Regulation And Mechanism Of Micro/Nano-silica On Alkalinity Sensitivity Of Supersulfated Cement

The high slag（75%-85%）and low clinker（1%-5%）composition characteristics give the supersulfated cement（SSC）system its green and low-carbon characteristics.However,small variations of clinker content leading to large fluctuations in compressive strength can seriously limit its application on a large scale.The high pozzolanic activity of silica-based micro-nano particles can effectively regulate the system alkalinity and calcium ion content,which is expected to regulate the hydration environment of SSC and reduce the clinker content sensitivity.In this paper,we systematically analyzed the effects of macroscopic and microscopic properties brought about by the variation of alkalinity in super sulfate cement,and explored the regulatory role of silica-based micro-nano particles,to provide new means and ideas for the performance regulation of SSC systems.Firstly,the effects of clinker blending（0.5%,1%,3%,5%,named C0.5,C1,C3,C5）and NS addition on the macro and micro properties of SSC were investigated.The results showed that the clinker admixture significantly affected the strength,and the 90 d compressive strengths of all samples were 40.45 MPa,29.71 MPa,17.64 MPa,and 13.93 MPa,respectively.1%NS effectively reduced the effect of clinker content on the compressive strength of SSC:the 90 d compressive strengths reached 42.08 MPa,43.41 MPa,32.38 MPa,and the difference in compressive strength between C0.5 and C1 samples at 90 d was reduced from 10 MPa to within1 MPa.However,NS retarded the early hydration of SSC,which may be related to its inhibition of gypsum consumption leading to hindered ettringite production（more pronounced in the low clinker group CO.5 and C1）.Compared with the effect in the low clinker group（CO.5 and C1）,NS increases the early and mid-late compressive strength,and the slag reaction rate reduces the porosity and refines the pore structure in the high clinker group（C3 and C5）.Meanwhile,the morphology of hydration products changed significantly for different clinker content systems,with 1%NS transforming ettringite from needle and rod-like to long columnar and C-S-H gel from thin film-like to thick form.Secondly,the effect of the different activities of Si O₂ on the performance of SSC was investigated.3%NS and silica fume（SF）were selected for the experiments.It was found that the compressive strength of SF was consistently better than that of NS in the low clinker group,and in the C0.5 sample,the addition of 3%NS would prevent it from setting and hardening,resulting in the inability to measure the compressive strength;while in the high clinker group,the strength of NS was better than that of SF.Comparative analysis of the hydration properties,microstructure,and pore solution of NS and SF samples showed that NS would advance the acceleration period of the hydration exothermic reaction,while SF delays the acceleration period.NS inhibits the formation of ettringite and gypsum consumption in the hydrated samples,while SF promotes the formation of ettringite in the early stage but suppresses the production of ettringite in the later stage.The difference between the two on ettringite formation is mainly caused by their different particle sizes（NS diameter is about 20 nm,SF diameter is about 100nm）,and SF causes a hysteresis of the hindering effect due to its larger particle size than NS.NS has a higher ability to refine the pore structure than SF,which is mainly due to the increased content of gel products and the corresponding increase in the number of gel pores,resulting in a denser pore structure.NS significantly reduces the p H（from 12 to about 10.1）and calcium ion concentration in the system,shifts the hydration products from oversaturated to unsaturated,which is the intrinsic reason why the C0.5 samples could not be solidified and hardened,and SF has no significant effect on this.By investigating the differences in the effects of nanosilica on the samples with different clinker contents in SSC,it was found that nanosilica acts differently on the hydration products（ettringite and C-S-H gels）,and its main mechanism of action in SSC is to inhibit the hydration of the aluminum phase and promote the hydration of the silicon phase,which leads to different compressive strength development trends in SSC.Both nanosilica and silica fume reduces the compressive strength fluctuations brought about by clinker variation,which is related to their high pozzolanic and ability to consume calcium hydroxide.However,silica fume and nanosilica cause different strength development in the high and low clinker content groups due to the high and low pozzolanic activity.Two reasons for this difference are the different degrees of influence of silica fume and nanosilica on the hydration reaction and the different effects on ion concentration and p H in the pore solution.The simulation of ion concentration revealed that nanosilica had a large effect on the saturation index of the pore solution of hydration products,and silica fume had no significant effect.