Convergent weeds out the concrete ‘virus’
01 April 2006
Belgium-based concrete specialist Convergent is targeting the Middle East market with its new product range the Pentra-Sil 244+, which can help project developers and contractors weed out ‘viruses’ from concrete.
The company, which has applied the product on more than 4 million sq m since 2003, is looking forward to enter into partnerships with regional players who could sell and apply the product, says company president Jean-Claude Biard.
“The new product that the company promoted at last year’s Big 5 can help prepare longer lasting surfaces,” according to Biard.
Explaining the properties of the product, he continues: “Pentra-Sil 244+ surface densifier is a lithium-based liquid compound that penetrates and reacts with calcium to form insoluble calcium silicate hydrate within the concrete. It works by impregnating the surface with lithium ions that can shut down ASR (alkali-silica reactivity) at the surface, and by reducing the ingress of moisture that will help prevent ASR from happening in the substrate.”
Why lithium?
“Lithium-based surface densifiers, unlike the older sodium and potassium-based densifiers used for decades, utilises the sophisticated lithium chemistry,” says a spokesman for Convergent. “Older sodium and potassium silicates form an expansive alkali-silica gel to seal the surface. This is why even properly finished and cured concrete can develop crazing or map cracking after the application of sodium and potassium products. Lithium’s reaction in concrete is non-expansive, so it won’t contribute to crazing or map cracking.”
“Being smaller than sodium and potassium ions, lithium ions can penetrate concrete better. In addition, lithium reacts more evenly throughout the surface zone, unlike sodium and potassium ions, which tend to react more violently and unevenly, leaving clumps of reacted and unreacted calcium scattered throughout the surface zone.
“Also, lithium silicates do not absorb water like sodium and potassium products can, making the resulting structure more water resistant and easier to maintain.
Pentra’s chemistry
“Pentra-products react with calcium compounds in the concrete to form insoluble silicate structures that strengthen the substrate. In addition, they also react with calcium hydroxide, which lines the microscopic capillary channels in the concrete to form insoluble complex tricalcium silicate compounds. These tricalcium silicate compounds block the capillaries and pores and virtually eliminate moisture absorption into the substrate.
“These reactions are somewhat similar in all surface densifiers, however, the degree to which these reactions occur can vary widely. Pentra-products utilise high-tech chemistry to control, slow down, extend and maximise the reactivity of the lithium with the calcium, resulting in fewer unreacted calcium compounds.
What is ASR?
“ASR is an enormous worldwide problem. In simple terms, a combination of alkali + silica + moisture = ASR (Alkali Silica Reactivity). Silica in the aggregate reacts with alkalies in hydrated cement to form a gel that expands when water is introduced. When this reaction occurs, huge pressures are produced that cause cracks to develop in concrete, which lead to the accelerated deterioration of the surface protective layer. These cracks and micro-cracks allow water to penetrate the substrate, causing corrosion of reinforcing steel, freeze-thaw damage, and other problems.
“The lithium in Pentra-Sil and Pentra-Sil 244+ helps shut down ASR-related reactions in the surface. These products also help keep moisture out of the substrate, and without moisture, the ASR reaction cannot occur. In addition, Pentra-Sil and Pentra-Sil 244+ are the only surface densifiers that do not add sodium or potassium to the surface (sodium and potassium can contribute directly to an ASR problem),” the spokesman claims.
Meets standards
“Pentra-Sil 244+ contains an effective chloride ion screen that meets the standards set by the National Cooperative Highway Research Program 244 (NCHRP 244). It can be used for a range of applications that include: parking decks, marine environments, pedestrian walkways, stamped/coloured, concrete, and salt exposed areas. It is a non-flammable, nontoxic, water-based formulation for use on most portland-cement-containing materials. It contains no solvents or harmful levels of VOCs, is non-carcinogenic and non-mutagenic, and may be used in food preparation areas. Pentra-Sil 244+ is odourless during and after application and also allows concrete to breathe,” he says.
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