| Ultra-high-performance seawater sea-sand concrete(UHPSSC)has similar mechanical and durability properties to traditional ultra-high-performance concrete(UHPC),and its cost is significantly reduced,so it has attracted more and more attention in recent years.UHPSSC has significant advantages in Marine engineering applications,especially in offshore projects where construction materials are difficult to obtain and transport costs are high.However,due to the low water-binder ratio of UHPSSC,a large amount of cement is still consumed in the production process,resulting in a large amount of carbon dioxide emissions.In this paper,a new type of ultra-high-performance concrete incorporating limestone and calcined clay(LC3-UHPSSC)is developed according to the advantages that the terpolymer system(LC3)can effectively reduce the dosage of cement.The effects of calcined clay content and curing mode on the working performance,mechanical properties,hydration process and chloride ion erosion resistance of LC3-UHPSSC were discussed.The following important conclusions are drawn:(1)The addition of limestone powder has no significant effect on the fluidity of the mixture,but the limestone powder can provide additional nucleation sites for cement hydration,promote early hydration and shorten the setting time.With the increase of calcined clay content,the fluidity and setting time of LC3-UHPSSC gradually decrease and advance,which is related to the increase of specific surface area due to the small particle size of calcined clay.(2)The compressive strength of LC3-UHPSSC firstly increased and then decreased with the increase of calcined clay content.The highest level is the replacement of 40%(10% limestone powder and 30% calcined clay)cement,the compressive strength does not decrease significantly.When the content of calcined clay is 20%,the compressive strength reaches 117.8MPa,and the compressive strength does not decrease with the increase of age.Unmixed steel fiber will lead to lower flexural strength in later period,which can be effectively solved by adding steel fiber.(3)Seawater curing had no obvious effect on the compressive strength of UHPC,UHPSSC and LC3-UHPSSC,and even slightly increased,but had adverse effects on the flexure strength.Hot water curing can improve the compressive strength and flexural strength of UHPC,UHPSSC and LC3-UHPSSC to varying degrees.The mechanical properties of hot water curing at 80℃ are more significantly improved than those at 40℃.(4)With the increase of the content of calcined clay,the time of the main heat release peak is gradually advanced,and the hydration process is accelerated.The addition of seawater can not promote the hydration process of LC3-UHPSSC,but can improve the hydration degree to a certain extent.The aluminate peak is related to the content of chloride ion and sulfate ion in the mixture,which reflects the chemical binding of ions in seawater by calcined limestone clay to some extent.(5)There is a good correlation between the chemically bound water and the compressive strength under different ages,so it is feasible to calculate the chemically bound water in the mixture by calcining in a horse boiler.Ettringite,brucite,hydrotalcite,Friedel’s salt,gismondine,hemicarboaluminate(Hc),and monocarboaluminate(Mc)were detected in the mixture by TGA,XRD and SEM.(6)Compared with common UHPSSC,LC3-UHPSSC has stronger resistance to chloride ion erosion,and the content of chloride ion in LC3-UHPSSC is lower than that in common UHPSSC under the same conditions.On the one hand,the addition of calcined limestone clay optimizes the pore structure and makes the matrix more dense.On the other hand,the calcined clay system of limestone will chemically combine with chloride ions in the matrix and reduce the content of free chloride ions.In addition,hot water curing can improve the corrosion resistance of external chloride ions and the chemical binding of internal chloride ions. |
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