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Glass blocks specialist mortar – Colmef Vetromix 

Colmef Vetromix is a specifically designed and formulated mortar for glass block construction. Using a prepared powdered mortar means that all the raw materials are added in a controlled factory environment. This ensures accuracy and consistency of performance. It can be used for all applications, internally, externally, straight, curved and fire-rated glass block walling. 

Premix mortars have a better thermal efficiency value than sand and cement, so U-values for glass block walling will be constant.

Colmef Vetromix has a fine texture, low slump and the whitest mortar available. It is used as bedding and pointing mix, therefore there are no bonding issues between building and grouting.

Traditionally, like brickwork, glass block joints were pointed up as the courses were laid. However Colmef Vetromix is designed so that a complete panel can be constructed raked back and then grouted in using just slightly diluted Vetromix with a squeegee. . Panel construction is quicker and more efficient and time can be spent ensuring a first-class finish is achieved for the joints on both elevations. Working with Colmef Vetromix

Before construction, a vital rule is to ensure that the surrounding temperature is not 5OC and falling or 30OC and rising. Also ensure the joint width does not exceed 22mm. 

Colmef Vetromix should be paddle-mixed to a consistency of slump 1 or less, worked in and compacted. This will ensure that the oval profile is totally filled in both vertical and horizontal directions as a bedding mortar. For grouting, a slightly diluted mix ensures that any voids are totally filled.

Mixing instructions are on the reverse of each bag. Always ensure that water and bucket are clean and use a paddle drill. Cold water is best as hot water may cause the mortar to set quicker and will reduce open time.

Other mortars may not be as stable, as cavities or voids may form within the joint structure and may contain large aggregate or sand particles that will scratch the glass.

The joint finish

Colmef Vetromix is a pure shade of white and a smooth consistency. The powder contains hydraulic binders and inert and synthetic resins. 

Other mortars are not always water-repellent or as white and differ greatly in colour, therefore need a separate grout to achieve a weather-resistant, white finish (the most commonly desired colour). 

Vetromix can be coloured by mixing it with special colouring compounds (supplied by others). It is advisable to prepare a sample to show the final colour after curing and record the ratio of colorant to mortar.

Joint performance

Colmef is designed to fully compact in the joint (it does not contain large chippings) to ensure weather tightness against water penetration. Cavities within joints would cause problems in freezing weather conditions.

Glass block mortar joints cure with reaction to air just like normal mortars, so it is important that the joint size is not too thin, to guarantee complete curing and maximum strength. Vetromix will become solid within hours of construction. Total curing is achieved after 21-28 days. Colmef Vetromix has no hydrated lime content and therefore cannot leach. 

The most critical rule of Rods & Mortar is after grouting, the expansion joint and any intermediate soft joints, must be raked back and silicone applied. If the soft joint is bridged by mortar, it negates having expansion fibre and this restriction will stop expansion and contraction, increasing the possibility of joints or blocks cracking.

Colmef Vetromix mortar – number of blocks per 10kg bag calculation

Blocks per m2

Blocks per bag

10mm joint

Blocks per bag

10mm joint curved

Blocks per bag

6mm joint

115x115x80mm 64 19 36 32
145x145x80mm 44 16 13 27
190x90x80mm 50 16 13 22.5
190x190x80mm 25 12 10 20
190x190x100mm 25 10 8 16
240x115x80mm 32 13 11 21
240x240x80mm 16 9.5 8 16
300x300x100mm 10 6 5 10












Technical characteristics of Colmef Vetromix glass block mortar:

Figures stated were taken at 23OC at 50% RH and in lack of ventilation

  • PH of mixture: 12.8
  • Minimum joint size for a curved panel: 6mm
  • Maximum joint size relating to the external joint for a curved panel: 22mm
  • Compressive resistance after 28 days: 19.5N/mm2
  • Flexible resistance after 28 days: 7.5N/mm2
  • COSHH Data sheets are available for Colmef Vetromix on request from Glass Block Technology
  • Colmef Vetromix retains the glass block panel at a U-value of 2.9W/m2OC
  • Wet sand and cement mix increase panel to 3.2W/m2OC

How a mortar joint works

Glass Block Technology mortar is a specially formulated premix bedding and finishing compound, available in one bag to be mixed with water. It is manufactured under factory controlled conditions so all additives are accurately blended and designed for maximum performance of strength, flexibility, water repellence and U-value. 

Mixing instructions are on the reverse of each bag. Always ensure that water and bucket are clean and use a paddle drill. For optimum open time mix with cold water. Hot water may cause the mortar to set too quickly.

Traditionally sand and cement were used. If not consistently mixed , this can increase heat loss leaching and may not be necessarily water- or frost-proof. 

A mortar joint will cure in reaction to air just like normal mortars, so it is important that the joint size is not below 6mm. This guarantees total curing and maximum strength. Vetromix will become solid within hours of construction. Total curing is achieved after 21-28 days.

 Glass is impervious unlike brick and concrete, therefore mortar is not absorbed into a glass block. The strength and support of a joint is created by the shape of the mortar profile (oval). The edge or collar of a block is concaved so when two are laid next to each other an oval joint is created. 

This oval joint enables the glass block wall to resist impact or applied loads resulting in the panel being stable and self-supporting, but not load-bearing. Stainless steel reinforcement bars are used to restrain the panel to the surrounding aperture, whilst also giving the panel integral support and a wind-loading value. 

The minimum recommended joint is 6mm and the most common used is 10mm. However, this is only the distance on show; the centre of the oval joint is always deeper from collar to collar. This area houses the stainless reinforcement bars, which should never be in direct contact with the glass surface.

rods & mortar joint profile head

rods & mortar joint profile cill detail

rods & mortar joint profile jamb


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