It entirely depends on what you mean by 'zero'. Usually in algebra the zero element of some set, with some 'addition' operation has the property that a+0=a where a can be any element of your set. That is, we use addition to define what zero is. If your set of things (affectionately denoted G) also has a 'multiplication' operation, and that operation behaves like multiplication on R then it can be shown that a*0=0 for all a in G. Since 0 is also in G it follows that 0*0=0.