ZURICH (Reuters) - U.S. environmental groups have stepped up a campaign to get cement maker Holcim to curb its carbon footprint, writing to its chairman and potential investors in its North American business before it is spun off next year.

Swiss sustainability-focused proxy adviser Actares, which represents individual shareholders in Holcim, has backed the Concrete Change campaign organised by U.S. nonprofit Industrious Labs.

Cement, considered a sector where reducing carbon emissions is difficult, is responsible for around 6% of global CO2 emissions. That has put cement producers firmly in the sights of environmentalists.

"Cement and concrete create nearly 8% of global emissions. That must change," Concrete Change wrote in a letter sent on Thursday to chairman Jan Jenisch.

Among its proposals, the campaign group wants the Switzerland-based company to phase out its use of energy-intensive wet kilns to make cement and to establish at least one net zero cement plant in North America by 2030.

Holcim declined to comment on the letter, which it has not yet received. The company plans to spin off 100% of its North American business in the first half of 2025, with a potential valuation of $30 billion.

Thursday's letter, seen by Reuters, follows criticism of Holcim from Concrete Change last month.

Signatories to the campaign, which include Michigan Climate Action Network, Clean Water Action from Baltimore and the Potomac Riverkeeper Network from Washington, said the spinoff was a good opportunity for Holcim to change its approach.

Holcim, which has been investing recently to improve its emissions profile, has previously defended its record.

It is currently establishing three projects in North America to capture 5 million tons of CO2 per year, which will come online from 2028.

Last month it took a stake in Sublime Systems, a U.S. tech start-up working on low carbon cement.