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.