Manipulation Mechanisms Of Micro/nano-silica On The Hydration Of Cement Minerals With Sulfate

Cementitious materials are the widely-used construction materials around the world.Nowadays,how to enhance the performance of cementitious materials and reduce its impact to the environment are becoming an emergency of the world.Normal approach to tackle this issue is adding supplementary cementitious materials?SCMs?which are produced from the industrial by-products,and the most widely-used type is silica-based SCMs.There have many studies according to the effects of silica-based SCMs on the performance and hydration of cement,and also on the hydration of tricalcium silica?C₃S?,which comprises around 60–70 wt.%of cement and dominates the early hydration process.However,there was a limited study according to the tricalcium aluminate?C₃A?hydration.Tricalcium aluminate,which represents less than 10%of the Portland cement,but has a great influence on the early age performances including the setting,rheology,durability,early age strength,and etc.When the pure C₃A meets with water,it will have a quick reaction and lead to the flash setting of cement.Thus,in order to obtain a normal workability,gypsum is added to delay the C₃A hydration.The main component of gypsum is calcium sulfate,and its hydration will not only adjust the setting time,but enhance the early mechanical performance.However,whether the presence of silica-based SCMs will influence the hydration between the aluminate and sulfate phases,and change the performance of cement,is still need further study.Therefore,this study mainly focuses on the effects of nano/micro silica-based SCMs on the hydration of C₃A and its resistance of external sulfate attack.We hope that we can optimize the sulfate content and further control the mineral hydration process combing with the investigation results and mechanisms,which will maximize the performance of SCMs-added cementitious materials.In this study,nano silica?NS?and silica fume?SF?were used to investigate its influences on the performance and hydration of C₃A-gypsum systems and C₃S-C₃A-gypsum model cement systems.As shown in results,the presence of NS and SF delayed the C₃A-gypsum systems hydration.With the addition of NS,negative-charged NS particles adsorbed on the positive-charged C₃A surface through the electrostatic interaction,which further blocked the dissolution site of C₃A and the nucleation site of ettringite,and thus delayed the hydration of C₃A.When the presence of SF,micro-sized SF particles cannot immediately adsorb on the C₃A surface.However,the negative-charged SF would also adsorb the positive-charged ions including Ca²⁺and Ca-S ions complexes,which further weaken the effects of gypsum on C₃A hydration,and thus,accelerated the dissolution of C₃A and declined the formation of ettringite.Furthermore,the effects of NS and SF on the external sulfate attack of C₃A were also studied.It can be found that the formation of external expansion hydrates was linked with the sulfate content,and the presence of NS and SF can efficiently delay the sulfate attack process due to the compactness of structure.Meanwhile,the electrostatically adsorbed of NS on the surface of ettringite can retard the sulfate ions exchanging and delay the growing of ettringite.However,the presence of SF cannot efficiently delay the growing of ettringite due to the micro particle sized.Based on the effects and mechanisms of NS and SF on C₃A hydration,a further study of the effects of NS on the optimal sulfate content were carried out.As shown in results,a trace amount of sulfate changing had a huge influence on the hydration process between silica and aluminate phases,and finally influenced the performance of cementitious materials.With the addition of NS,the optimal sulfate content will be further changed.This study physiochemically investigated the effects of silica-based materials on the hydration of aluminate phase and proposed a new hydration controlling mechanism between silica and aluminate phases.We hope this study can provide the possibilities and basic knowledge for the further regulation of optimal sulfate content and performance of cementitious materials.