Researchers
from North Carolina State University have, for the first time, used a
"micropillar compression" technique to characterize the micro-scale
strength of cement, allowing for the development of cement with desirable
strength properties for civil engineering applications.
"The
information collected using this technique can be used to better understand the
behavior of concrete when it fails, as well as providing key data for
'constitutive' models that are used for designing and determining the safety of
large-scale civil engineering structures," says Rahnuma Shahrin, a civil
engineering Ph.D. student at NC State and lead author of a paper on the work.
"The
research outcomes will lead to significant impacts in the study of failure of
materials containing cement," Shahrin says. "The production,
transportation and use of concrete accounts for between 5 and 9 percent of
total carbon dioxide emissions worldwide. The knowledge from this study can be
applied toward development of stronger, more sustainable materials for civil
infrastructure, reducing consumption of natural resources and production of
CO2."
Cement
is used to make concrete, one of the most widely used construction materials in
the world. The compressive strength of cement is a primary factor in
determining how much load concrete can bear -- a critical consideration for
civil engineering projects. Engineers have long known that cement derives its strength
from an ingredient called calcium silicate hydrate (C-S-H) -- the primary
product formed when cement powder is mixed with water. Researchers, however,
have not been able to measure the compressive strength of the C-S-H in a cement
sample -- the sample sizes needed for isolating and testing the C-S-H
components are too small to fabricate by conventional sample preparation
methods.
To
address this challenge, the researchers turned to a technique used in materials
science called micropillar compression. Normally used on crystalline materials,
micropillar compression uses very small samples to determine the compressive
strength of a material.
Because
cement is a heterogeneous material, made up of multiple components, Shahrin
used a scanning electron microscopy/X-ray technique to find the areas in cement
samples that had the highest ratio of C-S-H relative to other constituent
materials.
Once
the C-S-H sites were identified, they were milled into cylinders 2 micrometers
wide and 4 micrometers in height. These samples could then be subjected to
micropillar compression.
"There
are lots of ways to make cement, and it can be made with different constituents
in different ratios," Shahrin says. "We've shown that the micropillar
technique can be used to give us precise measures of C-S-H compressive strength
in these different type of mixtures. This information can be used to help us
understand how various processes, and the constituents added during cement
production, can affect the cement's strength. It's basically a tool that can be
used to develop better, stronger cement."
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