Study On Mechanical Properties And Microscopic Analysis Of Basic Magnesium Sulfate Cement Based ECC

Engineered Cementitious Composite（ECC）have excellent flexural toughness and crack control capabilities,and have been applied in pavement,tunnels,airport runways,bridge decks and other projects.In order to further improve the flexural and compressive properties of ECC materials and expand the application scenarios of ECC in water resistance,this paper uses basic magnesium sulfate cement（BMSC）as the matrix material,uses the endogenous5Mg（OH）₂·MgSO₄·7H₂O（5·1·7 phase）whiskers of BMSC,and cooperates with the external fiber doping of ECC to make the material reach multi-scale toughening and reinforcement at the micron and millimeter levels,and prepare basic magnesium sulfate cement based ECC（BMSC-ECC）with high flexural resistance,compression resistance and good water resistance.In this paper,the influence of two single factors,H₂O/MgSO₄·7H₂O and citric acid,on fluidity,flexural and compressive strength and hydration products was first studied to determine the ratio of BMSC,and then the life cycle evaluation of BMSC was carried out.The results showed that the optimal ratio was H₂O/MgSO₄·7H₂O of 16:1,citric acid content of 1.25%and Mg O/MgSO₄·7H₂O of 12.5:1.The LCA found that four items in the midpoint category had high impact values and three were low.The resource consumption impact value was highest in the endpoint category.Magnesium oxide in many categories of raw materials accounts for a relatively large influence.Among the inventory impacts,PM2.5,arsenic and carbon dioxide emissions were high,and the energy consumption of coal,oil and natural gas was high.Compared with the other four types of magnesium cement,BMSC has certain advantages.Secondly,the flexural and compressive performance of BMSC-ECC was studied,and it was found that when the H₂O/MgSO₄·7H₂O was 19:1～23:1,it was conducive to fiber dispersion and specimen molding,while the flexural strength fluctuated significantly,which was higher at 15:1,16:1,20:1,21:1,and higher compressive strength at 15:1～19:1.Among the fiber types,the flexural and compressive strength of two PVA and PE fibers was high.In the fiber volume content,the flexural strength increased with the increase of fiber volume,and the compressive strength did not change much.In the sand-binder ratio,it was conducive to fiber dispersion and specimen molding when it was lower than 0.6,and the flexural strength was higher when it was lower than 0.5,and the compressive strength was higher when it was lower than 0.3.In the CFBC ash,with the increase of the CFBC ash content,the fluidity decreases,the flexural strength increases first and then decreases,the 7d compressive strength decreases,and the 28d compressive strength first stabilizes and then decreases.Then,the load-deflection curve,bending resistance,bending toughness index and failure mode of BMSC-ECC were analyzed by four-point bending experiment.It was found that only 21:1 and 22:1 met the toughness index index in the H₂O/MgSO₄·7H₂O,and at this time,the strain hardening and multi-joint cracking were obvious,the peak deflection was large and the TI value was high.Among the fiber types,the strain hardening and multi-seam cracking of the two PVA and PE fibers were obvious,the brittle failure of PP,BF and CF fibers,the peak deflection and TI value of PVA-1 and PE fibers were higher,and the toughness index of PVA-1,PE and BF fibers met the index.In the fiber volume content,when it was greater than 1.5%,the phenomenon of strain hardening and multi-joint cracking was obvious,the peak deflection and flexural strength of 2%and 2.5%were higher,the toughness index meets the index when it was 0.5～2.5%,and the equivalent flexural strength strength and TI value of 1.5～2.5%were higher.In the sand-binder ratio,all specimens had strain hardening characteristics and all met the toughness index index,the peak deflection,flexural strength,equivalent flexural strength and TI value when the sand-binder ratio was 0.4～0.6 were higher,and the multi-crack cracking phenomenon of 0.3～0.6 was more obvious.In the CFBC ash,all the test pieces have strain hardening and multi-joint cracking,and all met the toughness index index,the peak deflection and flexural strength of the CFBC ash content within 10～20%were outstanding,the equivalent flexural strength and TI value within5～20%were better,and the number of cracks within 5～25%was more.Finally,the water resistance of BMSC-ECC was tested,and the strength change,softening coefficient and water absorption rate before and after water immersion were analyzed.It was found that the strength of H₂O/MgSO₄·7H₂O of 15:1～18:1 decreases significantly,the strength loss before and after water immersion at 19:1～23:1 was small and stable,the softening coefficient met the requirements of water resistance at 19:1～23:1,and the water absorption rate increases with the increase of the H₂O/MgSO₄·7H₂O,and was more stable at18:1～20:1.Among the fiber types,the strength loss of the two types of PVA fiber specimens is relatively low,and only the softening coefficient of the two types of PVA fibers meets the requirements,with a water absorption rate of about 10%.In the fiber volume content,the difference in strength after immersion was not large,the softening coefficient met the requirements,and the water absorption rate fluctuates with the increase of fiber volume content.In the sand rubber ratio,the strength was relatively stable after immersion,the softening coefficient meets the requirements,the water absorption rate first decreases and then increases and then decreases with the increase of the sand-binder ratio,and the water absorption rate decreases greatly when 0～0.3.In the CFBC ash,the softening coefficient met the requirements,and the strength gradually decreases after immersion with a content higher than 10%,and the water absorption rate decreases significantly at 0～10%,and was relatively stable at 10～25%.Multiple mechanical properties of BMSC-ECC were comprehensively evaluated,and the optimal ratio was obtained as follows:H₂O/MgSO₄·7H₂O was 21:1,PVA-1 fiber was used,fiber volume content was 2%,sand-binder ratio was 0.4,and CFBC ash content was 10%.