Hydrostatic pressure may make any topping fail. You may want to call the manufactuer of the product. Two companies that make stuff for this are Ardex and Custom Building Products. There are others. Check their websites, read the spec sheets, and then call and talk to a tech to find the best product. Some of them require a primer first (a bonding agent), and that usually requires the area to be dry. Depending on the weather and ground conditions, you may only have a narrow window of opportunity to do this.