Disadvantages Of Slag Cements

Slag cement

According to Silica Fume Association, “In the production of iron, the blast furnace is continuously charged from the top with iron oxide (ore, pellets, and sinter), fluxing stone (limestone or dolomite), and fuel (typically coke). Two products are gotten from the furnace: molten iron that accumulates in the bottom of the furnace (hearth) and liquid iron blast-furnace slag floating on the puddle of molten iron. To make the most of cementitious properties, the molten slag is cooled rapidly as it leaves the blast furnace. Quick quenching or chilling reduces crystallization and converts the molten slag into fine-aggregate-sized particles composed majorly of glass. This product is referred to as refined blast-furnace slag. Other types

Hydraulic cements (e.g. Portland cement) set and become adhesive due to a chemical reaction between the dry elements and water. The chemical reaction results in mineral hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This permits setting in wet condition or under water and further protects the hardened material from chemical attack. Cements that harden by reaction with water and form a water-resistant product. (Nemati, 2015)

2.2 Chemistry of Cements

Non-hydraulic cement such as slaked lime (calcium hydroxide mixed with water), hardens by carbonation in the presence of carbon dioxide which is naturally existing in the air. First, calcium oxide (lime) is fashioned from calcium carbonate by calcination at temperatures above 8250C (1,5170F) for about 10 hours at atmospheric pressure:

CaCO_3→CaO +CO_2

The calcium oxide is then expended (slaked) mixing it with water to make slaked lime (calcium hydroxide):

CaO+H_2

This process results in concrete with increased strength and decreased permeability. Curing is also a key player in mitigating cracks, which can severely affect durability.

2.4.2 Hardening

Cement hardens when it comes in contact with water. Hardening is not a drying process and can very well take place in water. Heat speeds up the setting and hardening of cement and cold slows it down and can even completely stop the processes. In order to crystallize or hydrate, cement requires a quantity of water equal to 25% of its weight. But in order for it to be laid and remain sufficiently workable, twice this amount is usually required. However, too much water can reduce the strength and durability of the concrete. The paste (i.e. the mixture of cement and water) acts both as a lubricant and an adhesive. Hardening does not start as soon as cement and water are mixed, but after a certain length of time known as the “initial set time” during which the concrete can be mixed, transported, laid and vibrated. (All Public Works,