Study On Long-term Of Chloride Resistance Of Alkali-activated Concrete

As is known to all,the durability of concrete is an important factor that determines the service life of engineering structural members.The huge economic losses caused by the durability of concrete in various countries worldwide account for 3 % – 5 % of the gross national product,of which about 80 % are seriously threatened by the corrosion of reinforcement.It is generally believed that chloride is the main reason for the corrosion damage to structures.In order to improve the corrosion resistance of reinforced concrete in the chloride environment,the development of more durable materials that can replace ordinary Portland cements(OPC)has attracted wide attention and interest in academia and industry.Alkali-activated materials(AAM)have the advantages of low CO2 emission,early setting,chemical corrosion resistance and high temperature resistance.The raw materials are silica aluminate solid such as fly ash and slag.It is a green environmental protection building material with cementitious properties by stimulating the activity of raw materials under alkaline conditions.Based on the existing research of our group,this paper studies the long-term performance of alkali-activated concrete(AAC)against chloride erosion,discusses the development law of macroscopic mechanical properties,microscopic product crystal phase,product polymerization degree and microscopic pore structure under different influencing factors(slag content,water cement ratio,Na2 O content and sand ratio).At the same time,the natural immersion method and the Rapid Chloride Migration test(RCM)with relatively simple experimental operation and short cycle are used to evaluate the chloride erosion resistance of AAC,and the quantitative relationship between the two methods is established,which provides a reference to the durability life prediction of AAC.The research shows that different influencing factors have obvious effects on the early compressive strength of AAC.With the extension of age to two years,the compressive strength of all ratios tends to be flat.For the long-term compressive strength,the influence laws of different influencing factors of the compressive strength are generally consistent.In addition to the phenomenon of strength regression when the slag content is 100 %(SL100),the watercement ratio is 0.4(W4),and the Na2 O content is 4 %(N4),the phenomenon of the intensity of the backward,the rest is still in the state of continuous alkali-activated reaction,the strength of slowly growth.Although the chloride coefficient measured by natural immersion method and RCM method is different in numerical value,the overall trend and change ruled have good correlation.The early chloride erosion resistance of the two methods increased from the increase in slag content and alkali content,and decreased to the increase in water cement ratio.With the extension of age,the influence degree of each influencing factor of the performance development trend of AAC decreases.The chloride coefficient of SL100 and W40 increases slightly at 28+365d,which is corresponding to the strength regression.W45 and N5 with moderate dosage showed better performance development.A little different,28+730d SL100 AAC chloride erosion resistance performance is better,but with SL70 little difference,almost the same.The influence of sand ratio has no obvious regularity from beginning to end.Slag content,water cements ratio and alkali content are important factors affecting chloride transport.In addition,except that the resistance to chloride erosion of W50 in the early stage is not as good as OPC,the chloride coefficient of other ratios of the same period is more than twice that of OPC.Compare with the results obtained by the natural immersion method and RCM method,it is found that the AAC under different influence factors shows a strong dependence on schedule.By introducing the aging coefficient and modifying the fitting analysis of the two groups of data,it can be seen that the linear relationship is developing,and the quantitative relationship between them has been successfully established.Once known to a certain proportion of the aging coefficient and the initial diffusion coefficient can be determined by RCM method and the quantitative relationship between the only served as a natural diffusion coefficient of t times immersing the real value,thus overcoming natural soaking method operation process complex and long cycle fault,at the same time can provide a reference for realizing life prediction model of AAC.The FTIR and XRD results show that with the increase in slag content,the wavenumber frequency decreases in turn,and the polymerization degree of the system decreases,which is related to the continuous alkali activation reaction of fly ash in the later stage.In addition to SL100,the diffraction peak broadens when the diffraction angle is about 15°–40°,which is related to the formation of sodium silicate hydrates(N-A-S-H)and calcium silicate hydrate(CA-S-H),indicating that its resistance to chloride erosion is strengthened.With the increase of water-cement ratio,the structure of some silicon-oxygen tetrahedron and aluminum-oxygen tetrahedron in the product changes,resulting in the decrease of chloride resistance,which is consistent with the regularity of macroscopic properties.With the extension of age,the polymerization degree and reaction products of N4,N5 and N6 tended to be stable,indicating that the effect of this influencing factor gradually weakened,which was consistent with the macroscopic performance characterization.In addition,there is a Friedel salt with strong binding capacity of chloride in OPC,which has a great influence on the binding test of chloride.However,there is no such salt in the alkali-activated material system.Therefore,it is generally believed that the binding capacity of chloride in AAM is weak and can be ignored.In addition,the nitrogen adsorption experiment shows that the chloride corrosion resistance and mechanical properties of AAC are also closely related to the cumulative pore volume and pore size distribution of the matrix.The more the total adsorption capacity is,the more pores in the matrix are.However,due to the influence of the ’ink bottle’ pore structure and the tubular pore structure of the neck,the system may also show higher performance advantages.