Quantitative Study of Hydration of PPC-FA Based using Powder X-Ray Diffraction
Abstract
Pozzolana is the most commonly used mineral in cement industry. Pozzolana doesn’t possess cementitious property on its own but derives its strength by reacting in the presence of moisture with calcium hydroxide formed as a result of hydration. The pozzolanic reaction is slow and required high pH and calcium ion content. This paper deals with study the PPC-FA based cement conforming to IS 1489 Part 1:2015. The LSM, AR, SR, Degree of sulphurization, Burnability Index, Burnability Factor and Percentage Liquid is obtained for the cement using X-Ray diffraction analysis. The hydration of the cement is also studied for 1 day,3 days,7 days,14 days,21 days,28 days and 56 days of curing. The samples for initial setting and final setting of cement obtained as per IS 4031 Part 5:1988 were also analysed for X-Ray diffraction analysis. The paper presents a procedure for sample preparation, X-Ray analysis of cement and cement slurries, identification of phases in cement, identification of silicate hydrates, AFm and AFt phases, percentage variation of these phases and rate of change. The changes in the phases occurring are checked by performing physical tests on cement and cement mortar cubes.
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Introduction
The most commonly used mineral mixture cement industry is pozzolans. The pozzolans are defined as the siliceous and aluminous materials which by themselves possess very little or no cementitious property but in finely divided form in the presence of moisture, they react chemically with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties. Chemically speaking, the pozzolanic reaction occurs between calcium hydroxide (CH) also known as portlandite and silicic acid (H4SiO4 or Si(OH)4), their crystal structure details are given in Table 1.
Ca(OH)2 + H4SiO4 CaH2SiO4.2H2O
Many pozzolans also contain Aluminate Al(OH)- 4,that will react with calcium hydroxide and water to form hydrates such as C4AH13,C3H6 or hydrogarnet or in combination of silica forming stratlingite (C2ASH6) given in Table1.In the presence of anionic group such as sulphates, carbonates or chlorides, Afm phases and Aft phases or ettringite phases can form. The process is a long term process, sufficient amount of free calcium ion and pH of 12 and above is necessary for the solubility silicon and calcium ions and to support pozzolanic reaction.The pozzolans are classified into natural and man-made. Natural pozzolans include trass, certain pumicites and perlite which are generally of volcanic origin. The man made pozzolans include flyash, blast furnace slag and silica flume. Flyash is the most frequently used man-made pozzolan in concrete. The name flyash came from being finely divided residue that results from the combustion of ground or powdered coal.
Conclusion
The Lime Saturation Factor (LSM) was found out to be 1.1. The Alumina Ratio (AR) is 4.8, Silica Ratio (SR) 3.08, Magnesia is 1.37 which is with the limiting value 6 as per standards, SO3 is 8.89 which much greater than that mentioned in the code and the alkali content is 4.38. The degree of sulphurization is about 0.62 at MgO content of 1.37.
The HI is the most for 1st 7 days which is around 0.386 and the second most is during the 2nd 7 days. In the 3rd 7 days the HI is -0.16 which can be attributed due to fall in HI of C-S-H by -0.035 and fall in HI of Afm by –0.004. During the 4th 7 days, there is a marginal increase of 0.003 in the HI of the hydration products. This is due to decrease in HI of C-S-H by -0.013,Aft by -0.006,CH by -0.005 and increase of HI by Afm by 0.027.
There is a decrease in hydration products on day 1 by 3.033%, on day 7 by 1.608% and on day 21 by -3.746%. The most of the products are formed during initial setting as this is the phase of maximum heat release. The percentage increase in hydration products form initial set to final set which is about 18.527%.
There is increase in compressive strength of the cement with age of curing. The chage being drastic during the initial stages which smoothes with passage of time. The maximum increase in strength was obtained during the period 1 day to 3 days of about 312.92% followed by 40.41% during the period 3 days to 7 days and 37.39% during 7 days to 14 days. The SI for the 1 st 7 days is 0.123, 2nd 7 days is 0.022, 3rd 7 days is 0.015 and for the 4th 7 days is 0.01756.
At the end of 56 days of curing the total amount of hydration products formed is 74.09%.At the time of initial setting the total amount of hydration products was 50.9%.The unhydrated products percentages include, Alite =7.97% , Belite = 4.5% , Aluminate = 10.66% ( the cubic aluminate is around 0.33% which the orthorhombic aluminate is 7.33% and monoclinic aluminate is 3%).