Sulphates & Their Effects on Concrete
Sulphates, or sometimes called sulfates, may be the cause of loss of strength in concrete and
mortar due to the chemical reaction of the presence of sulphate ions with calcium hydroxide,
forming gypsum. A sulphate attack can lead to expansion, cracking and loss of strength and
integrity, and in extreme case, disintegration of the concrete.
Sulphates are usually associated with clay soils, where they are carried by ground water,
leading to various problems which will affect the integrity of projects, including rendered
surfaces and manhole chambers, leaching calcium deposits which are unsightly and attritional.
If you suspect the presence of sulphates, or when working in clay ground, consider using a
Sulphate cement for all applications likely to be affected i.e. works below ground, foundations,
piling works and drainage systems. Known as Sulphate Resistant Portland Cement (SRPC) it
is available from all good Builder’s merchants.
It is recommended that any works below ground, including concrete rafts under paving
projects, or manhole chambers, are lined with 150mm or more shingle to allow water to
percolate downwards and away from the concrete/mortar, taking any sulphates with it. This
will also have the effect of relieving hydrostatic pressures and prevent water from eroding the
soil from under concrete foundations.
Working on clay soil is always a challenge, and care should be taken to use sulphate
resistant cement if you are aware of the likely problem. Due to poor drainage and
construction techniques and allowances in this project, water has laid against the laying
mortar beds, and erupted through the joints having destroyed the integrity of the mortar,
leading to many manifestations including loose treads, risers and paving, and capillary
action rising through the rendering, causing the coping stones to also fail and become loose.
