Backscattered electron imaging analysis on interfacial transition zone between steel fiber and cement paste in steel fiber reinforced mortar

http://www.euromat2009.fems.eu/index.htm Mortar is a composite material consisted of cement paste and fine aggregate less than 4mm, and is brittle without the inclusion of reinforcement. In order to increase the ductility and strength of the mortar, steel fiber and silica fume were incorporated into it in our study. Silica fume is used because more than 95% of the particles have the sizes of less than 1μm, which is around 100 times smaller than cement particles; this makes it a potentially good void filling powder between cement particles to increase the packing. In addition, it reacts with calcium hydroxide Ca(OH)2, a cement hydrant, through pozzolanic reaction to form calcium silicate hydrate C-S-H, and also densifying the cement paste. Steel fiber is used because it increases the ductility of the mortar when shear stress is properly transferred from the cement paste to the steel fiber through the microstructures in the interfacial transition zone. However, due to the wall effect, cement particles do not deposit compactly close to the steel fiber and a higher porosity is usually found in the interfacial transition zone compared to bulk paste. This greatly reduces the stress being transferred from the cement paste to the steel fiber. Therefore, it is important to perform quantitative analysis on the microstructures and the porosity in the interfacial transition zone between steel fiber and bulk paste so that the microstructural properties in that region could properly be understood and then improved so that the strength of the mortar could be increased. In our study, straight high carbon steel fiber with diameter 0.16mm and length 13mm, and silica fume were used. The backscattered electron imaging analysis (BSE-IA) technique was developed to quantify the unhydrated cement and the porosity on the interfacial transition zone between the steel fiber and the bulk paste of mortars with water and binder ratio (w/b) 0.3 and 0.5, with and without silica fume 10% by cement weight, and with 0.3 and 1 vol% steel fiber. 10-μm wide strips were successively cut from the edge of the steel fiber and the segmentation of features was performed using analySIS®. With this technique, the area percent of porosity and the area percent of unhydrated cement were successfully measured and plotted against the distance from the steel fiber’s interface respectively. From the graphs obtained, it was observed that steel fiber reinforced mortars with silica fume showed higher area percent of porosity but lower area percent of unhydrated cement compared to that without silica fume for both w/b. The higher area percent of porosity in steel fiber reinforced mortars with silica fume revealed a strong contradictory to the role played by silica fume in mortar as mentioned above. However, the porosity results supported well the compressive energy, fracture energy and debonding load measured from the three-point bending and compressive tests carried out on the steel fiber reinforced mortars in our study. The energies and debonding load for steel fiber reinforced mortars with silica fume, in fact, were measured either lower than or close to that of without silica fume. In addition, the agreement between porosity, energies and debonding load results for steel fiber reinforced mortars with and without silica fume also supported the validation of the quantitative technique on the interfacial transition zone using the backscattered electron imaging analysis developed in our study.