Effect of vacuum and Focused Ion Beam generated heat on fracture properties of hydrated cement paste

The paper focuses on quantifying of vacuum and heat influence as unavoidable effects that appear during preparation and in-situ monitoring of micromechanical performance of cement pastes. Experimental tests at micrometer scale employ microscopic techniques of Scanning Electron Microscopy (SEM) and Focused Ion Beam (FIB). High vacuum is applied to samples during SEM-FIB procedures causing dessication and densification of primary cement paste constituents, the C-S-H gels. Collapse of pores and microstructural packing leads to their local stiffening and change in their fracture properties compared to usual partially saturated conditions of atmospheric pressures. The effect of vacuum is quantified for individual paste constituents in terms of their elastic moduli, tensile strengths and fracture energies measured on 15-20 mu m long cantilever micro-beams inside SEM chamber. It was found that application of vacuum in SEM increases elastic moduli of inner and outer products by similar to 30%, tensile strength rises 2.3-2.5 times. The effect of local heating due to ionic interactions during FIB milling is studied by means of micromechanical measurements on micro-beams and finite element (FE) numerical model. It is shown that high energy milling (30 kV, 30 nA) causes substantial microstructural and subsequent mechanical changes leading to further stiffening and tensile strength increase in the micrometer scale. The effect originates from phase changes caused by elevated temperatures under the ion beam that can be locally very high (thousands of K according to the simplified FE model). The paper also reports micromechanical response received by low energy milling for which microstructural changes due to increased temperature are restricted to very small volumes and can be assumed to be negligible with respect to the micro-beam dimensions.