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

Abstract

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 CSH 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 μm 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 ∼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.

Introduction

Cement is an aluminosilicate binder that hydrates and forms cement paste which serves as a matrix phase of various cementitious composites like mortars and concretes. Majority of the composites is prepared from Portland cement whose hydration reaction and microstructure has been widely studied, e.g. Ref. [1]. The cement paste has a hierarchical microstructure with variable chemistry and morphology across various scales. Traditionally, mechanical and other physical properties of the material are assessed on large laboratory samples of millimeter dimensions. Results received from such scales give effective engineering properties of the whole (e.g. elastic modulus, strength, fracture energy) but cannot give properties of individual constituents at the level of their origin that is much lower. Therefore, continuous effort is given by researchers to extract intrinsic mechanical properties at lower scales of the material.

With the advances of experimental techniques it becomes possible to access nano-to micrometer dimensions and perform direct measurements of physical properties here. One of the very useful tools used nowadays for determining local mechanical properties is nanoindentation [2,3]. The technique allows to perform surface impression tests and access cement volumes of around 1 $\mu {\text{m}}^3$. The scale is appropriate for extraction of e.g. elastic modulus, hardness or short-term creep of lower level paste constituents (e.g. Refs. [[4], [5], [6], [7]]) but cannot give values of tensile strength and fracture energy that need to be tested in modes where significant tension stresses are developed (e.g. direct tension or bending). But nanoindenter can be used also as a loading tool of small scale (micrometer) specimens and can measure their load-deflection response with high accuracy. The small scale specimens can be prepared with advanced sample preparation techniques based on Focused Ion Beam (FIB) milling, e.g. Refs. [[8], [9], [10]]. The technique allows precise fabrication of micrometer sized specimens within a cement paste [11]. However, none of the techniques is artefact-free. While nanoindentation mechanically affects significant volume of a material, the interaction volume of FIB is usually not large. However, dissipated energy in the form of heat can, due to the thermal conductivity of the material, influence much larger material region. Since cement paste belongs to the group of materials that are sensitive to heat exposure the influence must be taken into account. Heat causes loss of water from the system, volumetric changes and, if high, may cause also irreversible phase changes. Another influencing factor is a low pressure (vacuum) used for observation or in-situ measurements of samples in scanning electron microscope (SEM). The vacuum causes dessication and internal collapse of the microstructure at nanometer scale. The influence is manifested on a higher scale of cement paste by creation of a network of microcracks observable e.g. in SEM and caused by local exceeding of tensile strength. The effects of heat and vacuum is often combined since not only SEM observations but also nanoindentation experiments are performed within evacuated SEM chamber.

Nowadays, the FIB milling is used routinely but less is known about its microstructural and micromechanical effects on the matter that is sensitive to vacuum and heat like cement paste. High energy FIB milling is effective to remove large volumes of the matter in a reasonable time. Lower energy milling, on the other hand, does not cause much damage in the material but the milling takes substantially more time. Therefore, preparation of a specimen is always a trade-off between time consumption and potential damage caused by the interaction. Thus, the paper aims at quantification of vacuum and heat effects caused by SEM-FIB during preparation of micrometer sized specimens of cement paste on their micromechanical behavior and various aspects of this process are discussed within the paper. Thus, the paper aims at quantification of vacuum and heat effects caused by SEM-FIB during preparation of micrometer sized specimens of cement paste on their micromechanical behavior and various aspects of this process are discussed within the paper.