Concrete. Coloured environmental scanning electron micrograph (ESEM) of gypsum crystals (brown) that formed in setting concrete (blue). Concrete consists of cement, water and aggregates such as sand and gravel. Adding water causes the cement to harden around the aggregates. Unlike a convention- al scanning electron microscope (SEM), samples put into an environmental SEM do not need to be desic- cated or coated in gold. Magnification unknown.