Study the Influence of Chloride and Sulphate Ions on The Hydration Characteristics of Various Blended Cement Pastes Containing Nano Metakaolin

Abstract

Seawater is one of the most aggressive environments for constructions made with Portland cement, due to the high concentration of dissolved salts such as (SO4-2, Cl-, Na+, etc.). Sulphate and chloride attack on cement pastes, concretes causing a serious deterioration. The damage usually starts at edges and corners and followed by cracking of concrete. This is due to the formation of ettringite and Friedel’s salt which precipitates with their large volume than reactant in voids and pores causing an internal stress, herby a decrease in compressive strength. Magnesium sulfate has a more damaging than other sulfates because it lead to decomposition of the hydrated calcium silicates as well as Ca(OH)2 and of hydrated calcium aluminates. Metakaolin (MK) obtained by the calcination of kaolinite at temperatures between 700 and 850° C can be used very commonly as a pozzolanic material in mortar and concrete which can react with lime liberated from the hydration of OPC forming calcium silicate hydrate. MK when ground in a high speed grinding mill, can produce particle of the nano scale and it is known as nano metakaolin (NMK). The nano particle size SUMMARY & CONCLUSIONS 135 increase the compressive and enhancing the mechanical and durability properties of mortar and concrete. Cement klint dust (CKD), a by-pass dust, is generated in large quantities during the production of PC. It is a fine powdery, highly alkaline material similar in appearance to OPC and relatively uniform in size. Silica fume (SF) is an amorphous mineral material composed of extremely small and chemically active particle of SiO2 that exist as by-product in silicon or ferrosilicon industries. SF increase the compressive strength compared to plain cement pastes. This may be attributed to the reaction of silica fume with calcium hydroxide liberated during the hydration of cement. The aim of this investigation is to study the effect of chloride and sulphate attack on the hydration characteristics of various blended cement pastes containing various amounts of NMK, NMK+CKD and NMK+SF. To achieve this aim, different ratios of NMK, SF and CKD; (0, 2, 4, 6, 8, and 10%) by weight of cement are used. The different pastes are prepared by mixing cement with the calculated percent of the various additives, then adding the amount of water (w/s = 0.27). After complete mixing, the pastes are molded into cubic specimens of one-inch dimension. SUMMARY & CONCLUSIONS 136 The moulds are shaken for one minute to remove any air bubbles. A thin-edged trowel settles the top surface of the pastes. The moulds are kept in 100% humidity for the first 24 hours to attain the final setting and getting hard. Then cubes are divided into three parts, and immersed in tap-water, 5% MgSO4 and 3.5% NaCl solutions for different times of hydration up to 180 days. The pastes were designated as a, b, and c respectively.