Interest in exploring the use of
seawater as the mixing water for
preparing concrete is increasing due to the lack of freshwater in
some coastal regions and remote islands, where seawater is more accessible.
However, up to now, the mechanism of the accelerating effect of seawater
on the hydration of portland cement (PC) remains unclear. In this
study, alite, a major clinker phase in PC, was hydrated with common
salt solutions (NaCl, Na2SO4, and MgCl2) in seawater to explore the mechanism of acceleration. The heat
release peaks of the salt-added systems shifted to an earlier hydration
time with a higher peak value, which indicated the faster hydration
rate of alite pastes compared to the deionized (DI) water system.
The addition of the single salts was found to increase the concentration
of Ca species in solutions, contributing to the increased formation
of calcium–silicate–hydrates (C–S–H) and
portlandite at early ages. In the Na2SO4 system,
gypsum was the new hydration product, while brucite was formed in
MgCl2 systems, which caused the sharp decrease of Mg species
in the solution. The morphology of the early formed C–S–H
was changed with the addition of the salts, and the C–S–H
were characterized as thinner and longer fibers. At later ages, the
incorporation of the single salts lowered the polymerization degree
of C–S–H, but no noticeable morphological change was
observed.
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