Billionaire-backed ArcelorMittal South Africa’s (AMSA’s) plans to shut down a century-old steel mill in South Africa are holding President Cyril Ramaphosa’s dream of fostering a $257-billion infrastructure boom to ransom. That plant — and two others it wants to idle — supply what builders need for the vaunted exponential rollout of power-transmission towers, rail lines and new roads — projects that the leader says will turn the country into “a construction site,” boost sluggish growth and provide much-needed employment.