Expert Witness Journal Issue 67 June/July 2026 - Flipbook - Page 120
Methods to Block or Immobilize Nitrate
in Gypsum
Polysaccharides in gypsum plaster can be blocked
or neutralized by using additives that reduce
organic nutrient availability and by modifying
the plaster matrix to make it less hospitable to
microbial enzymes. The most e昀昀ective strategies
involve polymeric binders, pozzolanic additives,
and chemical modi昀椀ers that either encapsulate
or denature polysaccharides, preventing fungi like
Stachybotrys chartarum from metabolizing them.
1. Chemical Binding Agents
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Aluminum sulfate (alum) or ferric salts
can precipitate nitrates into less soluble
complexes.
These additives are sometimes used in
cement and gypsum systems to immobilize
anions.
Why Polysaccharides Matter
2. Pozzolanic Additives
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Fly ash, silica fume, or metakaolin
react with calcium hydroxide and create
additional binding phases.
These phases can trap nitrate ions within
the microstructure, reducing leaching.
3. Ion Exchange Additives
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Certain zeolites or clay minerals can
exchange ions and immobilize nitrates.
This is more common in environmental
remediation but can be adapted in gypsum
formulations.
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Source: Gypsum plaster often contains cellulose,
starch, or paper 昀椀bres as part of its formulation.
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Problem: These polysaccharides act as a
nutrient source for Stachybotrys Chartarum ,
especially under damp conditions.
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Mechanism: Fungi secrete cellulases and
amylases that break down polysaccharides into
sugars, fuelling colonization of the Stachybotrys
Chartarum.
Methods to Block Polysaccharides in
Gypsum Plaster
4. Polymeric Barriers
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1. Polymeric Encapsulation
Acrylic or latex admixtures can reduce
porosity in gypsum plaster, physically
blocking nitrate migration.
This is a practical solution in board
production where e昀툀orescence control is
critical.
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5. Commercial Gypsum Additives
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2. Pozzolanic Additives
Companies like Sika and British Gypsum
supply performance additives (plasticizers,
retarders, water reducers) that indirectly
reduce nitrate mobility by lowering water
demand and tightening the matrix
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Practical Considerations
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Compatibility: Not all additives are compatible
with gypsum chemistry; some can interfere with
setting times or strength.
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Testing: ASTM C471M outlines chemical
analysis methods for gypsum products,
including nitrate and sodium content British
Gypsum.
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Certain borates or phosphates can inhibit
fungal cellulases and amylases.
Borates are already used in wood
preservation and can be adapted for
gypsum systems.
4. Superplasticizers & Water Reducers
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Summary: To block nitrate in gypsum, you can
use aluminum/iron salts, pozzolanic additives,
zeolites, or polymer admixtures. The most
e昀昀ective strategy is combining chemical binders
with moisture control to prevent nitrate migration
and e昀툀orescence.
EXPERT WITNESS JOURNAL
Metakaolin, silica fume, or 昀氀y ash react
with calcium hydroxide to form secondary
binding phases.
These phases immobilize organic particles
within a denser matrix, reducing fungal
access.
3. Enzyme Inhibitors
Moisture Control: Even with additives, nitrate
migration is driven by water. Keeping gypsum
dry is the most e昀昀ective prevention.
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Additives such as acrylics, styrenebutadiene latex, or PVA emulsions can
encapsulate cellulose and starch particles.
This reduces bioavailability by creating
a hydrophobic barrier around organic
matter.
Products like Sika® ViscoCrete® (PCEbased) reduce water demand and porosity
Lower porosity means less moisture
transport,
which
indirectly
blocks
polysaccharide degradation by fungi.
5. Surface Treatments
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118
Silane/siloxane coatings can be applied to
gypsum plaster surfaces.
These repel water and reduce microbial
colonization, even if polysaccharides
remain in the matrix.
JUNE 2026
