Sealing the back and sides of stone
For years the use of sealers has been expressly for the protection of the stone or tile surface. Most, if not all, adhesive companies warranty their adhesives only if the back of the stone is free from any sealer as sealers are seen as “bond breakers” adversely affecting the integrity of the adhesives ability to bond to the stone or tile surface. However this situation has meant that many of the problems created by water absorption through the back and the sides of stone have gone unresolved. New and current technology now offers sealers that can successfully be applied to the bonded surfaces of stone without becoming bond breakers. 
To look at these and how they work I firstly want to investigate the problems and issues relating to porous stone and specifically water absorption through the sides and back.
Picture framing, efflorescence, soluble mineral contamination (for example iron sulphides such as Pyrite) and prolonged water marking are some of the problems created when water is absorbed by the back and sides of some types of stone. The mechanics are as follows. When a stone is installed over a concrete substrate the concrete can contain potential soluble contaminants such as salts and other minerals. The underlying cement based screed or topping as well as the cement based adhesive and grout also have the potential to hold some of these contaminants. In most cases the contaminants will not react unless there is water present. Water is both the catalyst as well as the transport mechanism. The initial and most important source of water that triggers much of the reaction originates from the hydrating adhesive or mortar bed that is even more aggressive due to its high ph. With water the soluble minerals travel to the surface by way of evaporation and capillary action working their way through the stone and grout. In many cases the grout is more porous having higher water absorption than the stone creating an easier exit for the evaporating soluble minerals. This explains why in many cases the resulting stains are revealed as picture framing or at least concentrated around the sides of the stone and grout joint. Once on or near the surface the contaminants further react with the increasing rate of oxygen and ambient air temperature to form various compounds or simply evaporate or dissolve only partially leaving behind the unwanted stain or compound.
A good example of this mechanism at work is the soluble pyrite found in the granite Panha Fragola Corcovado (PF) as used on the just completed Dubai airport extension.
The water from the thick mortar bed under the stone once absorbed into the stone body easily reacted with the soluble pyrite to form highly visible iron blooms. In some cases the iron sulphide would turn the complete stone a light shade of yellow. As the PF was the selected stone for the entire job and had already been purchased it was imperative to find a solution to this problem. The answer was simple – if the stone’s natural water absorption could be reduced close to zero then the risk of iron contamination would be similarly reduced. The best and most cost effective way to reduce the stones water absorption was to seal the stone on all six sides. This is what was done and the result was better than expected. The first stone was sealed nearly three years ago with the airport opening just last month and in this time there has been no iron contamination or for that matter adhesion issues due to the sealing of the bonded side of the stone.
We have all known for many years that the trick to managing many of the water related problems of soluble mineral staining such as the pyrite in PF is to lower the stones natural water absorption by sealing it on all six sides while still maintaining good vapour transmission. (The ability for the sealer to breath is very important as any trapped water can create other issues such as surface delamination by way of excessive moisture expansion. However the formation of the contaminants is not only due to the presence of water but also the quantity of water and rate of evaporation. If the amount of water is reduced and the rate of evaporation high enough that the water does not condense then the soluble minerals will also exhaust through the surface rather than solidify). The problem however was that most sealers either did a poor job of repelling water in a high alkaline environment as that found at the interface between stone and cement mortar, or reduced the bond strength of the adhesive system. The latter is the reason why most adhesive manufacturers only warranted their adhesives when applied to clean unsealed stone. This claim in turn made clients reject any sealer solution to the problem as well as making sealer manufacturers uninterested in developing specific sealer technology. However as stone’s use increased globally so did the problems related to the high water absorption and chemistry of certain stones. All of this at last led to the development of sealers that could in fact both reduce a stone’s water absorption as well as maintaining the adhesive’s bond.
Aqua Mix® has designed the markets first sealer specifically for application to the back and sides of stone called Aqua Mix® ProBlock. It is designed to hit the main market requirements for such a product – low cost per m2 (or sq ft), highly water repellent, good vapour transmission and of course not being a bond breaker for the adhesive. However Aqua Mix® also designed in several other important features. Aqua Mix® realised that the best way to apply sealers to the sides and back in a cost effective manner was to dip the entire stone (as done on the Dubai airport project). However this presented a problem in that many clients wanted a low cost sealer for the back and sides (Aqua Mix® ProBlock is designed to fill this role) but wanted a premium product for the actual surface, which would be exposed to long-term dirt and contamination. Therefore any specialized back applied sealer had not only to be compatible with a premium sealer but also needed to allow the premium sealer to penetrate it so the correct quantity of premium sealer could be applied to the surface. Aqua Mix® ProBlock is designed to satisfy both these conditions allowing the secondary application of any of the Aqua Mix® penetrating sealer family (excluding Aqua Mix® Enrich’N’Seal™ & Aqua Mix® Stone Enhancer) directly over the applied Aqua Mix® ProBlock. The sub surface sealers will penetrate right through the Aqua Mix® ProBlock enabling the complete and correct quantity of premium sealer to be applied guaranteeing the long term performance of the final sealer.
Aqua Mix® has also engineered its other economical sealer, Aqua Mix® Penetrating Sealer, for application to all six sides. This enables the client to use only one product instead of two if they do not require a premium sealer to be used on the surface. Both of these sealers are tested regarding shear bond to ensure they do not act as bond breakers.
The contemporary existence of sealers that can be applied to the sides and back of a stone or tile now help to manage and greatly reduce the risk of the long endured problems created by moisture moving through the stone especially during installation and the process of final cure. The argument by both clients and adhesive manufacturers to not seal all sides due to the possibility of the sealer being a bond breaker is no longer valid now the technology exits to do so. Many thousands of metres of stone installed globally over nearly a 10 year period with Aqua Mix® products successfully managing these problems is testimony to the technologies performance.
Using a suitable sealer on all six sides is part of a total water management system that should be implemented to fully control the uses around leaching of soluble minerals. These include for example the use, where appropriate, of waterproof membranes, epoxy remediation systems, proper falls, factory prepared adhesives and grouts etc.