Uptake and reversibility of aluminium in calcium silicate hydrates

Student thesis: Master thesis (including HD thesis)

  • Inga Mathilde Tangen
4. term, Chemistry, Master (Scient) (Master Programme)
Production of Portland cement (PC) is responsible for a significant amount of
anthropogenic CO2 emissions, and ways to lower emissions are highly sought after.
One way of reducing emissions is by substituting part of the cement clinker with
supplementary cementitious materials (SCMs). SCMs typically contain more silica
and alumina and less calcium than ordinary PC, which could change the compositon
of hydrated cement and C-S-H. This study aims to determine how the kinetics and
reversibility of aluminium uptake in calcium silicate hydrates (C-S-H) changes, based
on its composition. Specifically, it investigates how the Ca/Si molar ratio of C-S-H
changes the kinetics of aluminium uptake and properties of the resulting solids, as
well as whether the aluminium uptake is reversible.
In order to investigate the kinetics of aluminium uptake, 240 samples of varying
Ca/Si and Al/Si molar ratio with varying hydration time were synthesised and
analysed. The results showed slower uptake of aluminium in low Ca/Si samples,
compared to high Ca/Si. Additionally, the inclusion of aluminium increases the
mean chain length (MCL) of the C-S-H by binding to bridging tetrahedra in C-S-H.
In target Ca/Si ≤ 1.0 and Al/Si ≤ 0.05, all the Al goes to C-S-H.
Furthermore, the reversibility of the aluminium uptake in C-S-H was tested by
synthesising samples of calcium aluminium silicate hydrate (C-A-S-H) that were
filtered and added to a solution without aluminium. The results showed that
aluminium is reversible in low Ca/Si C-S-H short-term, with an increasing stability
of Al in C-S-H with higher Ca/Si.
These results suggest that PC which is partially substituted by SCMs will have
increased MCL due to the incorporation of more aluminium. As PC systems are
complex, the reversibility of aluminium uptake in C-S-H does not necessarily mean
that aluminium will be equally reversible in PC systems.
Publication date6 Aug 2021
External collaboratorSwiss Federal Laboratories for Materials Science and Technology (Empa)
Barbara Lothenbach barbara.lothenbach@empa.ch
Information group
ID: 434060390