Boosting concrete compatibility

With the Paris Climate Agreement, adopted in December 2015 and ratified by almost 200 countries to date, many of the world’s nations committed themselves under international law to limiting global warming to 1.5 °C above pre-industrial levels, and to continuously reducing national greenhouse gas emissions.

The long-term goal is to achieve climate neutrality in the second half of this century. In Germany, according to a resolution passed by the federal government, the date set for this objective is actually 2045. But this demands a lot from industry, especially since the underlying conditions have become much more difficult in the past three years in the wake of the global coronavirus pandemic and, not least, due to escalating price increases following Russia's attack on Ukraine, an act that contravenes international law.

Ways to cut CO₂ emissions

Not all areas of industry are equally challenged. It is no secret that companies that cause particularly high CO₂ emissions due to their production activities are facing an especially difficult task. Included in this group are cement manufacturers, because the calcination process required to produce Portland cement clinker is still responsible for a large portion of global CO₂ emissions, not least because the manufacturing process is particularly energy-intensive.

Even if the process could be based entirely on renewable energies, the deacidification of the limestone during the calcination process still releases a significant amount of CO₂. One possible partial solution lies in the use of clinker-reduced cements, which have been entering the market in ever-increasing volumes for some time now. These do indeed have a significantly lower CO₂ footprint – but they also change the properties of the concrete. But – as we reported in MC aktiv 3/2021 – the use of special concrete admixtures goes a long way to compensating for such effects.

Challenges facing precast concrete producers

For precast concrete producers, the use of clinker-reduced cements in their concrete formulations poses certain specific challenges: The process of early strength development is retarded, thus lengthening the demoulding or stripping time. This longer occupancy of the formwork means lower output of precast concrete goods and components, which in turn leads to a reduction in yield and profitability. But concrete formulations with CEM I cements also pose further challenges for precast manufacturers against the background of rapidly rising raw material and energy costs. The early strength of precast concrete goods is often accelerated by heating the concrete, even when CEM I cements are used, e.g. with heated moulds, fan heaters or heating chambers. However, this requires a large amount of thermal energy – an enormous cost driver given the current dramatic increase in energy prices. Moreover, price increases of around 40 to 50 % have already been announced for cement as of the beginning of 2023. Conventional manufacturing processes are thus becoming so cost-intensive that any scope for optimising production must be worth examining for any company wishing to remain competitive in this field.

Cost-cutting rethink required

“The biggest challenges occurring in the precast market at the moment relate to the ever-rising cost of cements and GGBS (Ground Granulated Blast-furnace Slag) fillers. Both products have increased in price by approximately 50 % in Ireland in the course of 2022,” notes Steve McCormack, Managing Director of MC-Building Chemicals Ireland. “Energy costs have also risen dramatically, and burgeoning fuel prices have in turn caused transport costs to rise very sharply.” So, in order to prevent prices from exploding out of control, new ways of cutting costs in production are needed – including the use of modern concrete admixtures from MC-Bauchemie such as those of the MC-FastKick product family. This new generation of hardening accelerators not only ensures shorter curing times in winter construction, but also greatly reduced cycle times in the precast plant.

Faster form stripping with MC-FastKick hardening accelerator

With MC-FastKick, MC-Bauchemie has developed a completely new product family of hardening accelerators in which CS phases are stabilised with polycarboxylate ethers. This idea, based on a patent from MBCC, has been refined with the latest MC polymer technology, ensuring effective initial slump retention and accelerated hardening of the concrete as well as raising the concrete’s performance up a class.

Hardening accelerators such as the MC-FastKick products enable high early strengths by inducing a mechanism that considerably accelerates the curing process, while also imparting to the concrete very good workability properties. This is also confirmed by Martin Struk, concrete expert in the Concrete Industry (CI) division of MC-Slovakia: “We are currently using MC-FastKick with four customers and consider it to be the best hardening accelerator around. Only with MC-FastKick can we achieve a strength level of 30 N/mm² after 7.5 hours. We also get a better stripping result and much nicer concrete surfaces.”

The special PCE polymers that MC has developed for MC-FastKick hardening accelerator exert a positive effect on the crystal growth of the calcium silicate hydrate phases in the concrete. This leads to a significant higher in the crystallisation rate and thus to substantially higher early strengths without negatively influencing the other concrete properties. MC-FastKick is thus not only suitable for use in precast plants, but also wherever curing times need to be shortened.

Fast curing for CEM I, II and III with hardening accelerators

The acceleration effect is largely dependent on the reactivity of the cement type used. Vicente Diago, CI concrete technologist at MC-Bauchemie Spain, comments: “Cement manufacturers are trying to switch from type I cements to more environmentally friendly cements, which leads to longer formwork stripping times due to the lower initial strengths. Keen to compensate for this negative effect, concrete manufacturers are showing great interest in our MC-FastKick hardening accelerators.” The period of maximum acceleration effect is in the range of six to twelve hours for CEM I, roughly eight to 16 hours for CEM II and approximately ten to 20 hours for CEM III.

Numerous benefits for precast concrete producers with MC-FastKick hardening accelerator

Precast plants that use their formwork two to three times, i.e. produce around the clock, either heat their formwork or use heating chambers to accelerate curing. Using MC-FastKick hardening accelerators means heating temperatures – and therefore energy costs – can be reduced. These admixtures also offer another positive side effect: The PCE technology incorporated in the formulation gives rise to a tangible improvement in liquefaction and improved slump without affecting the stiffening behaviour. This means that the amount of superplasticiser or the water content can be reduced, in turn helping to further cut costs and protect the environment.

MC-FastKick hardening accelerators thus enable concrete producers from both the precast and ready-mix concrete industries to produce faster and more flexibly, with output increasing and production costs decreasing. Last but not least, these manufacturers are immediately put in a position of being able to better meet their CO₂ reduction targets. A real win-win situation for manufacturers, consumers and – not least – the environment.