Agriculture Reference
In-Depth Information
the initial surface hardening that takes place during the early drying period,
forming a very thin veneer over the brick face that is difficult to penetrate
with the float stone in order to achieve the final rubbed-smooth finish to the
enrichment.
The secondary hardening, as stated above, occurs over several months as the
rubber fully dries out. This is considered to occur as the internal minerals con-
tinue to be carried slowly in solution to the exposed face of the brick, where
they crystallise at or near the surface. This view is upheld by Prentice (2000),
who suggests:
…this is due to the movement of soluble-salt-rich water from the inside of the
brick - a process which sometimes manifests itself as visible 'scum', but which is
always present to some degree.
The cause of this case-hardening is considered to be the result of one of two
distinct processes, or possibly a combination of both:
The low-firing temperature of rubbers, at or below 900ºC, causes some of
the integral minerals, in particular the silica and alumina content, to become
highly reactive (i.e. similar to what occurs chemically when burning a lime to
create quicklime). This is a completely different mineralogical reaction to that
which occurs to the silica in the burning of standard facing bricks. Here, there
is the production of a glass-like bond, cementing the particles together, and
the formation of a protective 'fireskin' to the surface of the brick in a process
known as vitrification. This reaction, which destroys the reactivity of the silica,
occurs between 900 and 1,200ºC.
The highly reactive silica and alumina mineral content of these low-fired
bricks is being activated by contact with water. This is initiated whilst soaking
the rubber prior to setting, and later during the laying process, and leads to
a hydraulic reaction during the subsequent drying-out of the newly-laid brick-
work. This action could produce silicic acid, which could act upon calcium car-
bonate or chalk in the brickearth or clay (as in the malm cutter) to form an
additional silicate of lime. These hydraulic chemical reactions could eventually
form a siliceous texture to the exposed surface or face of the brick, giving it a
much harder property.
In order to investigate the cause of the case-hardening phenomena, samples
from ten different historic and contemporary rubbing bricks were analysed.
The performance of a building material in relation to site and environmen-
tal conditions is determined by its mineralogical composition and physical
properties. A brick is composed of minerals and pores arranged in a certain
pattern. The nature of these minerals and the relationships between them
