Effect of Metakaolin on Efflorescence

Turkchem 30 Jan 2018 41 3 dk okuma

TURKCHEM

Durability problems in cement-based systems affecting performance and aesthetic properties, with efflorescence being the most significant. There are two main types of efflorescence. The first is the emergence of calcium hydroxide during cement hydration and its transport to the surface due to evaporation. The second type of efflorescence occurs through the dissolution of portlandite in pores when a mortar or concrete is wetted.

1. Introduction

Ca(OH)2, present in large quantities in concrete and soluble in water, plays an important role in efflorescence. Water entering concrete through rainfall or groundwater carries salts within the concrete to the surface. C3S + H2o → C3S2 + H3 + 3CH C2S + H2o → C3S2H3 + CH

To prevent efflorescence;

• Use of lower cement quantities, • Selection of cements with lower C3S (calcium silicate) content, • Addition of mineral admixtures to reduce the amount of CaCO3 likely to migrate to the surface. Mineral admixtures have become an integral part of high-strength and high-performance concrete mix design. These can be naturally occurring materials, industrial waste, by-products, or materials requiring less energy to produce. Some commonly used supplementary materials include: ash, silica fume (SF), ground granulated blast furnace slag (GGBS), rice husk ash (RHA), and metakaolin (MK), among others. These products prevent efflorescence by reacting with water and CaCO3, eliminating unwanted phases in concrete. The C-S-H gel resulting from this reaction adds durability to concrete and increases its strength. CaCO3 + H2o + Pozzolan → C-S-H Gel As shown in Table 1, metakaolin is the pozzolan with the highest reactivity compared to other pozzolans. Table 1. Pozzolanic reactivity of pozzolanic materials Pozzolans Silica fume Ash Metakaolin Reactivity (mg Ca(OH)2 / g pozzolan) 427 875 1050

2. Metakaolin

Metakaolin (MK) is a pozzolanic material. Kaolinite clay is obtained by calcination between 500°C and 800°C. Unlike other SCMs that are secondary or by-products, kaolin is obtained by converting it to lime as a primary product. It is widely used as a pozzolanic material in mortars and concrete and has a significant effect on improving the mechanical and durability properties of mortar and concrete compared to other pozzolans.

2.1 Metakaolin Application

Metakaolin can be used in many cement-based products; • Water-proofing products such as repair mortars, tile adhesives, pool plasters, • Adhesives, • Thermal insulation systems, • High-performance and high-strength and lightweight concretes, • Prefabricated concrete for architectural, civil, industrial and structural applications, • Fiber cement and ferro cement products, • Glass fiber reinforced concrete. To date, many studies have been conducted on metakaolin application, and these studies have observed how metakaolin at different ratios substituting for cement affects the system (5, 10, 15, 20, and 25%). According to the results from the literature reviewed, MK is a pozzolan with high pozzolanic activity. The results are as follows. • In studies conducted on adhesives, mortars and concrete, metakaolin has been observed to increase strength in early aging tests as well as increase final strength. • In studies on partial replacement of cement with metakaolin, it was observed that water penetration into concrete was reduced. • MK reduces permeability significantly by changing the pore structure of cement, mortar and concrete; it prevents the transport of harmful ions causing deterioration in the matrix to the surface through water. • Metakaolin substitution for cement is effective in increasing concrete's resistance to sulfate attack. The sulfate resistance of MK concrete was observed to increase with increasing MK substitution levels. Concrete containing 10% and 15% MK replacement has been proven in research to exhibit excellent resistance to sulfate attack. Gizem Sarp Sales Manager Günkem A.Ş.