Expert Witness Journal Issue 67 June/July 2026 - Flipbook - Page 121
Hygroscopic salts are nitrates and chlorides that
absorb moisture from the air, causing walls to
appear damp even when dry. They are a by-product
of damp problems and must be treated by removing
contaminated plaster, applying salt-resistant
renders, and controlling moisture sources.
Practical Considerations
•
Moisture is the trigger: Even if polysaccharides
are present, fungi cannot metabolize them
without sustained dampness.
•
Compatibility: Additives must not interfere with
gypsum setting (CaSO4·2H2O crystallization).
•
Regulatory Note: In housing pathology,
blocking polysaccharides is part of a nonbiocide remediation protocol, aligning with
sustainability and public health standards.
In Building Materials (Gypsum Context)
To block polysaccharides in gypsum plaster,
use polymeric binders (acrylics, latex, PVA),
pozzolanic additives (metakaolin, silica fume),
borates/phosphates as enzyme inhibitors, and
superplasticizers to reduce porosity. Combined
with moisture control, these measures prevent fungi
from accessing and metabolizing cellulose or starch
in plaster.
Hygroscopic salts are soluble salts (commonly
nitrates and chlorides) that absorb moisture
directly from the air. In buildings, they concentrate
in plaster or masonry where water has evaporated,
and they can cause persistent damp patches, tide
marks, and staining even when the original water
source is removed.
•
Organic additives in plaster:
Gypsum plaster often contains starch or
protein-based additives (polysaccharides and
polypeptides) to modify workability.
•
Chloride contamination:
Chloride salts (from groundwater, cement
admixtures, or e昀툀orescence) are highly soluble
and hygroscopic. They can interact with organic
additives, altering moisture retention and
microbial susceptibility.
•
E昀昀ect on mould growth:
Chloride salts can increase water activity in
plaster, making polypeptide or polysaccharide
additives more available to fungi like
Stachybotrys chartarum.
•
Blocking strategies:
ɿ Use pozzolanic binders (metakaolin, silica
fume) to immobilize chloride.
ɿ Apply polymeric encapsulation (acrylics,
latex) to shield organic additives.
ɿ Introduce borates or phosphates to inhibit
enzymatic breakdown of polypeptides.
•
In biology:
Chloride ions interact with polypeptides
via electrostatics, hydration, and enzyme
modulation.
•
In gypsum plaster:
Chloride salts can mobilize moisture and
interact with organic additives (polypeptides,
polysaccharides), indirectly promoting fungal
colonization.
•
Control measures:
Immobilize chloride with mineral additives,
encapsulate organics, and reduce moisture
pathways.
What Hygroscopic Salts Are
•
De昀椀nition: Hygroscopic salts are salts that
attract and hold water molecules from the
surrounding environment.
•
Common Types: Nitrates (sodium, potassium,
calcium) and chlorides.
•
Formation: They accumulate in walls when
water carrying dissolved salts evaporates,
leaving the salts behind.
•
Behaviour: Unlike e昀툀orescent salts (which
crystallize on surfaces), hygroscopic salts remain
in solution and can repeatedly absorb moisture
from humid air.
Impact on Buildings
•
Persistent Damp Appearance: Even if the wall is
structurally dry, hygroscopic salts can make it
look damp by absorbing atmospheric moisture.
•
Tide Marks: Dark horizontal lines or patches on
plaster, often changing intensity during the day
depending on humidity.
•
Damage:
ɿ
ɿ
ɿ
Carbohydrates in Gypsum Plaster
Carbohydrates — mainly **polysaccharides**
such as starches and cellulose derivatives — are
deliberately added to gypsum plaster formulations.
They play important roles in **workability, adhesion,
and water retention**, but they also introduce
vulnerabilities in damp housing environments.
Bubbling or peeling paint and plaster.
Weakening of plaster cohesion.
Continuous
damp
conditions
that
encourage mould growth.
EXPERT WITNESS JOURNAL
Why Carbohydrates Are Added
Starch ethers & cellulose ethers are common
additives.
119
JUNE 2026
