Resin repair product? Cement repair product? Which to use?

Grout, mortar, filler, render, plaster – all these can be based on resins or cement.  In order to know when and what to use, you need an appreciation of the different properties of the materials.  Their weaknesses, strengths and limitations.

The following is by no means a detailed guide, it’s a basic comparison of the merits of both. When I deal with clients, Contractors, Surveyors and Architects, I sometimes find that these materials are miss-specified, usually due to a lack of appreciation of the fundamental differences between these amazing products.

Resin.

Very strong and hard wearing, water and vapour proof, can be fast setting and when mixed with a filler, lightweight too. Is not harmed by frost, even when just mixed. Resistant to a wide range of acids and alkalines. Does not shrink on curing.

Can be softened by temperatures as low as 40 degrees C. harmful when not cured. Expensive. Waste needs careful disposal.  Epoxy needs to be mixed in exactly the correct proportions.

Cement

Inexpensive, easy to use, forgiving, flexible range of uses. resists heat and cold when cured. Vapour permeable, can be made waterproof, can have good insulation properties when mixed with suitable filler.

Can be damaged by heat or frost when curing, shrinks as it cures, dust is harmful, it is highly alkaline when mixed; causes burns and dermatitis, cannot resist acids,

Which one then?

So how do these properties relate to us? Basically, cement based grouts and mortars are usually used where the chemical resistance or high strength of resin is not required.  Resins are expensive, so using resin in a standard situation is uneconomic.

Perhaps the best way is to do a simple test; which is, assume in the first instance that a cement based repair product will be used.  Then asses the risk of that.  If there are unacceptable consequences of using cement, then consider resin.

However, once resin is being considered, you then need to consider the potential issues and set any negative ones alongside the ones which were raised by cement – then make a choice.

Remember – resin is almost indestructible, once applied. So I’d never use it for stone repairs, listed building facades and such.  If you do, you’ll probably get into trouble with your conservation officer and, in a decade or two, the surrounding masonry will have eroded and aged, leaving the resin repair, untouched and standing out like a sore thumb.  In fact, cement is usually too hard and impervious too, so in these cases, natural lime products come into play (I will provide a separate article on these later).

Epoxy need exactly the right quantities of resin and catalyst (usually filler too), get these ratios wrong and your resin may remain ‘sticky’ forever –not so good, if it’s a showroom floor you’re working on.

If you want to grout in fixings or bolts, resin is king.  It can be thixotropic and will flow into cracks and holes and then stay put. In these situations cement based grouts sometimes dry out too quickly, before curing is complete; losing strength and shrinking, particularly in very dry or porous substrates.

Epoxy resin is an adhesive (Polyester is not), so when injected into cracks and such it not only fills, but bonds too. It likes water (Polyester does not), so can be used as a very powerful bonding agent; apply epoxy to prepared concrete or masonry and if rendered over with cement, whilst it is still tacky, it will fuse the render to the substrate – probably forever. This is sometimes used for ‘tanking’ basements.

Useful resources for resin product information are Fosroc and Weber sbd, both are excellent manufacturers with great technical help to back up any projects.

Dry Rot.