The efflorescence products of AAM are white hydrated carbonate or hydrated bicarbonate precipitates.The efflorescence not only affects the aesthetics of buildings,but also reduces the alkalinity,workability and durability of samples.Both of“internal composition”and“external environment”are two important factors affecting efflorescence.Therefore,herein,the ASFM were manufactured by optimizing the modulus of alkali activator,water-cement ratio,relative content of slag as well as fly ash.The influence of internal composition,curing temperature,environmental temperature and humidity conditions on ASFM efflorescence was analyzed.With the working of modern analysis techniques such as XRD,FT-IR,SEM,MIP as well as AAS,efflorescence products structure,sample mass loss rate,morphology and pore structure of ASFM could be concluded.The main mechanism of ASFM efflorescence underwent“high and low humidity cycle”and“semi-immersion”were discussed and proposed a method to inhibit the efflorescence by adopting hydrophobic agents.The main conclusions of this thesis are as follow:（1）The method of“high-low humidity cycle”was used to research systematically the influence of the modulus of activator,water/cement ratio and slag content on the efflorescence rate of ASFM.Moreover,the mechanism of efflorescence on ASFM in the condition of high-low humidity cycle was indicated.The results show:increasing the modulus of the activator,keeping the water/cement ratio to"0.38-0.42"and mixing an appropriate amount of slag was beneficial to suppress the efflorescence.These samples lose water and caused both drying shrinkage and auto-shrinkage during the process of efflorescence.At the same time,the volume of harmful pores was increased.Thus,compressive strength was decreased ultimately with the change of pores.In the condition of“high-low humidity cycle”in efflorescence on ASFM was mainly divided into three stages.The first stage was first 3 cycles of efflorescence,which was mainly manifested by the loss of moisture on the surface or near the surface of the sample.The matrix was corroded by moisture dissolved carbon dioxide as well as the formation of a small amount of bicarbonate.The middle stage was the 3^rd to 7^th cycles of efflorescence.The process of migration of moisture dissolved free Ca²⁺and Na⁺inside the sample along the pores to the surface began.The last stage of efflorescence was 7^th to 14^thcycles.The process of diffusion and water loss continued,bicarbonate was transformed into crystallized carbonate and grew,white crystals began to adhere to the surface.（2）The method of“semi-immersion”was used to research the effects of“curing temperature”and“temperature and relative humidity of efflorescence”on ASFM efflorescence.Moreover,the mechanism of efflorescence on ASFM in the condition of semi-immersion was indicated.Then summarized that:a curing temperature of 50°C and a high-temperature with high-humidity environment（30°C,80%RH）was conducive to the efflorescence phenomenon.Meanwhile,compressive strength was promoted with increasing of volume of harmless pores.It’s easy to induce efflorescence with a low-temperature and low-humidity environment（10°C,60%RH）,then formed efflorescence products to destroy the structure of matrix as well as the mechanical properties decreased at the same time.When the degree of efflorescence was relatively high,the hollow structure of the"fluffy"product Na₂CO₃·n H₂O was easy to form on the surface.However,Na₂Ca（CO₃）₂·n H₂O crystals were easily formed when the degree of efflorescence was relatively low.The efflorescence process of the method of“semi-immersion”found that the process was mainly divided into four stages.The first one was the migration of pore solution and ions in the sample under the condition of humidity difference.The second was that the saturated carbon dioxide in the stomata reacted with the pore solution containing sodium and calcium ions to form bicarbonate nuclei.The third was the growth of the bicarbonate crystal nucleus due to the migration of matter,accompanied by the appearance of microcracks.The fourth was to continue mass transfer,accelerate efflorescence,and destroy the matrix structure.（3）The efflorescence method of“high-low humidity cycle”was adopted to study the influence of the contents of hydrophobic agent on ASFM efflorescence by adding different hydrophobic agents,and the“area method”was introduced to evaluate the degree of efflorescence.It was found that the incorporation of the liquid hydrophobic agent and solid hydrophobic agent with proper contents could efficiently inhibit efflorescence.The addition of 1 wt%liquid hydrophobic agent into ASFM with a low water-cement ratio,and addition of2.5wt%solid hydrophobic agent into ASFM with a high water-cement ratio were both beneficial to hinder the efflorescence.The hydrophobic agents formed a“protective film”on sample surface,effectively hindering the migration of some sodium and calcium ions.In addition,the partial“Lock-pore”effect hindered the migration of Na（H₂O）ⁿ⁺to the surface,thus inhibiting the efflorescence.