Construction World April 2018

CEMENT & CONCRETE TECHNOLOGY

Environmental conditions such as temperature, moisture and chemical factors can hasten the deterioration of concrete, causing problems such as steel corrosion, delamination, cracking, carbonation, chemical attack, scaling, spalling, abrasion and cavitations. Working with customers to ensure the durability of concrete structures is a key Chryso offering, and the group brings a depth of expertise to help maintain the operational function of 1 and 2. The finer fractions are used as a PC extender. Ultrafine FA is sold as a separate product. FA reacts with calcium hydroxide, in the presence of water, to form cementing compounds consisting of calcium silicate hydrate. This reaction is called pozzolanic and FA may be described as an industrial pozzolan. The hydration of PC produces significant amounts of calcium hydroxide, which does not contribute to the strength of the hardened cement paste. By extending PC with FA the calcium hydroxide can be used to form additional calcium silica hydrate. Silica fume (SF) is the condensed vapour byproduct of the ferro silicon smelting process. SF must comply with SANS 53263: parts 1 and 2. SF reacts with calcium hydroxide in the presence of water to form cementing compounds consisting of calcium silicate hydrate. This reaction is also pozzolanic. Because the hydration of PC produces calcium hydroxide, the combination of SF and PC is a practical means of using SF and improving the cementing efficiency of PC. In addition to the chemical role of SF, it is also an effective 'fine filler.' The extremely small SF particles in the mixing water act as nuclei for the formation of calcium silicate hydrate which would otherwise form only on the cement grains. SF will also change the microstructure of the interfacial zone. The result is a more homogeneous microstructure that has greater strength and lower permeability. Limestone filler is limestone, finely ground but not chemically processed. There is no separate specification for limestone as it can only be added during the production of cement. When mixed with PC and water, finely ground limestone is chemically virtually Enhancing durability of concrete While concrete is one of the most durable and long lasting of building materials, there are a number of both internal and external factors that can reduce its life-span unnecessarily, but these challenges can be solved with technical solutions from Chryso Southern Africa.

The iconic Nelson Mandela Bridge where Chryso admixtures were used.

concrete – protecting its aesthetic properties and limiting future maintenance costs. Among the solutions in the Chryso range are options to prevent corrosion of steel reinforcement in the concrete, and to withstand the aggressive action of water or the action of salt water on concrete. The effects of freezing and de-icing salts can also be avoided, as well as the phenomena of alkali-aggregate reaction and internal sulphate attack. For instance, Chryso manufactures and distributes two integral waterproofing products – Chryso® CWA 10 and Chryso® Fuge B – which are added directly to the concrete mix at the time of batching. These admixtures reduce the permeability of concrete structures, making them more durable. With a low permeability, concrete is less likely to experience frost damage, acid and sulphate attack, steel corrosion, alkali aggregate reaction and efflorescence. The active ingredients of Chryso® CWA 10 react with moisture to form crystals deep within the pores and capillary tracts of the concrete structure, while Chryso® Fuge B is a pore-capillary blocking admixture, combining with components of the concrete and effectively forming deposits which line the walls of the pores and capillaries. The result is that water ingress is effectively reduced. While Chryso’s solutions can be applied ‘upfront’ in the batching of the concrete to enhance durability, the group’s expertise and products are also extensively used in the maintenance and repair process – further strengthening concrete structures and extending their life. Chryso group company a.b.e. Construction Chemicals has the experience and products to save customers considerably through effective concrete repairs. Based on epoxies, synthetic resins, silanes and cementitious materials, Chryso products can be applied to protect and waterproof the concrete sections of bridges, silos, building and water containment structures; the skilled team at a.b.e. can also advise on concrete crack injection, fairing, reprofiling, priming, mortar repair, and the rehabilitation of spalled concrete surfaces. 

inert. Depending on its fineness, limestone may however act as a 'fine filler' in fresh paste. Limestone may be used as a filler in common cement or as a workability improver in masonry cement. The effect of limestone on the properties of concrete or mortar depends on the specific limestone, whether a grinding aid is used in production, and the fineness of the limestone. 

Bryan Perrie, MD of The Concrete Institute (TCI).

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CONSTRUCTION WORLD APRIL 2018