Macro-and Micro-Properties of Concrete for Century-Long Service
Introduction A long-term service capacity of Portland cement is important for cement mineral composition manipulating,concrete mix proportion design,and building maintenance.In 2017,beam cores were sampled on the Hugee building as one of the first reinforced concrete structures in China built in 1916.Macro-and micro-properties of concrete core samples were tested,i.e.,mechanical properties,anti-permeability properties,microscopic morphology,chemical composition,mineral composition and thermal stability properties.Meanwhile,concrete mix proportion parameters were disclosed by a reverse analyze method.In this paper,the macro-and micro-properties of concrete after a century of service and the evolution law of concrete properties were investigated.The results show that although the water-cement ratio of the concrete used in the construction of the Hugee building is relatively large,the mechanical properties and anti-permeability properties of the concrete are satisfying after a century of service.In addition to conventional C-S-H gel,CH crystals,AFt crystals,and AFm crystals,the concrete hydration products also contain a large amount of calcium carbonate crystals(i.e.,calcite,aragonite and vaterite),indicating that the concrete is gradually carbonized during its service.The results of SEM and quantitative XRD analysis show that after a century of service,the concrete still contains unhydrated Belite clinker minerals(hydration degree of 69.4％),which provides a material basis for the continuous improvement of concrete strength,showing that concrete prepared with Portland cement has a service capacity of more than 100 years.Methods Chemical composition,mineral composition,microscopic morphology,mechanical properties,anti-penetration ability of century-long service Hugee building concrete were systematically measured.Especially,the mix proportion of Hugee concrete was analyzed by a reverse analysis method in the absence of historical records of the material and mix proportion for the Hugee concrete.Core specimen obtained from Hugee building was first dismembered to obtain its coarse aggregate,fine aggregate and cement paste(i.e.,unhydrated clinker mineral and hydration products).The particle size distribution of coarse aggregate and fine aggregate were analyzed via sieving.The chemical composition,mineral composition and microscopic morphology of cement paste were analyzed by X-ray fluorescence(XRF),X-ray diffraction(XRD),scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).In addition,the hydration degree of the Portland cement clinker minerals in the Hugee concrete was also quantitively calculated based on the analysis by quantitative X-ray diffraction(QXRD)and thermal gravimetric-differential thermal analysis(TG-DTA).Results and discussion The results of reverse concrete mix proportion analysis show that the water to cement ratio(W/C)of the Hugee concrete is 0.69,and the bulk volume ration of cement:sand:aggregate is 1.0:2.1:4.0,which is well in line with the concrete preparation guidelines in 1920s.Although a high W/C value in the Hugee concrete restricts its strength and density according to the Abrams W/C-strength law,after a century of service,the compressive strength of the Hugee concrete is 23.4 MPa and the passed charge of the Hugee concrete is 936 Coulomb,which is satisfactory for a centenarian concrete.The results by SEM and QXRD show that after century-long service,the Hugee concrete is severely carbonated.The carbonation products are calcite,aragonite and vaterite,resulting from the carbonation of CH crystal,C-S-H gel and Aft crystal from cement hydration products.The hazard of the carbonation is the consumption of cement hydration product,and the benefit of the carbonation is the densification effect of concrete,preventing the penetration of harmful liquid into concrete.The guiding value of the carbonation measurement is that the W/C ratio can be lowered by chemical admixture to optimize the pore structure in concrete and enhance its anti-penetrating ability,which is important for guaranteeing concrete durability.Also,after century-long service,cement clinker minerals like alite,celite and ferrite are completely hydrated.However,partial of cement clinker mineral like belite still exists,which guarantees the continuous strength growth and micro-crack healing ability of concrete.Based on the results by TG-DTA,the hydration degree of belite calculated is 69.4％after a century-long service.The results of cement clinker hydration degree analysis show that high Belite cement is of unique value for guaranteeing concrete long-life service.Conclusions The reverse concrete mix proportion analysis method promoted could well reveal concrete mix proportion starting from scratch,which is important for understanding concrete mechanical,anti-penetration and micro-properties.The century-long service Hugee concrete is of well mechanical strength and anti-penetration ability,resulting from well preparation and casting at its birth age.After century-long service,the hydration products in the Hugee concrete include CH crystal,C-S-H gel,AFt crystal,AFm crystal,gypsum crystal and types of calcium carbonate crystals.The calcium carbonate crystals are resulted from the carbonation of Portland cement hydration products.After century-long service,there is still 30.6％of belite mineral to be un-hydrated,which is a material basis for the continuous improvement of concrete strength,manifesting that concrete prepared with Portland cement has a service capacity of more than 100 years.
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