BASF’s Construction Chemicals division has recently announced the European launch of its new ‘Crystal Speed Hardening’ (CSH) admixture technology in the tunnelling market, developed to achieve substantial increases in efficiency for concrete casting, both for precast segments and cast in situ linings.

The fast-curing concept has been designed to allow the removal of formwork at an earlier stage of the casting process to increase productivity commensurately, either by shortening the length of time that formwork or shuttering has to be kept in position or by reducing the number of sets of formwork required.

In addition to supporting efficiency, the system supports sustainability by addressing the key issues of energy demand reduction (lower carbon dioxide emissions), material optimization and high quality specifications. It does so by providing high early strength development whilst balancing the requirement for an optimum mix through the use of more binding material and reduced clinker content, says BASF.

The use of X-SEED

The CSH concept uses a new hardening accelerator – X-SEED – to boost early strength development by modifying the action mechanism of cement hydration in a unique way. X-SEED is made of suspended CSH-seeding crystals: the material added is of the same nature as the final hydration products from the cement reaction with water. As the suspended crystals are considerably smaller than microsilicas, the surface is much more beneficial for accelerating the crystal growth during the early stages of concrete hardening.

Many international and local companies already appreciate the added value provided by using X-SEED. One example of the application of X-SEED in the casting tunnel concrete lining is on the high-speed railway tunnel between Madrid, Bilbao and Donosti in the Basque Country of northern Spain. The results have demonstrated a 20% increase in productivity as well as better workability. But also investment plans in heat curing facilities for new lining segment plants are revised, in order to optimize capacity use based on mix designs containing X-SEED.

“The CSH concept, of which X-SEED is a key component, is compatible with BASF admixture technologies like the ‘Zero Energy System’ and ‘Smart Dynamic Concrete’. Although primarily developed for improving precast production efficiency, it also offers distinct advantages in cast in situ operations,” says Michael Kompatscher of BASF.

“The abbreviation ‘CSH’ also refers to calcium silicate hydrates; the cement hydration products responsible for the compressive strength of concrete. Hydration speed depends on the chemical composition of clinker and other cementitious materials. To simplify the science, by using BASF’s X-SEED admixtures, a liquid suspension of synthetic crystal seeds on the nano scale, the growth of calcium silicate hydrate crystals is significantly accelerated. We have demonstrated that the CSH concept works perfectly to boost the hydration process of early-age concrete (6-12h old) and within a wide range of temperatures.

“The CSH crystals grow quickly between cement grains and promote also improved properties in the final microstructure through natural hardening, without the typical drawbacks on durability of common acceleration methods.”

The cost implications of using X-SEED vary strongly by application, as does the generated added value. However, the elimination of heat curing, the optimization of the mix design for lower CO2 emissions and the acceleration of the production process bring considerable benefits that are attractive for any user, says the company.

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