EWJ FEB 59 2025 web - Flipbook - Page 39
Factors that limit distribution
and abundance of black pin
mould in dwellings
A dissertation by Dr Aaran Marriner-Clark
FRICS, FISSE, FCABE C.BULD.E, FNIBME, PNIBME, FRSPH, MRPSA, MBMS,
EurBE, MCIEH MMSA.
effectively reducing sodium concentrations over time.
This eduction plays a critical role in the subsequent
colonisation dynamics of black pin mould.
Introduction
Black pin mould represents one of the most significant challenges in indoor mycology due to its capacity
to colonise a variety of building substrates and its association with serious health risks. This dissertation
examines the interplay between construction material
composition, particularly gypsum plaster and various
paint components, and the chemical environment
that influences mould proliferation. Emphasis is
placed on the role of sodium within these substrates,
which appears to act as a limiting factor in the growth
and distribution of black pin mould. Additionally, the
work reviews pertinent legal precedents and regulatory frameworks, highlighting the implications for determining a property’s fitness for human habitation.
Refined gypsum in the anhydrite form (no water) is
29.4 percent calcium (Ca) and 23.5 percent sulphur
(S). Usually, gypsum has water associated in the molecular structure (CaSO4·2H2O) and is approximately
23.3 percent Ca and 18.5 percent S (plaster of paris).
Gypsum used as fertiliser usually has other impurities
so grades are approximately 22 percent Ca and 17
percent S. Gypsum is sparingly soluble (the reason
wallboard gets soft but does not immediately dissolve
when it gets wet, at least if only damp occasionally).
Gypsum is the neutral salt of a strong acid and strong
base and does not increase or decrease acidity. Dissolving gypsum in water or soil results in the following
reaction: CaSO4 Na2SO4·2H2O = Ca2+ + NaSO42+ 2H2O. It adds calcium ions (Ca2+) and sulphate
ions (SO42-) but does not add or take away hydrogen
ions (H+). Therefore, it does not act as a liming or
acidifying material. The Ca2+ ions simply interact
with exchange sites in soil and sulphate remains
dissolved in soil water.
Masonry Construction and Material Composition
Paint Components
Paint is a complex composite material typically
comprising four primary constituents:
Pigments:
Responsible for colouration, white paints frequently
utilise titanium dioxide, while additional hues are
derived from iron oxides and metallic salts.
Gypsum removes sodium by the interaction between
the sulphate ions and sodium. Sodium ions are positively charged due to their lost electron, so they are attracted to the negative electrode which has an excess
of electrons. This attraction occurs because opposite
charges attract each other, according to the fundamental principle of electrostatics. The negatively
charged sodium ions attract with the positively
charged sodium ions, forming sodium sulphate,
which is highly leachable from the masonry (efflorescence).
Gypsum is sparingly soluble, which is why plaster
boarding gets soft when it gets wet but doesn't immediately dissolve. As gypsum is a neutral salt and does
not act as an acidifying material. It is made up of
calcium sulphate (CaSO4) and water (2H2O). When
gypsum is dissolved in water it adds calcium ions
(Ca2+) and sulphate ions (SO42-), but it does not add
or remove hydrogen ions (H+).
Resins:
Serve as the binding agent to hold suspended
pigments and facilitate film formation. Both natural
resins (e.g., linseed oil) and synthetic variants (e.g.,
acrylics, epoxies) are employed.
Additives:
It functions primarily as fillers (for instance, calcium
carbonate) and may also confer anti-fungal properties.
Solvents:
These carriers (ranging from organic solvents like
acetone and alcohols to water-based solutions) ensure
the appropriate viscosity for application and aid in the
even distribution of the paint film.
Gypsum Plaster and Its Interaction with Sodium
Gypsum plaster, predominantly composed of calcium
sulphate (CaSO4), is utilised widely in interior
construction due to its favourable drying characteristics and relative moisture tolerance. The material exists in various hydration states: Owing to its sparing
solubility, gypsum can interact with environmental
moisture without immediate dissolution. The process
of sodium leaching not only results in efflorescence
but also alters the chemical milieu of the plaster,
EXPERT WITNESS JOURNAL
Given this gypsum can remove sodium from masonry
through a reaction between the sodium and sulphate
ions. The sodium ions are negatively charged, and the
sulphate ions are positively charged, so they attract
each other to form sodium sulphate. This sodium
sulphate is then easily leached from the masonry.
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