An introduction to lime - The Hempcrete Topic: Designing and Building with Hemp-lime - page 61

a seasonal activity, dependent on the minimum temperatures required while lime mortars
and renders slowly re-carbonated. Building in the colder months meant an increase in pro-
ductivity and profits, and if the weather became colder still, the proportion of cement in
the mix could be increased from 4:1 to 3:1 (sand:cement) to decrease curing times still
further. Of course these advantages were already available to builders through the use of
natural cements, but since these tend to have a natural buff colour on drying, they were not
ideal for one of their major uses: the imitation of stone in, for example, decorative cornices
or statues. It was the desire to create a cement with a similar grey colour to that of many
stones that was the driving force behind the first innovation in artificial cements.
Significantly, in contrast to those made from lime, mortars and renders made from Portland
cement have very little permeability to moisture vapour. This characteristic was initially
welcomed, and cement enthusiastically used to apply a 'waterproof ' barrier, both in en-
gineering works and to houses and other buildings as an external render.
The merits of the ubiquitous use of Portland cement in the new construction of the post-
war period is itself questionable with hindsight, but its use for the repair of older buildings
- originally built with lime mortars and renders - was disastrous. The application of ce-
ment stopped these buildings from working properly, allowing moisture to become trapped
in the fabric of the building and leading to the rapid deterioration of the natural materials
used in their construction.