Study On Repair Property Of Modified Sulfoaluminate Cement Mortar Under Typical Northwest Environment

As a new kind of microporous cement,sulfoaluminate cement has the advantages of low alkalinity,small volume expansion,fast setting speed,high impermeability,high frost resistance,high corrosion resistance,etc.It has been widely used in road repair and emergency engineering.In recent years,with the advocacy of low-carbon and environmental protection concept,sulfoaluminate cement has attracted the attention of civil engineering field because of its low energy consumption,which can effectively save energy,reduce carbon emissions and protect the environment.To study its applicability and durability in Chinese construction industry has become one of the hot issues.Due to the large difference of temperature and humidity between day and night,cold winter,and erosive ions in soil or water,reinforced concrete structures in northwest China mainly face the compound action of sulfate attack,dry-wet cycle and freezethaw cycle,which is one of the main reasons for the disease of concrete structures in this region.Therefore,it is an effective attempt to use the advantage of sulfoaluminate cement to repair the structural defects of reinforced concrete in service in the complex environment in Northwest China.Considering the shortcomings of sulfoaluminate cement itself and the characteristics required by repair materials,this paper firstly modifies sulfoaluminate cement mortar through different factors to study its basic performance as a repair material for damaged concrete,focusing on the mechanical properties and volume stability of modified sulfoaluminate cement mortar.Based on the optimized modified mix ratio,the mechanical properties of modified sulfoaluminate cement mortar at different temperatures were studied.In view of the compound action of sulfate multiple factors,two corrosion mechanisms of sulfate and dry-wet cycle and sulfate,dry-wet cycle and freeze-thaw cycle were designed.The durability of modified sulfoaluminate cement mortar under the compound action of sulfate multiple factors and the bonding property with old concrete substrate were tested.This paper attempts to understand its service behavior under the action of sulfate multiple environmental factors after repairing concrete damage.The main research results of this paper are as follows:(1)From the two indexes of mechanical property and volume stability,polypropylene fiber(PPF),UEA expansion agent and sodium silicate were used to modify sulfoaluminate cement mortar.The optimal mix ratio A3B1C3,namely 0.1%polypropylene fiber(PPF),0.4%UEA expander and 0.3%sodium silicate,was optimized by the orthogonal test of three factors and three levels,combined with range analysis and efficiency coefficient method.(2)Based on the two groups of excellent mix ratios(group M1 and group M2)screened out in(1),combined with the unmodified mix ratio(group M0),the mechanical properties of each group of mortar at different temperatures were studied.At-25℃,the compressive strength of sulfoaluminate cement mortar with different mix ratio increases gradually with the extension of placement time;At 25℃,the compressive strength has no obvious change.At 80℃,the compressive strength increases first and then decreases and gradually decreases according to the different mix ratios.(3)Based on the two groups of excellent mix ratio(group Ml and group M2)screened out in(1),combined with the unmodified mix ratio(group M0),the durability of sulfoaluminate cement mortar under sulfate compound attack was studied.Under the action of dry and wet cycles of sodium sulfate,with the increase of dry and wet cycles,the unmodified sulfoaluminate cement mortar sample(group M0)has the lowest degree of spalling and the smallest mass change,and the compressive strength increases first and then decreases,which is the best group of the three samples.With the increase of freeze-thaw cycles,the spalling of specimens is more serious than that of the two factors,and the unmodified sulfoaluminate cement mortar has the lightest spalling.The mass,relative dynamic elastic modulus and compressive strength of all samples decreased gradually with the increase of the number of freeze-thaw cycles.The mass change of unmodified samples was the least,the decrease of relative dynamic elastic was the least,and the decrease of compressive strength was the least.In summary,the unmodified sulfoaluminate cement shows good comprehensive performance.(4)The modified sulfoaluminate mortar has good ductility,which can be broken without cracking at failure.Under the combined action of sodium sulphate and dry-wet cycles,with the increase of the number of dry-wet cycles,the bonding strength of the C30 old substrate and before and after modified mortar increases first and then decreases,and the modified mortar has a higher bonding strength.Under the action of sodium sulfate,dry-wet cycles and freeze-thaw cycles,with the increase of the number of freeze-thaw cycles,the bonding strength of the C30 old substrate and the modified mortar gradually decreases,and there is little difference before and after modification.In summary,the modified mortar has a significant effect on the bonding between the old substrate and new materials.