Study On Capillary Water Absorption And Sulfate Resistance Of Seawater Sea-sand Cement-based Material

The construction on marine islands and reefs far away from the mainland is faced with the problem of high transportation cos t of construction materials.The use of seawater and sea sand as raw materials of cement-based materials has low cost advantage.However,seawater and sea sand have unique physical and chemical properties,and their effects on the mechanical properties and durability of cement-based materials still need to be studied.For this purpose,samples of seawater sea-sand mortar（SSM）,fiber reinforced seawater sea-sand cement-based materials（SSC）,and nano-silica modified fiber reinforced SSC specimen were prepared in this study.Firstly,the dynamic elastic modulus,compressive and flexural strength,capillary water absorption and microstructure of SSC at different ages were studied by using mechanical properties tests,weighing method test and low field nuclear magnetic resonance technology（LF-NMR）.Then,the effects of sulfate attack on the mechanical properties and capillary water absorption of SSC during wet and dry cycles were studied.Finally,nano-silica was used to modify fiber reinforced SSC,and its mechanical properties,capillary water absorption,micro structure characteristics and sulfate resistance were studied.The main research contents and achievements were summarized as follows.（1）Compressive and flexural strength tests were carried out for SSC with different curing ages and water-cement ratio,capillary water absorption tests were carried out by weighing method,and the microstructure of SSC was studied by using LF-NMR technology.The results showed that the strength of SSM developed rapidly in the first 3 days.For SSM samples with water-cement ratio 0.4 and 0.6,the compressive strength after curing for 3 days was 56.2%and 62.0%of the compressive strength after curing for 28 days,and the flexural strength after curing for 3 days was70.3%and 63.2%of the flexural strength after curing for 28 days,respectively.The capillary water absorption coefficient decreased with the decrease of water-cement ratio and the increase of curing age.When the water temperature rose from 20℃to40℃,the capillary water absorption coefficient of SSM was increased by 1.2 times.In the long term water absorption process,there was a linear relationship between the water absorption mass per unit area and the 0.25 power of water absorption time.With the growth of curing age,the porosity of SSM sample decreased,and the porosity at28 d was significantly lower than that at 3 d.The estimated tortuosity and measured capillary water absorption coefficient were substituted into the capillary model to calculate the representative capillary diameters,and were compared with the pore diameters estimated based on the transverse relaxation time of LF-NMR.In addition,it’s found that the mechanical properties of polyvinyl alcohol（PVA）fiber reinforced SSC were better than those of basalt fiber reinforced SSC,but the capillary water absorption coefficient of PVA fiber reinforced SSC was higher.（2）The mechanical properties,capillary water absorption and microstructure evolution of SSM and fiber reinforced SSC after sulfate attack under dry-wet cycles conditions were studied.The results show that the compressive and flexural strength of SSM specimens decreased obviously after 45 sulfate attacks.However,the compressive and flexural strength of fiber reinforced SSC specimens showed an increasing trend.When other components were consistent,the compressive and flexural strength of PVA fiber reinforced SSC was higher than that of basalt fiber reinforced SSC.As for SSM specimen,the capillary water absorption coefficient increases with the increase of sulfate attack times.After 45 sulfate attacks,the capillary water absorption coefficient of SSM specimen increased to 3 times of that of non-eroded specimen.Within 15 cycles of sulfate attack,a large number of needle rod ettringite was generated in SSM specimens to fill the pore structure,and small pore increased while large pores decreased.After 45 cycles sulfate attacks,too many products were generated in the pores,which exceeded the bearing capacity of the pores,resulting in pore wall cracking.For fiber reinforced SSC,porosity decreased,the peak value of T₂ shifted to the left,and pore structure was refined.（3）The mechanical properties capillary,water absorption propert ies of nano-silica modified fiber reinforced SSC and its relationship with microstructure were studied before and after sulfate attack.The results show that the mechanical properties and impermeability of fiber reinforced SSC were improved by adding nano-silica into the SSC.In addition,the nano-silica modified fiber reinforced SSC has good resistance to sulfate attack.The mechanical properties and impermeability properties of nano-silica modified PVA fiber reinforced SSC showed an enhanced trend after 15 and 45 times of sulfate attack under wet and dry cycles but the porosity gradually decreased.After sulphate attack under the condition of 15 and 45 dry-wet cycles,the mechanical strength of nano-silica modified basalt fiber reinforced SSC increased slightly,while the porosity and capillary water absorption coefficient of nano-silica modified basalt fiber reinforced SSC decreased first and then increased.