Research On Effect Of Superplasticizers On Properties Of AFt-rich Cement-based Repair Materials

Ettringite is one of the main strength sources of cement-based repair materials,and its growth process is greatly affected by the hydration medium environment.The superplasticizer is one of the key factors that affect the environment of the hydration medium.The growth medium environment of the hydration products of the cement-based system will be changed by the addition of superplasticizer.And the impact between superplasticizer and cement-based repair materials with ettringite-rich has not been studied in depth.In this paper,two repair materials-sulphoaluminate cement(CSA)and ordinary Portland cement-aluminate cement-gypsum system(OPC-CA-C(?))were selected,which all generate a large amount of ettringite in its hydration products.To study the effect of naphthalene-based superplasticizer(BNS)and polycarboxylic acid superplasticizer(PC)on setting time,fluidity,and compressive strength of CSA and OPC-CA-C(?) system.Xray diffraction analysis(XRD),thermogravimetric analysis(TG),scanning electron microscope(SEM)are used to study the type,content and morphology of hydration products.Research results show that:(1)In CSA,with the increase of BNS in the range of 0.8～2.0 wt%,the setting time is prolonged,the fluidity at 5 min increases,and the compressive strength of 2h,1d,7d and 28 d is obviously improved.When the PC dosage is changed within 0.08～0.20 wt%,the setting time of CSA is changed within 30 min,and the fluidity at 15 min is too small to be measured.When the PC dosage is 0.20 wt%,the 2h compressive strength of CSA is decreased by 53.6% compared with the blank,and the phenomenon has not been improved with the age prolonged.(2)In the OPC-CA-C(?)system,with the increase of BNS in the range of 0.08～0.20wt%,the overall setting time is prolonged,the fluidity at 5min and 15 min is increased,and the compressive strength changes insignificantly.However,there is an optimal dosage of BNS.When PC increases within 0.12～0.24 wt%,the initial setting time is prolonged,the final setting time is shortened first and then prolonged,and the fluidity increases.When the PC amount was 0.24 wt%,the 1d compressive strength of the OPCCA-C(?) system decreased by about 19.7% compared with the blank.This situation did not improve with the hydration age more.The 7d and 28 d compressive strengths of the OPC-CA-C(?) system mixed with 0.24 wt% PC are still reduced by 31.6% and 31.3%respectively.(3)XRD analysis shows that with the addition of BNS and PC,the main hydration product of CSA and OPC-CA-C(?) system is still ettringite,indicating that the addition of superplasticizers did not make CSA and OPC-CA-C(?) system hydration generate new hydration products,the types of hydration products have not changed.(4)The addition of BNS and PC will promote the formation of early ettringite in the CSA system.In the OPC-CA-C(?) system,both BNS and PC content all have no significant effect on the amount of ettringite amount in the OPC-CA-C(?) system.(5)In CSA,the morphology of ettringite changed from needle rod shape to a large number of flakes and layered ettringite stacks when PC was 0.08 wt% and 0.20 wt% in2 h.In the OPC-CA-C(?) system,the growth morphology of ettringite at 28 d when PC is0.12 wt% is a pencil-tip polygonal prism.When the PC content is 0.24 wt%,only a small amount of rod-shaped ettringite attached or wrapped on the surface of the hydrated product can be observed in the visual range of the OPC-OPC-CA-C(?) system of 28 d hydration.These results show that the morphology change of ettringite is the reason of the significant decrease in the early compressive strength of CSA and OPC-CA-C(?)system added by PC.