Activated clay to make its mark on cement production

Clay Technology magazine
15 May 2020

A cement plant in Cameroon is set to produce calcined activated clay with the aim to save around 110,000 tonnes of carbon dioxide emissions a year. Shardell Joseph reports.

German-based multinational company, thyssenkrupp, claims to be fitting the first cement plant with a system to produce calcined activated clay on an industrial scale. Building the plant in Kribi, Cameroon, the company believes it will reduce emissions in cement production by 40% when it comes online in Autumn 2021. 

Termed as polysius activated clay (PAC), the material will replace one third of the cement clinker, which thysenkrupp explains emits around 790kg of process-related carbon dioxide (CO₂) per tonne. Two-thirds of these emissions are due to limestone use and its chemical reaction during production. The limestone is also heated with other aggregates at more than 1,400°C. 

By producing PAC, the process requires far lower temperatures of 800°C, and the changed chemical composition of the thermally-activated clay results in around 110,000 tonnes less CO₂ emitted during production. ‘The sustainability challenge is to develop green technologies that contribute to both environment and business performance,’ says Dr Luc Rudowski, thyssenkrupp’s Cement Technologies Business Unit Head of Product Management and Innovation. 

‘The stronger the business case – i.e. reduction of cement production costs – the faster activated clay development will contribute to reduce the carbon footprint of the cement industry.’ 

According to Rudowski, the firm’s engineers have tackled two issues that previously restricted industrial use of activated clay. Firstly, they have confirmed that ‘the calciner technology works to activate various types of clay at the proper temperature with solid fuel firing. 
‘Secondly, we have succeeded in removing the unpopular terracotta colouring that cements with calcined clay have suffered from in the past. Even with some difficult raw material components – e.g. iron oxide – our customers can achieve a grey product, if desired,’ 
says Rudowski. 

‘With regard to the desired effect, the term “clay activation” is preferred to “clay calcination”, as a certain temperature window must be reached in the process. In this window, a meta-phase of kaolin can form as an active supplementary cementitious material, so that [it] will make a positive contribution to the final cementitious product. 
‘While a minimum temperature is required for the reaction, a prolonged excessive temperature will again deactivate and burn the clay material,’ Rudowski adds. 

Thyssenkrupp believes that if there is increased adoption of the technology from cement manufacturers in the future, the reduction in CO₂ emissions could be huge. ‘For example, if 50% of today’s conventional cement production [is] converted to ternary cement using PAC, this would lead to a global CO₂ reduction of roughly 0.6 billion tonnes,’ Rudowski claims. ‘This corresponds to 1% of today’s global CO₂ emissions. The introduction of new cement norms for ternary cement is of essence.’