The invention provides an aqueous suspension/concentrate of a sparingly soluble borate, preferably as disclosed in Mondshine U.S. Pat. No. 4,619,776, which is effective in crosslinking aqueous solutions of a water soluble, borate-crosslinkable polymer, preferably guar gum and derivatives thereof. The aqueous suspension/concentrate comprises a saturated aqueous solution of a sparingly soluble borate, a suspension agent, suspended particles of a sparingly soluble borate, and optionally a stabilizing agent, wherein the saturated solution of the sparingly soluble borate is prepared prior to the particles of the sparingly soluble borate being suspended therein. The preferred suspension agent is a palygorskite clay such as attapulgite and sepiolite. The invention further provides a method of hydraulically fracturing a formation with the aqueous borate suspension/concentrate.