Research On The Influence Of Seawater On The Properties Of Sulphoaluminate Cement Concrete

Marine concrete is the most basic and the biggest amount of materials that used in marine engineering construction.While due to the long-term corrosion by seawater,wave splash,sea breeze blowing and other physical and chemical effects,marine concrete has higher requirements on mechanical properties and durability,especially on permeability and resistance to salt corrosion.In this paper,by adding a certain dosage of retarder and admixtures to optimize the setting time,compressive strength and microstructure of sulphoaluminate cement?SAC?and ferroaluminate cement?FAC?.Than,SAC and FAC based C40,C50,C60marine engineering concrete ware prepared to study the influence of three different mixing or curing methods:fresh water mixing-fresh water curing,fresh water mixing-seawater curing,seawater mixing-fresh water curing on concrete strength development,the resistance to chloride permeability and volume stability,and made comparisons with Ordinary Portland cement?OPC?.The microstructure,composition of hydration products and interfacial transition zone?ITZ?were measured by X-ray diffraction?XRD?,scanning electron microscope?SEM?,thermal gravity-differential scanning calorimetry?TG-DSC?tests.Experimental results show that:?1?Single or compound appropriate amount of mineral admixtures metakaolin?MK?,fly ash?FA?,Slag have little effect on setting time,while FA and Slag will reduce the strength of cement.Appropriate amount of inorganic retarder boric acid?H₃BO₃?or borax and single?Na₂B₄O₇·10H₂O?or compounded with mineral admixtures can significantly lengthen the setting time of SAC and FAC,but H₃BO₃would reduce SAC mortar compressive strength.Therefore,Na₂B₄O₇·10H₂O is choosed as retarder,and the optimal dosage in SAC and FAC is 0.35%and 0.25%respectively.After adding Na₂B₄O₇·10H₂O,the induction period and acceleration period of cement hydration will be significantly lengthened,hydration heat quantity of early age?24h?decreased,while later age increased.?2?The compressive strength of concrete curing in seawater increases along with age growth.Compared with curing in fresh water,there is no obvious difference,and the 28d and 90d age strength slightly increased.The compressive strength of concrete mixedwith seawater increases along with age growth,the increasing range follows the order of OPC>>SAC>FAC.Compared with mixed with fresh water,mixed with seawater will promote the development of early strength,but reduce the later strength by about 10%,and the influence of high strength grade concrete is greater than the low grade concrete.?3?The main compositions in SAC and FAC hydrationsystem are AFt,AH₃ gel,C-S-H gel,CaCO₃and a small amount of unhydrated C₄A₃S^—,CaSO₄,C₂S,without Ca?OH?₂.After adding Na₂B₄O₇·10H₂O,AFt productionin SAC sample is slightly reduced,while inFAC sample,AFt production has no obvious change.Under the condition of seawater curing,SAC and FAC concrete hydration products are as the same as the fresh water curing,while OPC concrete hydration products containFriedel's salt,and Friedel's salt content increases gradually along with age growth.Under the condition of seawater mixing,Friedel's salt are found in hydration products of three kinds of cements,the chloride binding capacity of three kinds of cements follows the order of OPC>FAC>SAC.?4?The 14d resistance to chloride permeability of fresh water mixing SAC,FAC,OPC blank samples?without admixtures?and C40,C50,C60 concrete?containing admixtures?follow the order of R·SAC>FAC>OPC,and improved at 28d age.Appropriate amount of MK or FA can improve the resistance to chloride permeability.SAC and FAC concrete mixed withseawater has a poorer resistance to chloride permeability than that mixed with fresh water,while OPC is almost the same.The 28d age resistance to chloride permeability of SAC,FAC,OPC C40 concrete?containing admixtures?thatmixed with seawater follow the order of OPC>>SAC>FAC.?5?The shrinking rate of SAC,FAC and OPC increasesalong with age growth,and become stabile after 28d.No matter mixed with fresh water or seawater,the 28d shrinking rate of different cement follows the order of OPC>>FAC>SAC.The shrinking rate of different mixing methods follow the order ofblank samples>seawater mixing>fresh water mixing.An appropriate amount of MK or FA can improve the volume stability and seawater mixing would increase the shrinkage of concrete.