Activist hedge fund Corvex Management has built a stake in Clariant AG in a move designed to undo the Swiss chemicals maker’s $6.4 billion planned takeover of Huntsman Corp., according to people with knowledge of the situation. Corvex, which has built a stake in Clariant that exceeds the disclosure threshold of 3 percent, plans to push the company to explore alternatives to the deal including a potential sale, said the people, who asked not to be named because the situation is private. The activist investor believes the Huntsman acquisition, which was championed by Clariant Chief Executive Officer Hariolf Kottmann, lacks strategic rationale,...