Compressive Strength and Permeability of Concrete Made by Utilizing Type I Portland Cement and Portland Pozzolan Cement

Compressive Strength and Permeability of Concrete Made by Utilizing Type I Portland Cement and Portland Pozzolan Cement

by:

I Made Alit Karyawan Salain

Civil Engineering Department, Faculty of Engineering, Udayana University

INTRODUCTION

Concrete is a very popular building material used in the construction world. Many researches on concrete have already been realized and it will continue to answer the development of science and technology as well as environmental conditions. It is known that the strength of the concrete is strongly influenced by its constituent materials (water, cement and aggregates) so that the quality control of these materials must be considered carefully in order to obtain concrete as expected.

In concrete technology, Portland cement is the main component that serves as a hydraulic binder to bind and unify aggregate to become a solid mass. Various types of Portland cement have been developed in accordance with the type of building and environmental requirements. Type I Portland cement (PPI) is commonly used to make concrete. This type of cement is used for the buildings which do not require any special requirements [1]. With the development of technology and efforts to save costs production as well as to reduce environmental problems, it has been produced Portland Pozzolan cement (PPC), which is a mixture of Portland cement clinker with a material having properties of pozzolan [2].

Pozzolan is a mineral materials, mainly containing silica and alumina, which when grounded equivalent to granular cement and mixed with water at normal temperatures will react with lime to form a compound of calcium silicate hydrate C-S-H and calcium aluminate hydrates C-A-H, similar to the compound produced in the hydration process of cement [3, 4]. It is well known that the reaction of Portland cement major minerals i.e. C3S and C2S with water will produce hydration products C-S-H and free lime Ca(OH)₂. Free lime less contributes to the strength and even tends to disadvantage in terms of durability, when in the course of time the hydration products react with aggressive elements such as sulphate [3-5]. By blending Portland cement clinker and pozzolan, in certain proportions, it will obtain PPC having different character compared to PPI [3-5]. This different character will affect the mechanical properties of concrete made by using PPC.

This paper presents the results of a study which compare the compressive strength and permeability between concrete made by using PPC and PCI at the age of 3, 7, 28 and 90 days.

MATERIAL AND EXPERIMENTAL

Two type hydraulic binders used (PPC and PCI) originated from one of Portland cement industries in Indonesia. Natural sand and crushed stone were used as fine aggregate and coarse aggregate respectively. The grain distribution of sand and crushed stone were designed according to SNI 03-2834-2000 [6] to meet respectively the gradation zone 2 for fine aggregate and the granules with a maximum diameter of 40 mm for coarse aggregate. For each type of binder, concrete was made to produce cylindrical specimens with a diameter of 150 mm and a height of 300 mm. Concrete was made by using a proportion, by weight, of 1.0 cement: 2.0 sand: 3.0 crushed stone. The water cement ratio was 0.4. The compressive strength and permeability tests were realized after 3, 7, 28 and 90 days of hydration utilizing 3 cylindrical specimens respectively.

RESULTS AND DISCUSSION

·         Compressive Strength

The results of the compressive strength test of concrete made by using PPC and PCI at the age of 3, 7, 28 and 90 days indicate that the compressive strength of concrete increases with the increase of the hydration time. This happens on concrete made by using PPC and that made by using PCI.

However, it is also seen that the development of the compressive strength of concrete using PPC is relatively slower at early age compared to that using PCI. Up to the age of 7 days, concrete using PPC produces lower compressive strength, about 15%, compared to that using PCI. By the increase of the hydration time, especially after more than 20 days, concrete using PPC can produce higher compressive strength compared to concrete using PCI. Even, at the age of 28 and 90 days, concrete using PPC produces compressive strength respectively 41 and 46 MPa, which are higher respectively 10% and 8% compared to that using PCI. The development of compressive strength tends to continue after the age of 90 days.

·         Permeability

The results of permeability test of concrete made by using each type of binder at various ages show that coefficient permeability of concrete, kp, decreases with the increase of the age of hydration. A sharp decrease of kp mainly occurs from age 3 to 7 days. This phenomenon occurs on both concretes.

At the early age kp of concrete using PPC is relatively higher than that using PCI. At the age of 7 days, kp of concrete using PPC is 30% higher than that using PCI. However, after the age of hydration is more than 20 days kp of concrete using PPC becomes lower than that of using PCI. When the hydration time reaches 28 and 90 days, kp of concrete using PPC is respectively 6.07 E-10 and 9.36 E-11 cm/sec. This is respectively only 80% and 50% of kp of concrete using PCI.

·         Discussion

The results of compressive strength and permeability test of concrete made by using PCI and PPC show the same tendency i.e. the compressive strength increase and the permeability coefficient decreases with the increase of the hydration time. The development of this mechanical properties of concrete can be attributed to the growth of the hydration products with the increase of the hydration time.

It is well known that with the increase of the hydration time and under condition of adequate curing, the quantity of hydration products, especially C-S-H produced from the reaction of C3S and C2S of the binder with water (H₂O), increases. The increase of the quantity of C-S-H makes the bond between binder and aggregate getting stronger and the empty spaces that are originally occupied by water and cement particles are replaced by C-S-H so that the porosity of concrete reduces. This condition provides a major contribution to the increase in the compressive strength in line with the reduce of the permeability of concrete with the increase of the hydration time [3-5].

From the test, it is also obtained that at early age, concrete using PPC initially produces lower compressive strength and higher permeability coefficient compared to that using PCI. It is strongly related to the presence of pozzolan in PPC. It is known that the reactivity of the pozzolan is generally lower compared to Portland cement clinker. Blending pozzolan and Portland cement clinker will influence the reactivity of binder produced (PPC), and of course, the development of the mechanical properties of concrete made.

With the increase of the hydration time, it appears that concrete using PPC produces better mechanical properties than that using PCI, especially after the age of hydration has passed 20 days. Pozzolan blended in PPC contains mainly reactive silica and alumina. The purpose of a certain amount of pozzolan blended with Portland cement clinker in producing of PPC is to reduce the Ca(OH)₂ produced by C3S and C2S hydration and simultaneously to produce additional hydration products, in particular C-S-H. Free lime produced from C3S and C2S hydration will react gradually with the reactive silica and alumina of pozzolan to form additional C-S-H and C-A-H. This process will reduce the size of large crystals such as Ca(OH)₂ and subsequently the additional C-S-H and C-A-H will fill large capillary cavities which are formed on a common Portland cement hydration process. So that, the porosity of the cement paste hydrates as well as the transition area between the cement paste and aggregate hydrates will be reduced significantly. Consequently, compressive strength of concrete increase and its permeability decreases simultaneously [3, 4]. However, it appears that, only after the period of hydration is more than 20 days, the contribution of pozzolanic reaction to the mechanical properties of concrete using PPC can be seen. This condition gives better mechanical properties than concrete using PCI.

CONCLUSION

1.      At the early age, the development of the mechanical properties of concrete using PPC are slower than that using PCI.

2.      Concrete using PPC can show better mechanical properties compared to that using PCI only after the age of the hydration time is more than 20 days.

3.      At the age of 90 days, concrete using PPC can produce about 8% higher compressive strength and about 50% lower permeability coefficient compared to concrete using PCI.

REFERENCES

[1] National Standardization Board (2004), “Indonesian National Standard: Portland Cement (SNI 15-2049-2004)”.

[2] National Standardization Board (2004), “Indonesian National Standard: Portland Pozzolan Cement (SNI 15-0302-2004)”.

[3] Lea F.M. (1970). The Chemistry of Cement and Concrete. Third edition, Edward Arnold Ltd., London.

[4] Mehta, P.K. (1986), “Concrete Structure Properties, and Materials.” Englewood Cliffs, New Jersey.

[5] Neville, A.M. and Brooks J.J. (1998), “Concrete Technology.” Longman, Singapore.

[6] National Standardization Board (2000), “Indonesian National Standard: Procedure for Making Normal Concrete (SNI 03-2834-2000)”.