Expert Witness Journal Issue 64 December 2025 - Flipbook - Page 95
Using Bleach to Eliminate
Stachybotrys Chartarum
on Masonry
by Dr Aaran Marriner-Clark, FRICS, FISSE, FCABE C.BULD.E, FNIBME, FRSPH,
FAMI, MRSB, MRPSA, MBMS, MCIEH, MMSA MBSMM, MIBBS MBTS
It was a昀케rmed that in a study by Kelly A. Reynolds,
Stephanie Boone, Kelly R. Bright, Charles P. Gerba
2012 (Journal of Occupational and Environmental
Hygiene, 9: 663–669 ISSN: 1545-9624 print / 15459632 online Copyright 2012 JOEH, LLC).
Gypsum exposure to bleach and
sodium transport
Bleach introduces sodium into porous gypsum
systems; it doesn’t cause sodium to “leach out” of
gypsum’s crystal lattice. Instead, gypsum boards
absorbs the bleach solution, and sodium remains
as dissolved salts that can migrate, concentrate,
and crystallize during drying e昀昀ect of e昀툀orescence
formation. What actually happens chemically is the
gypsum calcium and sulphate are already in stable
oxidation states; sodium does not substitute for
calcium in the crystal under ambient conditions.
There is no meaningful ion exchange of Na+ into the
gypsum lattice to do this. What actually happens is
the bleach in aqueous solution, sodium hypochlorite
exists as Na+ and OCl-. Over time—especially on
porous, catalytic surfaces—it disproportionate to
chloride (Cl-) and chlorate (ClO3-). Carbon dioxide
uptake can also convert alkalinity to carbonate/
bicarbonate. Giving a net e昀昀ect of the sodium
remaining paired with anions as soluble salts. It is
this that leaches the sodium from the gypsum when
leaching or saturation occurs through structural
defects and the drying process of e昀툀orescence
formation.
That the results of this study support those of others
concluding that hypochlorite disinfectants are
e昀昀ective antimicrobials against moulds. Where in
the study, Trichophyton mentagrophytes was not
culturable in 10/10 trials following 10-min exposures
to 2.4% NaOCl on porous surfaces.
I am of the opinion this may occur in a laboratory
setting and the results though encouraging for
Penicillium Spp and Cladosporium Spp have little
e昀昀ect or relevance to Stachybotrys Chartarum,
(black pin mould) because on gypsum walls and
gypsum plaster boards, the bleach has a very
di昀昀erent e昀昀ect.
The Sodium hypochlorite (NaOCl) does not
chemically react strongly with gypsum, calcium
sulphate dihydrate, CaSO4·2H2O, but when mixed
or used for disinfection, it can alter the physical
properties of gypsum products when applied.
Salt migration and e昀툀orescence:
Studies show that immersion or mixing of gypsum
with sodium hypochlorite reduces its compressive
strength and surface quality.
Capillary transport draws solution into and
through paper facings and pore networks. As water
evaporates, NaCl, NaClO, NaClO3, and occasionally
Na2SO4 (from sulphate-rich environments) can
crystallize at or near the surface, presenting as
e昀툀orescence. Once leaching of the sodium occurs
through structural defects the sodium is reduced
over time, the calcium is then available to the
Stachybotrys Chartarum with a reduced sodium
content high calcium and cellulose.
Also, gypsum composition: CaSO4·2H2O, calcium
sulphate dihydrate and Sodium hypochlorite: NaOCl
has no signi昀椀cant chemical reaction noted under
normal conditions, since gypsum is relatively stable
and NaOCl mainly acts as an oxidizer. However,
some practical e昀昀ect such as the interactions NaOCl
solution penetrating the gypsum and disrupting
its crystalline structure and hydration balance as a
result of application.
Practical material e昀昀ects in buildings
Surface contamination:
Which is more physical than chemical but allows for
the leaching of sodium from the gypsum the limiting
factor in Stachybotrys Chartarum colonisation.
EXPERT WITNESS JOURNAL
Residue formation: Sodium salts persist after
bleaching, increasing surface conductivity and
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DECEMBER/JANUARY 2025-2026
