Concrete — the essential material used by the world’s $3.8 trillion construction industry — accounts for 5 percent of the world’s man-made carbon dioxide emissions; each ton of cement emits about 800 kg (1,763 lb.) of CO2 during manufacture — and every year, some 3 billion tons of cement turn into nearly 30 billion tons of concrete, a British start-up has devised a new cement — based on magnesium silicates rather than limestone — that absorbs and stores CO2 when it is produced