No, they were not. It has become evident from recent digs that NONE of them were slaves, that it was, in fact, the ancient Egyptian equivalent of Roosevelt's CCC, a means of keeping agricultural workers occupied before the planting season, as well as a means for common people to participate in a state religion and gain their own merit in the afterlife.
The principal objection to slavery as an institution in the modern world (outside of a moral one) is that slavery can only succeed in support of an activity that is human-labor-intensive to the point where it is infeasible to automate it. In the antebellum South this was cotton agriculture before Eli Whitney upset that applecart, one reason for the persistent claim that slavery was a "dying institution" in that area - in fact, although it was on the way out for that industry, it had found another agricultural activity that was at the time impossible to automate, tobacco farming, and transferred the bulk of labor effort there. But it was living on borrowed time.
The main survival of slavery in the modern world is in the only industry yet impossible to automate - sex. That aside, there really isn't much in the way of economic activity that it is cheaper to do with captive human labor than with machines. There are exceptions. In the Soviet Union slave labor was feasible because making slaves was free for the state, and their survival was hence not real high on the state's priority list (less than breakeven, in fact, as for political reasons the state actually preferred many of them dead). This is a different and artificial form of "slavery." One form of what might be so described is still taking place in the form of prison labor, but special conditions are necessary to maintain this rather artificial arrangement - the state must sanction and enforce the prisoners' status and discourage free outside competition in the subject industries.
Aside from state coercion there was one additional motivation for slave labor in the ancient world which may persist in very poor areas today - it was sometimes the only way for a family to get their children fed in the presence of famine and the absence of any other form of social safety net. (Some libertarian theorists need to be reminded of this point on occasion when concocting social systems that function best in times of plenty.)
In summary, slavery doesn't work these days because (1) it can't make a decent buck, and (2) nobody wants to be one. If those change, we can talk.