PREVENTING CONCRETE BRIDGES FROM FALLING APART

A new study examines the adverse effects of the adsorption
of natural gas constituents found in our environment — and mixtures of several
such gases — into one of the materials that make up concrete: cement hydrate.
The author found that the preservation of concrete infrastructure from the
corrosive effects would require effective pre-treatment.

Concrete degradation from sulfuric acid can be avoided by
finding ways of preventing its gas precursor from adsorbing into concrete.

Extremes of temperature, rain, exposure to corrosive
substances — all of these environmental factors contribute to the degradation
of concrete. Specifically, a gas present in our environment, called hydrogen
sulphide, turns into sulphuric acid, a corrosive substance, when combined with
rainwater. In a new study published in EPJ B, Matthew Lasich from
Mangosuthu University of Technology, Durban, South Africa, examines the adverse
consequences of the adsorption of natural gas constituents found in our
environment — and mixtures of several such gases -into one of the materials
that make up concrete: cement hydrate. Lasich found that the preservation of
concrete infrastructure from the corrosive effects would require a
pre-treatment targeting the adsorption sites in cement hydrate, where the
majority of hydrogen sulphide molecules become attached. However, this approach
could prove difficult because of their wide distribution.

What makes concrete vulnerable to natural gas adsorption is
its porous nature. Its structure is made up of a cement matrix binding together
aggregates of particles of sand. In this study, the authors perform a nanoscale
analysis based on Monte Carlo simulation to mimic the migration of gas
molecules into the cement hydrate structure.

They first recorded the adsorption level across various
temperatures for methane, ethane, ethene, and ethyne to determine the uptake of
each gas species in cement hydrate. This allowed them to study the effect of
molecular size and molecular shape on the sorption of gases in cement hydrate.
They then performed a similar analysis for natural gas constituents including
nitrogen, carbon dioxide, and, most importantly, hydrogen sulphide.

Their simulations suggest that a specific combination of molecular size and surface area is required for good uptake into cement hydrate. While hydrogen sulphide adsorbed most favourably of all gases considered in this study, ethyne adsorbed more favourably than methane, despite being a ‘heavier’ molecule, because its molecular shape lent itself better to the task. https://www.sciencedaily.com/releases/2018/12/181219124316.htm