This article focuses on some of the applications being developed by the Civil Engineering Department at the Massachusetts Institute of Technology (MIT), for whom Anton Paar developed an innovative humidity-controlled nanoindentation system. This group focuses on the study of geomaterials or natural composites, i.e., materials which have complex heterogeneous structures and those that occur naturally. Examples of such materials are shales (which cover oilfields) and cement pastes (used for concrete).
The need for in-situ analysis of chemically complex phases obviates conventional mechanical testing of bigger specimens representative of these material components. It is thus possible to use nanoindentation as a 2D mapping tool for examining the properties of constituent phases independently of each other.
C-S-H is a nonstoichiometric compound and the average Ca/Si ratio in ordinary hardened cement paste is about 1.7. It has a layered crystal structure similar to that of the mineral tobermorite, and this is the reason why this mineral is frequently used as a model material as it is analogous to the main hydrated phase of cement paste and can be artificially created. The multiscale heterogeneity of concrete eventually determines its in vivo mechanical performance (strength, stiffness) and degradation (damage, failure, fracture). It is possible to divide the microstructure into four levels as shown in Figure 1, from the scale of mortar (10-2 m) down to the C-S-H solid phase (10-10 m) which represents the smallest material length scale that is presently accessible by mechanical testing (nanoindentation). By experimentally investigating the mechanical properties of cement paste at varied length scales provides a means of correlating such microscale properties to macroscale applications.The last decades have seen a gradual improvement of the mechanical properties of cement, but this has been achieved more by trial and error than by an in-depth understanding of what is happening at the micro and nanoscales. The setting of cement is not a drying process, as sometimes believed, but in fact, it is the exact opposite. When cement is mixed with water, it goes through a dissolution reaction generating silicate, calcium and aluminate ions in the interstitial solution. New products (hydrates) then precipitate after reaching their solubility limit and also after a nucleation period. In a common cement, such as Portland cement, this dissolution-diffusion-precipitation process produces calcium hydroxide (Portlandite) and calcium silicate hydrate (C-SH). During hydration, the slurry coagulates after the cement is mixed with water, after which setting occurs. Some proportion of the anhydrous cement is converted into C-S-H and other hydrates.
Figure 1. Four-level microstructure of cement-based composite materials
Grid Indentation Technique
A novel grid-indentation technique can be used to attain significant mechanical properties of heterogeneous materials (such as C-S-H) at a specific length scale, and it also provides access to the volume fractions of independent phases. Consider a material to be composed of two phases of varied mechanical properties and characterized by a length scale D, as shown in Figure 2. If the indentation depth is much smaller than the equal to the surface fraction occupied by the two phases on the indentation surface. On the other hand, an indentation test performed to a maximum indentation depth that is much bigger than the characteristic size of the individual phases, h > D, senses the average response of the composite material, and the properties extracted from such an indentation experiment are representative in a statistical sense of the average properties of the composite material.
These large matrices of indentations offer a statistical analysis comprising of distributions and their derivatives (e.g. frequency diagrams or histograms) of mechanical properties determined by a large number of indentation experiments at a particular scale of material observation defined by the indentation depth. Hardened concrete always comprises of a major fraction of liquid water from the capillary condensation of water vapor in the intergranular pores, hence it is of particular interest to carry out grid-indentations with accurate Relative Humidity (RH) control in order to quantify the influence of water fraction on mechanical properties. This indeed was the motivation for designing a completely-automated nanoindentation instrument, capable of making hundreds to thousands of indentations while maintaining the ambient humidity to an accuracy of +/- 0.1 % RH.
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This information has been sourced, reviewed and adapted from materials provided by Anton Paar GmbH.
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