Effects Of Polycarboxylate Superplasticizer And Mineral Admixtures On The Workability,Hydration And Microstructure Of Cement Paste

Fly ash（FA）and ground granulated blast furnace slag（GGBS）are ecologically damaging industrial solid wastes which,if discarded and deposited at random,can aggravate environmental pollution and endanger human health.However,their resourceful use in the cement and concrete industry has gained widespread attention and has become an essential active mineral admixture component in the preparation of high performance concrete.Concrete co-disposal is a viable technology for the environmentally sound and comprehensive use of solid wastes such as FA and GGBS.This study focuses on the mechanism of the effects of FA,GGBS and polycarboxylate superplasticizer（PCE）reducing agents on the workability of cement slurry during concrete co-disposal,and the effects of hydration and microstructure of FA and GGBS on the porosity and strength of cement slurry.The packing density of cementitious materials,containing different contents of FA and GGBS,was determined under different dosage of PCE,then the WFT of cementitious materials was calculated.The relationships between WFT and the cohesiveness,flowability and flow rate of cement paste were established,respectively.Simultaneously,the compressive strength and porosity of cement paste were measured and a correlation was established based on different blending methods.Finally,the research explored the Ca（OH）₂content,the composition and the morphology of hydrated products of cement paste,so as to explain the macro changes of compressive strength and porosity of cement paste.The main conclusions are as follows:（1）WFT is an important factor affecting the workability of cement paste.The addition of PCE can increase the packing density and the WFT of cementitious materials,thus decreasing the cohesiveness of cement paste whereas increasing the flowability and flow rate of cement paste.To a certain extent,the incorporation of FA can improve the packing density of cementitious materials,and then increases the WFT of cementitious materials,thus increasing the flowability of cement paste.However,FA is able to retain water by fine particles,so the cohesiveness of cement paste was increased and the flow rate of cement paste was decreased.GGBS exhibited poor effect on increasing the packing density of cementitious materials and reduced the WFT of cementitious materials due to its large specific surface area.Nevertheless,GGBS has a weaker adsorption capacity on PCE than FA and cement,which leads to a decrease in the cohesiveness and an increase in the flowability and flow rate of cement paste added with GGBS and PCE.Compared with the cement paste mixed with FA,PCE is more sensitive to the cement paste mixed with GGBS.（2）PCE is capable of improving the comprehensive strength of cement paste under a certain dosage,but excessive PCE will cause a decrease in the comprehensive strength.Both the 7d strength of cement paste mixed with FA or GGBS was reduced,but the 7d strength of cement paste mixed with GGBS was higher than that of cement paste mixed with FA.The 28d strength of cement paste mixed with FA or GGBS reached a huge improvement,and the 28d strength of cement paste mixed GGBS was close to that of blank cement paste.The strength of the cement paste mixed both FA and GGBS was higher than that of the cement paste mixed FA and closed to that of the cement paste mixed GGBS.（3）The porosity of cement paste decreased with the increase of PCE.The porosity of cement paste increases with the increase of FA or GGBS,while the porosity of cement paste added with GGBS was lower than that of cement paste mixed FA.The porosity of cement paste added both of them is lower than that of cement paste mixed FA but higher than that of cement paste mixed GGBS.The porosity of cement paste was negatively correlated with the strength of cement paste,which can be well fitted based on Schiller formula.（4）The results of XRD,TG/DSC and SEM shows that the addition of PCE can promote the cement to hydrate and produce more Ca（OH）₂and C-S-H gel.PCE can also accelerate the secondary reaction of hydration of FA and GGBS for generating more hydrated products,which reduces the porosity of cement paste and enhances the compressive strength of cement paste.Because the secondary reaction of hydration of FA or GGBS is relatively slow,less hydrated products were produced and the pores were less filled,resulting in a decrease in the early strength of cement paste.In the later stage,the hydration degree of FA and GGBS was enhanced,and more hydrated products are produced accordingly,thus filling the pores and improving the strength of cement paste.The co-addition of FA and GGBS mutually promoted the pozzolanic reaction of FA and GGBS.With the addition of PCE,the pozzolanic reaction of mineral admixtures was accelerated and more hydrated products were produced,thus filling the pores and increasing the strength of cement paste.