Effect Of Micro-nano Admixtures On Properties Of High Sulfate Resistant Portland Cement Concrete

The report of the 18th National Congress of the Communist Party of China clearly put forward the strategic goal of building a maritime power.The report of the 19th National Congress of the Communist Party of China pointed out"adhere to the overall planning of land and sea,speed up the construction of a maritime power".The development of marine engineering had been raised to the national strategic height.As a traditional building material,cement concrete played an indispensable role in marine engineering construction.At present,most of the cement used in concrete was ordinary portland cement,which contained high content of tricalcium silicate（C₃S）and tricalcium aluminate（C₃A）.C₃S hydration generated a large amount of calcium hydroxide（CH）.CH could react with sulfate ion in seawater to form gypsum or with C₃A to form ettringite.The crystal expansion of gypsum and ettringite would lead to internal stress,which would cause cracks in concrete and affect the compactness and durability of concrete.Therefore,it is very important to select suitable cementitious materials to improve the compactness and durability of concrete.As a kind of non-uniform porous material,when concrete was used in the marine environment,the harmful ions and water from the outside can enter into the concrete through the concrete pores,causing structural damage and shortening the service life.Therefore,it is an effective way to improve the durability of concrete to optimize the pore structure of concrete and improve the density of concrete.At present,adding mineral admixtures can effectively improve the density of concrete.In this paper,High sulfate resistance Portland cement（HSRPC）was used as cementitious material to enhance the sulfate resistance of cement-based materials and reduce the structural damage caused by seawater erosion.Based on the performance of cement after hydration,micro silica fume（SF）,mineral powder（GGBS）,fly ash（FA）and nano silica（NS）with different particle sizes were added respectively,and the composition and particle gradation of the admixture were designed by using the concept of cycle reverse,so as to achieve the optimal matching of"pore structure"and"admixture"and achieve the"maximum"densification of concrete structure.Aiming at the special hydration characteristics of HSRPC,the hydration mechanism of micron mineral admixtures and nano silica in HSRPC system was studied.The main results are as follows:（1）In the SF-NS system,5 wt%SF and 0.3 wt%NS could improve the pore structure of cement-based materials,and the total porosity was 16.3%lower than that of blank sample.Accordingly,the ultrasonic amplitude and compressive strength of concrete with 28 days of curing age reached the maximum,and the ultrasonic amplitude and compressive strength of concrete increased by 120.1%and 28.3%respectively compared with the blank sample.The corrosion resistance of sulfate and chloride ion was the highest,the corrosion resistance coefficient of sulfate was 45.8%higher than that of blank sample,and the migration coefficient of chloride ion was reduced by 52.0%.Compared with HSRPC,the content of CH in 28d of HSRPC with 5 wt%SF decreased by 4.4%,because CH produced by HSRPC hydration could effectively stimulate the volcanic ash activity in the later stage of SF,and react with SF twice to generate more hydration products.The total porosity was reduced by 6.1%,and the compressive strength was increased by 14.4%.When 0.3 wt%NS was mixed with 5 wt%SF,the total porosity of 28 days was 16.3%lower than that of blank sample.Accordingly,CH content in TG-DTG curve was 24.7%lower than that of blank sample,which was consistent with the decrease of CH characteristic peak.It was indicated that the synergistic effect of SF and NS could further promote the secondary hydration reaction and reduce the porosity in the later hydration process.（2）In the GGBS-NS system,35 wt.%GGBS combined with 0.3 wt.%NS could significantly improve the pore structure of cement-based materials,and the total porosity reached the minimum,which was 23.8%lower than the blank sample.The ultrasonic amplitude and compressive strength of concrete with curing age of 28 days reached the maximum,which were increased by 134.5%and 27.4%respectively compared with the blank sample.Compared with the blank sample without mineral powder and nano silica,the sulfate corrosion resistance coefficient increased by 61.5%,and the chloride ion migration coefficient decreased by 52.0%.GGBS and NS could improve the stability of the paste and significantly reduce the slump of concrete.The results showed that the mass loss rate of the sample added with GGBS was significantly lower than that of the blank sample at 3 days,and there was no significant difference between the sample added with GGBS and the blank sample at 28 days.It indicated that the hydration of GGBS mainly occurs in the later stage.However,the mass loss of NS was higher than that with GGBS,which indicated that the hydration of NS mainly occured in the early stage.（3）In the FA-NS system,FA was not conducive to the improvement of the compactness of concrete,but NS could make up for the bad effect of FA on the concrete pore structure.When10 wt%FA was mixed with 0.3 wt%NS,the total porosity was 20.7%lower than that of the sample with 10 wt%FA.The fluidity of cement-based materials was improved obviously by FA with ball effect.When the FA content was 40 wt%,the slump decreased by 17.9%.FA could improve the sulfate resistance of concrete obviously.When 20 wt%FA was added,the corrosion resistance coefficient of the specimens increased 72.5%compared with the blank sample.