How do silicates work?
Sodium, Potassium, Lithium and Colloidal Silicates react with CaOH to form insoluble Calcium Silicate Hydrate gel (CSH gel, Ca-SiO2) in the pores – some companies will refer to this as a hydrogel.
The additional CSH gel formed by applying a Sodium, Potassium, Lithium or Colloidal Silicate will improve a concrete slab in several ways.
The pros and cons of water
Concrete is made with the optimal amount of water – not too much or too little.
So, concrete mixes are made with an exact amount of water, BUT, if some of this the water can evaporate out of the poured slab (or prefabricated vertical) before it reacts with the cement to create bonds, this has multiple negative effects on the concrete:
- Where water evaporates out of the concrete it leaves air pockets, creating weaker concrete while increasing porosity, capillary suction and water ingress.
- Some cement is left unreacted (too little water) – this means fewer bonds, weaker concrete and an increase in efflorescence (calcium carbonate leaching out of the concrete) when water moves through the capillaries.
- When water evaporates out of the concrete it also causes shrinkage, leading to spider cracks.
- Evaporation increases closer to the surface, leaving the surface of the concrete particularly weak. Also, this means that the surface of the concrete will shrink at a higher rate and spall (pull away from the concrete underneath in pieces) as well as spider cracking. If you knock this concrete it sounds hollow.
- Egress or water or water vapour (evaporation) can damage floor finishes and lift floor coatings. This means an untreated slab is usually left to cure fully for 60 days before floor finishes are applied, this is too much time (and money) for most building projects.
Benefits of treating a fresh concrete slab with a suitable silicate
Slabs and other concrete structures vary greatly in density, porosity, and water content (depending on how recently the slab was poured). Silicates range in molecular size, structure and penetrating ability, and the correct silicate should be matched to the project. You would not treat a 25MPA slab with the same silicate as 80MPA density structural concrete.
The correct silicate is applied to the exposed surface of a concrete slab and penetrates the pores from a few millimetres to a couple of centimetres (depending on the type of silicate, the concrete density, the amount of water in the forming capillaries and the amount of silicate applied).
- Silicate treatment as a curing compound for concrete slabs.
When treated with a Sodium, Potassium or Colloidal Silicate, CSH gel forms in the pores of the impregnated (exposed) surfaces of a concrete slab, substantially reducing water and water vapour permeability.
This greatly reduces the loss of water by evaporation, so the moisture remains in the slab and continues to facilitate the ongoing reaction and curing of cement in the concrete. The more of the cement which is reacted and forms hydrates and then fixed bonds in the material, the stronger and less porous the concrete will be, making it more durable.
- Silicate treatment as a membrane to allow immediate installation of floor coatings and other finishes on “green” (freshly poured) slabs.
Floor slabs usually need to be cured to 60 days before finished flooring, such as tiling, wood, carpeting or floor coatings are applied.
This is to prevent these finishes from being damaged, becoming mould infested or lifting due to water or water vapour coming out of the concrete slab.
Correct treatment with a suitable silicate will limit evaporation or water egress through the top of the slap sufficiently to enable floor finishes and coatings to be applied as soon as the silicate has cured, rather than having to wait until the concrete is completely cured.
Silicates form more of the same structures already present in concrete and so are compatible with most floor coatings, adhesives and finishes.
- Silicate treatment to densify cured concrete to facilitate diamond polishing.
Polished concrete is one of the toughest, most durable, practical and aesthetically pleasing floor finishes. A raw slab is ground with rough diamond pads to flatten it completely and then with progressively finer diamond pads until the suitable honed (matte) or polished finish is reached.
This requires treatment with suitable sodium, potassium, lithium or colloidal silicate densifier. As explained previously, silicates react inside the concrete to form extra silicate gels in the pores. Think of it as glass structures forming in the pores. This reduces the porosity and stain resistance of the concrete and allows it to be honed or polished to an aesthetically appealing gloss level.
Many of the premier concrete polishing companies will apply silicates of different types 2 or 3 times to facilitate and dense floor with a beautiful finish. They may treat the concrete first with larger molecular size sodium or potassium silicate when it is still fairly rough and the pores have been nicely opening by the initial few grades of grinding.
Then as finer diamond pads are used to achieve a smoother and smoother surface, a lithium or suitable small particle colloidal silicate, which can penetrate more easily into the previously treated concrete and fill more of the nano-sized pores.
- Silicate treatment of engineered concrete structures to reduce salt spalling and rebar corrosion from saltwater ingress, freeze-thaw action and carbonation.
High MPA concrete can be treated with a lithium or small molecular size colloidal silicate (4-8nm).
A small test area should be treated and a small core sample (3cm diameter, 3cm depth) drilled from the centre of the sample area and tested to check if there is sufficient depth of penetration before proceeding with the silicate treatment.
- Silicates for concrete crack repair.
Most concrete crack injection systems for concrete use an epoxy or urethane polymer, but there is an argument that because these differ in their physical properties so much from concrete, that this can cause them to degrade over time.
Colloidal silica, forms the same hydrogel compound found in cured concrete and has the same physical properties, including reaction to changes in temperature and moisture and will move with the concrete. A colloidal silicate of small size will also penetrate the walls of the crack, anchoring the filling very securely.
Colloidal silicate crack fillers are 2 part – the colloidal silica and a catalyst. Colloidal silicates cure particularly quickly but this can be moderated to some extent by adding more or less catalyst. Micromesh sand and some cement can also be added for filling large cracks that require some solid aggregate to help fill the gap.