Classical
antiquity has not only offered us a rich cultural
heritage, that has also seen a lot of scientific achievement. When one has an opportunity to admire the architecture that
originated from the Roman Empire, one should not overlook the fact that the
science of building materials from the ancient times have provided the starting
point for further development that is still highly relevant to this day. With
previous experience in the research of Construction Materials, I find the
science of Roman concrete (Latin: opus caementicium), responsible for
the wonder of Roman architecture, really fascinating. For this time, it is all
about science.
While
most of us can easily appreciate the importance of cement and concrete for our
daily lives, we may not necessarily know the scientific and historical background
of these useful materials. To start with, concrete is a mixture of a few chemical components: a binder
material, some form of aggregate, water, and some cases, further chemical
additives. The binder is often called a cement, and it is like the ‘backbone’
of the concrete. The aggregate is often some form of sand of various diameters, acting as a filler material. Concrete is prepared by mixing
cement, sand, and water in a well-defined proportion, when chemical reactions lead to the setting, and eventually hardening of the resulting concrete. Though it may sound trivial,
this has always been the the basis for the building industry throughout human history.
Let’s look a bit further about the historical and scientific aspects of
cement. While forms of cement have been known since ancient Egypt, Greek, and the Roman
empire, the key composition of cement almost always contains some calcium
compounds, like lime or calcium carbonate. The cement we know nowadays, the
most classic form being the Portland Cement, was a creation from England back in
the 19th century. While there are many variations since the first development of this classic form of cement (Ordinary Portland Cement, abbreviated as OPC), the key chemical
components of the cement itself are often very similar, albeit varying in
proportions – inorganic compounds of calcium (Ca), silicon (Si), aluminum (Al) and iron (Fe). The Roman
cement, as we will see, also contained many of these chemical components, yet its
specific composition led to some interesting properties of its own.
Before
we move on to the ancient science let me address two further issues regarding
some properties of cement. The first issue is the production of cement itself.
The ingredients of cement, containing inorganic salts of calcium, silicon, aluminum
and iron, is heated at very high temperature (>1500 degree Celsius), and as a result a
number of chemical reactions take place during the burning process, forming new chemical products. The resulting mixture is ground into a
fine powder and leads to the greenish grey appearance very typical of modern cement. While
we do not have to go into the chemical details, often 4 types of complex
inorganic salts result: two of them are calcium-silicon salts, designated as C3S
and C2S; one is a calcium-aluminum salt, C3A; and a
calcium-aluminum-iron salt, C4AF. The reason why this is mentioned
is because the proportion and the specific properties of the 4 salts will
determine the resulting properties of the cement, and hence impacts the setting time, mechanical
and durability of the concrete itself. As an example, ‘high-alumina
cement’ is a version that increases the concentration of aluminum salt in the
precursor mixture and leads to a richer aluminum content (more C3A)
in the resulting cement. Indeed, the various types of special cement that have emerged from the past 200 years of so originated from tuning of these
properties.
The
second issue is the paradoxical observation of water’s action towards concrete.
Water is needed in the setting and hardening of the concrete. Yet, the strength of the resulting
concrete is developed and enhanced by immersion of the concrete in water, which may sound counter-intuitive to many. That is because
the calcium-based cement is a hydraulic-setting cement, meaning that the resulting concrete
develops its mechanical strength in water. That would certainly sound strange
to many people not in the field of construction engineering.
Yet,
there is a caveat in the water immersion issue. While calcium-based concrete
can develop strength in a water medium, we mean water only. If the aqueous
environment contains other inorganic salts, just like in the marine environment, it
will lead to a deterioration of the structure in terms of mechanical strength and issues in durability.
Thus, underwater construction has always posed a problem for the field of
building materials, and throughout history many has worked towards solving the problem. That is where Roman Concrete enters the story.
Roman
Concrete, which has been used in the construction of many architectures since
Ancient Rome, was reputed to be a very durable building material. Also, the ancient
building material was in particular famous for application in underwater
building work, including bridges and areas near water. The Roman concrete was
also based on a calcium hydraulic-setting cement, like the Ordinary Portland
Cement. The wonder behind this ancient cement can be summarized by one word:
Pozzolana.
Pozzolana (Latin: pulvis puteolanus) is the volcanic ash found from
Pozzuoli, Italy.
While it is known that other civilizations, like Egypt and Greek, has adopted
similar approaches in their building work, that was the Romans who have really
popularized the idea and committed to large scale construction. Volcanic ash is
a product of a relatively explosive volcanic eruption, often due to the rhyolite-natured
magma (more felsic / acidic magma due to a higher SiO2 content), and that
contains more gas products and hence a more violent eruption. When the liquid magma is released into the
air through a pyroclastic eruption, the resulting tiny fragments of glass
formed from that are the volcanic ashes. The pozzolanic volcanic ash is rich in
silica and alumina, and by reacting with lime, Ca(OH)2, further
products are obtained, giving rise to its specific properties. The resulting
cement has a higher silica content than the standard version of modern cement,
and they are not indeed very different certain specific types of cement nowadays, where
additives like fly ash and fumed silica are added into the precursor mix.
The
resulting concrete, prepared from the cement (Lime and Pozzolana), various aggregate
and water, has offered a number of advantages. First, the concrete is very
durable. That can be attributed to the high silica and alumina content of the
cement. Also, it is very resistant to the action of salt water. Thus, in the
marine environment, even if the concrete structure was submerged under water,
the mechanical strength of the structure was not compromised. This can be
explained as high-alumina cement is often resistant to sulfate attack and the
action of other inorganic salts. One will always feel with a sense of awe how
a volcano-related material can contribute so much to a long-standing construction
problem.
The
ancient cement from the Roman Empire may be from a long, long time ago, yet the
concept behind such an achievement is in no way antiquated. The type of ancient
cement has inspired a modern type of cement known as 'Pozzolanic Cement', which
is still used in a number of regions around the world. The approach is
immortalized by a named chemical reaction, and a number of international
standards regarding the use of these materials in building industry. You will
never view the volcanos in the same way again!
by Ed Law
Conatus Classics
