It's a fairly common and very useful component of mathematical analysis, set-theoretical approaches to the real number line, etc. The most common version of algebraic treatment of ∞ as a "number" that can be negative or positive and to which algebraic properties are applied goes under the name "extended real numbers" or "extended real number line" or similar names. As an example:
(from appendix A.1 of Rana, I. K. (2002).
An Introduction to Measure and Integration (
Graduate studies in Mathematics Vol. 45) (2nd Ed.). American Mathematical Society.)
More compactly (from Beals, R. (2004).
Analysis: An Introduction. Cambridge University Press, p. 40).
Also keep in mind that almost all real numbers (in both the everyday sense of "almost all" and the measure-theoretic) require manipulations of infinite sequences or series by which almost all real number must necessarily be defined. This is because the set of rational numbers, while dense, has measure 0 and more importantly isn't complete (i.e., every irrational number is the limit of a sequence of rational numbers converging to that unique irrational number, which means that unless one embeds ℚ in ℝ one cannot even talk about 2*π). Since the set of irrational numbers is a greater/larger infinity than that of the rationals, most of any interval in ℝ is made up of irrational numbers. In fact, one can remove every single rational number from a given non-empty interval in ℝ and the measure (or length) of the interval would remain unchanged.
So we kind of half to deal with a variety of infinities and procedures that require infinitely many "steps" of one sort of another just to get to the point where we can reasonably define pi in order to attempt to define what the product 2*π means, if anything.