22-09-2016, 11:29 AM
Economical feasibility of new generation chloride-free Vs traditional chloride-based performance improving cement additive.
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ABSTRACT
The purpose of this study is to evaluate the performance and cost effectiveness of new generation chloride-free and traditional chloride-based performance improving cement grinding additives (CGA) on the early age compressive strength of Portland Pozzolana Cement (PPC) {Part 1-Flyash based} [1]. Grindings were done in laboratory ball mill at standard condition and tested according to IS 4031[2]. The result indicates that chloride-free CA shows better performance and cost effectiveness than traditional chloride-based CA.
KEYWORDS: Cement, Clinker, SCM, CO2, Cement Additive, Accelerator, Chloride, Chloride-free
INTRODUCTION
Portland cement manufacture is one of the most intensive CO2 emitting industry. With cement production exceeding 272 million ton/year, it contributes to approximately 7% [3] of India’s man-made CO2 emissions.
Thermal energy represents about 90% of the total specific energy consumption and the pyroprocessing for converting raw materials to clinker formation accounts for nearly 800 to 900 kg of CO2 per ton of clinker. In India, the specific electrical energy consumption is averaging to 82 kWh/ton of cement and specific thermal energy consumption is 725 Kcal/kg [4] clinker.
The CO2 emission is mainly depends on the clinker factor in cement and energy required to grind clinker along with other additives to produce cement to a specified fineness to attain required strength.
The introduction of Cement Grinding Additive (CGA) in the grinding process of cement reduces total CO2 emission by partially replacing clinker with SCM and by reducing the power consumption needed for producing high-early strength cement.
Internationally liquid Grinding Aids (GA) were in use in the manufacture of Portland cement for past more than 80 years. Initially GA’s were used in the cement manufacturing process only for increasing mill production rate, for reducing specific power consumption of grinding and for grinding cement significantly finer to obtain high-early strength cement.
Over the years, Performance Improvers (PI) were intensively developed from traditional GA’s to improve the quality of cement [5-8] which shall be used to manufacture high-early strength cement by promoting the hydration of cement at early ages (chemical activation) without the need of grinding the cement to higher fineness (i.e.) without increasing the grinding cost/specific energy consumption. The use of PI’s gained more popularity in the recent years, because this is one economical way where clinker which is the main component emitting CO2 during its manufacturing process can be replaced partially SCM [9-14] lowering the cost of cement production without affecting the properties of cement and by reducing the overall CO2 emission from the manufacture of cement.
Most commercially available PI’s have both set accelerating and hardening accelerating effect which significantly reduces the setting time of cement along with the increased rate of development of early age strength.
The advantage of high-early strength cement with lower setting time is its usefulness in completing the work in shorter time in the construction industry. The use of setting time accelerator at normal conditions speeds up the setting and hardening process for earlier finishing or mould turn round [15 or 16] and it’s very useful in winter construction.
- Set accelerator (setting time accelerator) [17] is defined as additive which decreases the initial setting for the transition of the mix from plastic to the rigid state.
- Hardening accelerator (strength accelerator) [17] is defined as additive that increases the rate of development of early strength in the cement with or without affecting the setting time.
Many of the commercial available PI’s have a dual benefit of improved strength and lower setting time.
Because of its economic advantage [], traditionally all the PI’s includes major composition as chloride. Chloride has an ability to reduce the setting time along with the strength gain benefit which avoids any construction delay []. But the main problem pertaining the use of chloride is its disadvantage in bringing corrosion in steel reinforced concrete []. Excessive dose of chloride leads to accelerated corrosion with subsequent spalling and possible structural failure.
Thus the idea of using non-corrosive PI’s came in practise. A number of inorganic and organic compounds [1] including formates, nitrates, nitrites, thiosulfates, thiocyanates, silicates, sulphates, alkanolamines and calcium salts are available. All these compounds have different solubility and cost effectiveness []. Thus, when liquid Cement Grinding Additive is formulated, dilution of these compounds along with its cost implementation also plays a major role in incorporating the same into it.
Of these, organic accelerators including DEA, TEA, DIPA, TIPA, MDEA, etc., are widely included in formulation because of their low effective addition to cement and its ability to modify particle size distribution of cement, tendency to reduce agglomeration by neutralizing charges during grinding process and along with promoting hydration process to increase strength, both in initial and late age of hardening.
EN 197-1 [], clause 5.5 defines additive as constituents other than clinker, gypsum, SCM added to improve the manufacture or properties of cement during its manufacture, the total quantity of which should not exceed 1.0% by mass of cement (expect for pigment) and the total quantity of organic additives on a dry basis shall not exceed 0.5% by mass of cement. IS 1489 (Part 1): 1991 [], clause 4.4 permits the usage of additives to a limit of 1.0%.
The chloride content in PPC is restricted to 0.05%, determined by the method given by IS 12423: 1988 [] for use in reinforced concrete,
For simplification, we will label any liquid Cement Grinding Additive including grinding aid and performance improver, etc., commonly as Cement Additive (CA) hereafter in this paper.
The aim of this study is to compare the performance and cost-effectiveness of chloride-based and chloride-free CA’s in PPC (Part 1 - Flyash based) in laboratory condition. For this purpose, 4 CA’s were formulated predominantly containing Chloride and 4 CA’s chloride-free.