Abstract
Magnesium oxide
board, “MgO-board”, is a factory-made sheathing board product, which has been
widely used in the last 5 years in ventilated facades on new or renovated
buildings in Denmark. In winter 2014/15, a number of problems began to appear
with these boards since the boards and adjoining building elements seemed to
suffer from some sort of disease, which manifested itself by damages such as
significant moisture, boards leaking salty water (‘tears’), corrosion of
fittings and anchors and mould growth. The damages were caused by the fact that
MgO-boards absorb moisture from outside air in periods with high outdoor
humidity (90-100% RH) and form water drops on the surfaces. The drops contain a
high amount of soluble chloride ions and appear on the surfaces of the boards
and may often run down the boards and to adjacent structures. Metal fixtures
for the MgOand siding boards may corrode heavily within a few years. The binder
in MgO-boards is formed by chemical reaction between MgO and MgCl2, known as
magnesium oxychloride cement or Sorel cement. Also organic matter can be found
in the material. The paper presents results of investigations of properties for
moisture retention and transport of MgO-boards.
Introduction
In winter 2014/15, many moisture damages were observed
inside ventilated facades of new or newly renovated Danish buildings. The
moisture damaged facades were constructed with sheathing made of magnesium
oxide-boards (MgO-boards) facing the air gap behind the material used for
exterior siding. MgO-boards have often been used as sheathing behind a
ventilated air gap as shown in Figure 1 and Figure 3. A photo of a MgO-board is
shown in Figure
Figure 1: Sketch of a typical façade construction where a
sheathing board is used as wind barrier. Adapted from [1]. Figure 2: Photo of 8
mm thick MgO-board with a glass fibre mesh on both surfaces. The observed
damages were caused by the fact that MgO-boards absorb moisture from outside
air. Water drops containing chloride ions appear on the outside of the boards –
often so many that they run down the vertical boards and out via the air gap
behind the siding boards (see Figures 4 and 5). The metal mountings for the
MgO-boards can corrode heavily within a few years, cf. Figure
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