Cryogenic treatment for hard substances
Above a certain temperature, concrete becomes more difficult to process. To prevent this, concrete and its additives can be cooled by means of cryogenic gases. Messer supplies the construction industry with the necessary equipment and know-how.
Concrete is undisputedly the principal building material worldwide. According to the German Association of Cement Works (VDZ), 27 million tons of cement were processed into roughly 90 million cubic metres of concrete last year in Germany alone. This quantity is enough to build 225 cathedrals each as large as the Dom in Cologne. Concrete is capable of bearing high loads, permits flexible processing, remains stable for an eternity, and is available in many different variants.
By modifying its formulation, concrete can be tailored to meet diverse requirements posed by the construction of underground structures, roads, and buildings. Whether the requirement is to cast a storey ceiling or tunnel wall, prefabricate load-bearing elements for a market hall or produce watertight pipe sections for public sewage systems: concrete can be brought into every conceivable shape and processed, in conjunction with adequate reinforcement such as steel, to produce strong foundations, dams or bridge piers. As a prerequisite, however, no faults must occur at any stage of processing. Fresh concrete should ideally be processed at between 5 °C and 25 °C, as temperatures outside this range spoil the quality of the concrete. Based directly on the chemistry of cement, this aspect is explained by considering the relevant details. Concrete has three essential ingredients: (1) aggregate such as sand, gravel, crushed stone or chippings, (2) water and (3) cement. Cement is a mixture of limestone, clay, sand and iron ore burnt at 1450 °C and subsequently ground. In its dry state, grey cement powder is completely non-reactive. When mixed with water, however, cement forms a kind of glue which gradually hardens or “sets”, as a construction engineer would say.
THE RIGHT TEMPERATURE RANGE
Cement sets in a chemical reaction with water (hydration) accompanied by a release of heat energy (hydration energy). In this process, the cement’s ingredients form mainly stable, needle-shaped crystals which gradually grow and mesh together. The sand, gravel and reinforcement steel, i.e. all basic materials intended to increase concrete’s durability and strength, are bonded together firmly as a result.
For hydration to yield satisfactory results, it is necessary to set certain basic conditions such as a temperature of at least 5 °C during processing of fresh concrete; below this temperature, the cement loses some or all of its bonding capability. Measures to ensure that the building material hardens properly, especially in winter, include pre-heating the additives or using thermally-insulated formwork. Whereas the tempering effect of hydration is beneficial in winter, it can pose problems during the warm summer season. At high temperatures, say above 30 °C, the concrete’s additives start to lose their liquefaction capability, thus spoiling fresh concrete’s fluidity and processability. Moreover, the heat generated by hydration proves significant especially in the case of bulky components. Thermal expansion of concrete can cause strain, resulting in cracks extending deep into the concrete core. Air and moisture can pervade these cracks and attack the concrete as well as its enclosed reinforcements.
THE REQUIREMENTS DEFINE THE COOLING TECHNIQUE
To maintain fresh concrete’s temperature in summer within the optimal processing range of 5 °C to 25 °C, Messer offers efficient solutions incorporating cryogenic gases: liquid nitrogen (LN2) or liquid carbon dioxide (LCO2).
In numerous experiment series, the company’s specialists investigated a variety of now proven techniques for cooling concrete and its additives.
To cool small and medium quantities of concrete by a few degrees, for instance, LN2 is introduced via lances on the truck mixer into the building material. Termed lance cooling, this rapid process can be executed directly at the construction site.
A much more efficient method, however, especially at high ambient temperatures, is to instead cool the cement (we have named this method cryoment). Two variants are available here: firstly, the cement’s temperature can be adjusted to the required level directly during storage in the local silo (we have named this method cryoment-flow). Secondly, it is possible to cool the cement just in time, i.e. immediately before processing. Advantage: no need for storing cold cement, so that cooling losses are lowered at all events.
Which of these methods is more suitable for a particular application depends significantly on the quantity of concrete requiring cooling as well as the anticipated cooling period and rate. The necessary fine adjustments are determined in meetings between Messer’s experts and the customer.