Posted by : Sajith Sreedharan on February 15th, 2020 in Sustainable Environmental Solutions
All structural engineering consultants know that cement, the primary ingredient in concrete, is not only an extremely important construction material for infrastructure development in particular, but also a key to economic growth in general. The strength, durability and other characteristic of concrete depends on the properties of its ingredients, proportion of mix, method of compaction and other controls during placing and curing. The production of cement is an energy-intensive process, resulting in the emission of greenhouse gases which adversely impacts the environment. Moreover, from a financial perspective, the cost of producing cement is increasing at an alarming rate, and the natural resources that provide the raw material for cement manufacturing are exhausting – two facts that really worry international engineering consultants. For both these reasons and more, the use of industrial by-products having cementitious properties as a replacement for cement in cement concrete has become the need of the hour in the global construction industry.
Ground Granulated Blast Furnace Slag (GGBS) can be effectively used as a partial replacement of cement. GGBS is a by-product from the blast furnaces used to make iron, which can be used as partial replacement of cement in concrete due to its inherent cementing properties. When iron ore, coke and limestone melt in the blast furnace, two products are produced – molten iron and molten slag. The molten slag is lighter than molten iron and floats on top of it. The process of granulating the slag involves cooling it with the use of high-pressure water jets. This rapidly quenches the slag and forms granular particles. The granulated slag is further processed by drying and then ground in a rotating ball mill to a very fine powder, i.e. GGBS.
GGBS is now commonly recommended by structural engineering consultants as well as independent engineering consultants and used with increasing frequency due to its superiority in ensuring concrete durability and impermeability, as well as for its ability to extend the life span of concrete structures. GGBS substantially improves the ability of concrete to resist deterioration from all major threats; it reduces the risk of damages caused by alkali-silica reactions, it offers higher resistance to chloride; and is also highly resistant to attacks by sulphate and other harsh chemicals. The use of GGBS is very beneficial, particularly in coastal areas where the aggressive nature of the saline water and sulphates in the sub-soil can severely degrade concrete structures. It can also be used in higher grades of concrete (more than M40 grade). The cost of GGBS is also lower than the cost of cement. All these advantages provide an economical alternative material for concrete production.
In a country like India, where the development of infrastructure projects such as those for large irrigation and road construction that are either in progress or in the final stages of planning and design; such use of waste material in cement concrete will not only reduce the emission of greenhouse gases but also provide a sustainable method for waste management. Like GGBS, there are other pozzolanic materials which can also be used (separately or in combination) as partial replacements for cement such as fly ash, silica fume, rice husk ash etc.
International engineering consultants are aware of a number of studies currently going on in India and abroad to study the impact of use of these pozzolanic materials as cement replacements. So far, the results of these studies are encouraging.
Nowadays, good sand is not readily available so it has to be transported from long distances. Like a lot of other natural resources, this is another resource that is also exhausting very rapidly. Because of its limited availability, the cost of river sand has skyrocketed and its consistent supply cannot be guaranteed. Under these circumstances, the use of crusher sand becomes a reliable alternative, as independent engineering consultants have found. It can be utilized as a 100% replacement to river sand. Crusher sand obtained from local crushers can be used in concrete to design the concrete mix, subject to satisfying the requirements of relevant codal provisions.
It is found that concrete with partial replacement of cement by 50% with GGBS and river sand with 100% crushed stone sand, helps in improving the strength of the concrete substantially compared to normal mix concrete with 100% cement.
Based on the design mix and observations, the following conclusions can be drawn;
- The combined uses of 50% GGBS and 100% crusher sand gives cost saving of 15 to 20 % per cubic meter of concrete produced as compared to 100% cement and river sand.
- GGBS is a by-product of steel manufacturing process and an economical partial replacement for cement.
- GGBS gives low permeable concrete mix and reduces chloride-induced corrosion to the reinforcement steel.
- GGBS reduces heat of hydration which reduces surface cracks.
- GGBS reduces water requirement and acts as a mineral admixture, reducing the water cement ratio which then results increase in compressive strength.
- GGBS effectively resists sulphate and chloride attacks.
- GGBS increases the retention time of mix, thus allowing extra time for the same work compared to work that’s executed with 100% cement.
- Crusher sand can be used as 100% replacement for river sand in all grades of concrete mixes.