“Across Sahelian Africa, there are thousands of villages like Koumi—and in the dozens I’ve visited in several countries, concrete is ascendant. Architects, officials, and villagers confirm the trend: People are discarding traditional materials, mostly mud, in favor of concrete, as soon as they can afford it. As living standards increase making concrete more accessible, some of the world’s hottest, poorest landscapes are rapidly morphing from brown to cinder block grey.”