Investigation Of Moisture And Chloride Ion Transport Behavior Of Unsaturated Aeolian Sand Concrete

As China’s rapid development of the construction industry and the implementation of"One Belt,One Road"and"Ocean Power"strategy,the demand for raw materials for concrete is expanding.Aeolian sand(desert sand)as an extra-fine sand,if aeolian sand replaces river sand.On the one hand,it can reduce costs,and on the other hand,it can promote social development.In this paper,we study the effects of chloride salt capillary absorption and long-term natural immersion environment on the durability of aeolian sand concrete/mortar,and reveal the transport process of chloride ions and moisture inside windblown sand concrete/mortar to give theoretical basis for its practical engineering application.In this paper,we designed three kinds of aeolian sand concrete/mortar with water-binder ratio to analyze the effect of chlorine salt concentration and water-binder ratio on capillary absorption,and analyzed the internal pore structure change pattern with the help of nuclear magnetic resonance(NMR)technique.At the same time,a long-term natural immersion test with chloride salt was conducted to analyze the deterioration process and mechanism of aeolian sand concrete under chloride salt environment,and the microscopic morphology was analyzed by scanning electron microscopy(SEM)and nuclear magnetic resonance(NMR)to reveal the chloride ion transport pattern and pore structure evolution in aeolian sand concrete.The moisture and chloride ion transport test of aeolian sand concrete shows that:with the increase of capillary absorption time and Na Cl solution concentration,the capillary absorption mass of aeolian sand concrete specimens gradually increases,the chloride ion intrusion also gradually increases,the depth of moisture and chloride ion intrusion gradually increases,and the porosity gradually decreases.Within 1d of capillary absorption,the growth rate of capillary absorption is fast,and the square root of capillary absorption time is linear,and the depth of moisture and chloride ion intrusion is linearly related to each other.After 1d of capillary absorption,the capillary absorption quality curve slowly showed a stable state,and the depth of moisture and chloride ion invasion showed obvious non-synchronous.After 28 d of capillary absorption,the moisture intrusion depth of ASC-3 was about 2 times of the chloride ion intrusion depth,and the moisture intrusion depth of ASC-2 and ASC-1 was about 1.5 times of the chloride ion intrusion depth.The moisture and chloride ion transport tests of aeolian sand mortar showed that the capillary absorption increased with the decrease of saturation of aeolian sand mortar and increased with the increase of Na Cl solution concentration.The capillary absorption process includes two stages of rapid absorption period and stable absorption period,and the function relationship between capillary absorption coefficient S and initial saturationθof aeolian sand mortar with different water-binder ratio is established:S=a(1-θ)~b.With the increase of capillary absorption time and water-binder ratio,the depth of moisture and chloride ion invasion gradually increases,and with the increase of initial saturation,the depth of moisture and chloride ion invasion gradually decreases.The long-term natural immersion test with chlorine salt of aeolian sand concrete showed that the specimen block appearance and morphology remained intact after 12m of natural immersion in 3%Na Cl and 6%Na Cl solutions,indicating that the chlorine salt had a light erosion on it.The mass loss rate of the test blocks was always negative throughout the soaking cycle,and the mass of the test blocks kept increasing.The mass loss rates of ASC-1,ASC-2 and ASC-3 test blocks after 12m soaking were-0.55%,-0.58%and-0.62%;-0.67%,-0.7%and-0.75%,respectively.With the increase of soaking time,the compressive strength of the specimens showed a gradual increase,and the relative compressive strength values also gradually increased and were all greater than those of the reference group.After 12 m of soaking,the compressive strengths of ASC-1,ASC-2 and ACS-3 specimens were:63.66 MPa,55.64 MPa and 48.24 MPa;66.05 MPa,57.34 MPa and 49.87 MPa,respectively.After 12 m of chloride salt soaking,the ITZ structure was improved,with better compactness and significantly reduced porosity.Considering the effects of temperature and humidity,age,deterioration effect and chloride ion binding effect on the chloride ion diffusion coefficient,the Fick’s second law was modified to derive a chloride ion transport model for aeolian sand concrete,and the difference between the theoretical and measured values was small.