Isocyanate-free foamable mixtures suitable for filling construction voids and for other uses, comprise isocyanate-free, alkoxysilane-terminated prepolymers (A) which have a silane end group of the formula [1], where X and Y are each an oxygen atom, an N—R3 group or a sulfur atom, R1 is an alkyl, cycloalkyl, alkenyl or aryl radical having 1-10 carbon atoms, R2 is an alkyl radical having 1-2 carbon atoms or an ω-oxyalkylalkyl radical having a total of 2-10 carbon atoms, R3 is a hydrogen atom, an alkyl, alkenyl or aryl radical having 1-10 carbon atoms or a —CH2—SiR1z(OR2)3-z group, and z is 0 or 1, with the proviso that at least one of the two groups X and Y is an NH function, and (B) blowing agents.