Experimental Study On The Mechanical Properties Of The Interfacial Zone Of Cement Based Composites

The character of micro composition structure of cement-based materials,the typical heterogeneous materials, plays an decisive role to the macroscopic properties.The Interface Transition Zone which is the weak area of the composite is(Interface Transition Zone-ITZ) only about tens microns.And its property is weaker than the basis material.In this paper, the effect of different substitution rate of silica fume instead of cement on cement-based materials compressive strength was studied with different water-binder ratio. And by using the nano indentation technique,the hardness and elastic modulus of the aggregate,fiber and their adjacent regions were tested,the micro-mechanical property and structure of the aggregate and ITZ. By the environmental scanning electron microscope(environmental scanning electron microscope- ESEM) test technology and back-scatter image gray statistical analysis,the micro-structure,the inner product,the feature of interface layer and the effect on interface layer enhancement were observed with different substitution rate of silica fume instead of cement and water-binder ratio.In this paper, the conclusions are drawn below:(1) With the increase of silica fume, the compressive strength of the sample presented firstly increased and then decreased; With the increase of water-binder ratio,the compressive strength decreased; Prolonged curing is helpful to improve the later strength when the silica fume mixing amount is high.In this experiment, the compressive strength of the specimens was the highest when the silica fume substitution ratio was 10% and the water- binder ratio was 0.18.(2) The ITZ which was observed by nano indentation test was about 20um-60 um in this experiment.With the increase of the amount of silica fume substitution, the elastic modulus and indentation hardness gradually increased and then decreased under the same water-binder ratio.When silica fume mixing amount reached 30%, the interfacial transition zone is not obvious; when it is not change, with the increase of the water-binder ratio, the elastic modulus and hardness of the interfacial zone decreased gradually but the interface thickness increased.(3) By using scanning electron microscopy test images(SEM) and back-scatter(Back scattering-BSE) image analysis, it showed that when the water-cement ratio was unchanged, with increasing content of silica fume, the big holes of macroporous interfacial zone and connecting holes decrease, the small holes and porosity increased, the volume of calcium hydroxide(CH) decreased,the number of unhydrated cement particle and the average gray value decreased, calcium silicate hydrate(C-S-H) gel quantity increased, the number and extended range of cracks declined and the interface structure becomes relatively dense; when the silica fume is certain, the increase of water-cement ratio could lead to more and more holes of cement paste and interface part, unhydrated cement particles and the average gray value decreased, CH crystal increased, interface structure became relatively loose; The thickness of interfacial zone,determined by back scattering image gray scale analysis, is about 20um-50 um, which is close to the conclusion of the nano indentation test.