Gravar-mail: Crystal structures of catena-poly[[μ-aqua-di­aqua(μ(3)-2-methyl­propano­ato-κ(4) O:O,O′:O′)calcium] 2-methyl­propano­ate dihydrate], catena-poly[[μ-aqua-di­aqua­(μ(3)-2-methyl­propano­ato-κ(4) O:O,O′:O′)strontium] 2-methyl­propano­ate dihydrate] and catena-poly[[μ-aqua-di­aqua­(μ(3)-2-methyl­propano­ato-κ(4) O:O,O′:O′)(calcium/strontium)] 2-methyl­propano­ate dihydrate]