Concrete can be made freeze/thaw resistant using one or more of several techniques. Waterproofing can be accomplished through the use of additives know as integral crystalline waterproofers, such as Penetron or Xypex. These products can be mixed as an additive to the original concrete mix or applied as a coating that penetrates concrete. They work by reacting with infiltrating water to form silicate crystals that grow into the pores of the concrete and block water penetration.
My personal experience has been with using three specific additions to the concrete mix: Polyvinyl alcohol (PVA) fibers, metakaolin, and acrylic emulsion admix. PVA fiber is available from concrete countertop suppliers. It greatly strengthens concrete and greatly reduces shrinkage cracking, which is one of the sources of water penetration that causes failure in freeze/thaw environments. Metakaolin is a powdered additive that is classed as a pozzolan, a substance that, by itself, is not a cement, but reacts with cement products to create more cementitious product. Other pozzolans include fly ash and silica fume, but metakaolin is more effective. When added to the concrete mix as a 10 to 20 percent cement replacement, it greatly increases concrete's compressive and flexural strength, fills spaces between cement particles, leading to decreased porosity, and, by reacting with the calcium hydroxide product of curing, reduces the alkalinity of concrete and eliminates efflorescence. The decreased alkalinity and efflorescence is important to maintaining desirable pond chemistry. Acrylic emulsion, also sold as acrylic bonding and curing agent or as acrylic fortifier, is sold under many brand names, such as Thoro's Acryl 60, Laticrete, Dayton-Superior J40, or Quikrete Acrylic Fortifier. I buy a brand that is 50% solids and dilute it in the ratio of 3 parts water to 1 part acrylic. This is the liquid that is used, then, to make the concrete mix. The acrylic additive increases concrete strength, acts as a curing agent to reduce shrinkage cracking, speeds the attainment of cure strength, and reduces the porosity of the concrete, creating a cured concrete that stands up well to the stresses of freeze/thaw environments. I also recommend sealing the concrete with a penetrating solvent-based sealing compound. Look for a sealer that contains xylene (xylol) as the solvent. You may have to settle for a less-desirable water-based product, as some states have banned the solvent-based sealers for environmental concerns.
With the use of additives like metakaolin and PVA fiber in the dry mix, the use of acrylic in the liquid part of the mix design, and sealing the finished product, the concrete should stand up to even the harshest of freeze/thaw conditions.
