Masons were the skilled individuals who first mastered the art of using materials like stone to construct sturdy, safe structures.
Mankind needs shelter to survive the harsh conditions in most of the Earth’s climates and our shelters have grown to become monuments to our personal wealth, our business power, and our government’s prestige.
Today, construction and engineering is very big business, employing around 7% of the global workforce and generating about 10% of the global GDP. This giant industry demands vast quantities of building materials that are manufactured from cement, steel, glass, aluminium, clay, resin and other materials.
Industrial gases play a vital role both in the production of these materials and also in the manufacture of construction components.
Calcium – cement – concrete
The shells and bones of most creatures, including humans, all consist primarily of calcium – an extremely abundant element on Earth. The high reactivity of the metal calcium makes it a rather unstable organic element, so it is always found combined with other elements in complex molecules.
Calcium carbonate (CaCO3) is a common mineral forming the major component of the sedimentary rock known as limestone and combines calcium with carbon, the fundamental element of organic compounds.
Limestone blocks were the construction material of choice during the Middle Ages and many churches, monuments and even the pyramids survive today, built of limestone. It is also the raw material for what is the most widely used construction material today: concrete.
Extracted most often from an open-pit mine or quarry, limestone is the source of several calcium based compounds that are the basic ingredients of Portland cement.
Cement sets when mixed with water by way of a complex series of chemical reactions that are still only partly understood. The different constituents slowly crystallise and the interlocking of their crystals gives cement its strength.
Carbon dioxide is slowly absorbed to convert the portlandite (Ca(OH)2) into insoluble calcium carbonate. After the initial setting, immersion in warm water will speed up the setting process.
Cement is the essential ‘glue’ in concrete, a fundamental building material for society’s infrastructure around the world. Concrete is second only to water in total volumes consumed annually by society. Cement production also co-produces carbon dioxide (CO2) and this contributes around 5% of current global man-made CO2 emissions.
In 2006 global cement production was 2.6 billion tonnes, having increased by 54% since 2000. The demand for cement, mainly in developing countries, is forecast to grow to at least 3.6 billion tonnes by 2050, but could exceed 4.4 billion.
Rotary cement kilns are used for the pyroprocessing stage of manufacturing Portland and other types of hydraulic cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates. As the main energy-consuming stage of the process their efficiency is a central concern of the cement manufacturing industry.
CO2 liberated during the calcination of limestone is the source of the majority of emissions in cement production and this would remain unabated even if pre-combustion technologies were used, therefore the focus is on CO2 capture technologies.
**Chemical absorption is most promising and high CO2 capture rates have been achieved in other industries
**Membrane technologies may also be used in the long-term, if suitable materials and cleaning techniques are developed
**Carbonate looping is an adsorption process in which calcium oxide is put into contact with the combustion gas containing carbon dioxide, to produce calcium carbonate. This technology is currently being assessed
**Oxy-fuel technology, using oxygen instead of air in cement kilns, would result in a comparatively pure CO2 stream suitable for recovery and re-use in other industrial applications. Extensive research is still required to understand all potential impacts on the clinker burning process
Carbon capture technologies in the cement industry are not likely to be commercially available before 2020.
Steel for structure & support
Strength, toughness and flexibility are the qualities of steel that have enabled many remarkable manmade constructions.
It is of interest to note that the element carbon again plays a central role in determining the properties of steel and is manipulated together with other alloying elements, to produce a wide range of construction materials and components.
Iron is the world’s most commonly used metal, because as the key ingredient in steel it represents almost 95% of all metal used per year. Iron is another reactive metal that combines with oxygen readily to form oxides, the most common being known as rust.
The ore from which iron is extracted for later conversion into steel, typically contains oxides and carbonates of iron, but could also contain hydroxides of iron and may contain water molecules.
The use of iron during the Iron Age, along with bronze and other copper-based alloys, helped man to develop tools which aided his crucial primal activities, and in modern times it supported the development of industrial processes and machinery.
The first step in producing steel from iron ore is the smelting of iron ore into pig iron in a blast furnace, with coke that acts as the fuel source and reducing agent, and air that may be enriched with oxygen. The basic oxygen steelmaking process converts the molten iron from the blast furnace – with up to 30% steel scrap – into refined steel.
High purity oxygen is blown through the molten bath via a water cooled lance to lower carbon, silicon, manganese, and phosphorous content of the iron, while various fluxes are used to reduce the sulfur and phosphorous levels. The impurities and a small amount of oxidised iron are carried-off in the molten slag that floats on the surface of the hot metal.
Modern steelmaking plants depend fundamentally on the capacity of the industrial gas industry to provide high purity oxygen (>99.5%) in quantities of up to several hundred tonnes per day. Steel plants are often supplied by dedicated air separation plants, built under an over-the-fence or supply-scheme contract.
Aluminium for windows and doors
The durability, corrosion resistance and aesthetic appearance of extruded aluminium explain its popularity in modern building constructions.
In the first phase of aluminium production, an oxide of aluminium called alumina (Al2 O3), a fine white powder, is extracted from bauxite. In the second phase, alumina is reduced to aluminium metal by electrolysis.
While pure aluminium is a relatively soft metal, its properties are modified by alloying with elements like copper, zinc, magnesium, silicon, manganese and lithium. Small additions of chromium, titanium, zirconium, lead, bismuth and nickel are also made to provide specific mechanical properties required for structural applications.
Aluminium is extruded into hollow sections required for the manufacture of architectural components like window and door frames, bathroom accessories and ventilation louvres.
Nitrogen gas is used to cool the extrusion process, prevent wear, and enhance the surface appearance of the product. This is another important application of industrial gas in the manufacture of construction materials.
Glass for elegance and light and insulation
After hundreds of years of use as a decorative material, the twentieth century ushered in an era when glass is now recognised as a material for construction.
Glass is perceived as a warm, friendly material superior and more cost effective compared to concrete and steel in many applications. Glass cladding of buildings is not just design-oriented, but fulfils functional requirements of lighting, heat retention and energy saving.
Float glass manufacture was explored earlier in the December 2009 issue of gasworld magazine and also depends critically on continuous supplies of industrial gases.
Composite structures for endurance
The height limit of buildings constructed from load bearing masonry was first exceeded, by using a frame of steel girders that was riveted together and strong enough to carry the weight of the entire building.
Until the early 1970s the limited compressive strength of concrete made steel the only material suited to the construction of ever-taller structures. Today, however, many of the tallest skyscrapers are built almost entirely from reinforced high-strength concrete.
One structural attribute that engineers have come to understand during the past 30 years as being critical to effective seismic design is ductility: the ability of a structural member, or a connection between structural members, to bend in response to earthquake-induced forces while simultaneously continuing to support the loads it was designed to carry.
The ductility of concrete columns can be increased by including either horizontal or transverse steel reinforcing, as well as vertical steel. Lack of ductility in columns, beams, and connections has been blamed for the most serious damage to major buildings and transportation structures that occurred during recent major earthquakes.