Mechanical Properties and Durability of Concrete Made with Portland ...

SP 202-27

Mechanical Properties and Durability of Concrete Made with Portland Limestone Cement by E.F. Irassar, V.L. Bonavetti, G. Menéndez, H. Donza and O. Cabrera Synopsis: European countries have a great deal of experience in the use of Portland Limestone Cements (PLC). In Latin American, most of the cement plants use limestone as a raw material and an increase in cement production is expected in the next few years. The manufacture of this cement would represent a rapid increase of production without environmental consequences. This paper synthesizes data from a research program carried out over two years to determine the effects of limestone filler on concrete and mortar behavior. At early age, the influence of limestone filler on workability, bleeding, initial curing and mechanical behavior (modulus of elasticity, compressive and tensile strength) was studied. Sulfate resistance and chloride penetration, the most important durability problems related with PLC, were also studied. The addition of slag was also investigated to improve the longterm strength and the durability of PLC. Results show that cements containing around 10% of limestone filler provide similar or better mechanical behavior than portland cement concrete, without compromising their durability properties where low chloride diffusion and high sulfate resistance is required. In this case, the environmental impact of cement manufacture decreases because the energy consumption and the CO2 emission are reduced per ton of cement and the combination with other supplementary cementing materials (slag, fly ash or natural pozzolan) can improve these aspects.

Keywords: blast-furnace slag; chloride diffusion; curing; dilution; hydration degree; limestone addition; limestone filler cement; modulus of elasticity; strength; sulfate resistance; water absorption

431 E.F. Irassar, V.L. Bonavetti, G. Menendez, O. Cabrera & H. Donza, “MECHANICAL PROPERTIES AND DURABILITY OF CONCRETE MADE WITH PORTLAND LIMESTONE CEMENT”, Proc. International Syposium on the Sustainable Development and Concrete Technology, V.M. Malhotra Ed, ACI Special Publication 202-27,Farminton Hill, MI, USA, pp. 431-450, 2001.

432 Irassar et al. ACI member E.F. Irassar is an associate professor in the Department of Civil Engineering at the University of Center of Buenos Aires State, Olavarría, Argentina. He is Specialist on Concrete Technology of University of La Plata and head of INMAT Research Group. His research interest includes blended cements and sulfate attack. V. L. Bonavetti is lecture in the Department of Civil Engineering at the University of Center of Buenos Aires State. She has received a MSc in Concrete Technology. Her current research interest is concretes with limestone blended cements. G. Menéndez is fellow in the Department of Civil Engineering at the University of Center of Buenos Aires State. He is MSc. candidate in Concrete Technology and his research interest is ternary blended cements. H. Donza is lecture in the Department of Civil Engineering at the University of Center of Buenos Aires State. He is MSc. candidate in Concrete Technology and his research interest includes high strength concrete. O. Cabrera is an associate professor in the Department of Civil Engineering at the University of Center of Buenos Aires State. His research interest includes concrete production and marginal aggregates. INTRODUCTION The growth of Portland Limestone Cements (PLC) utilization is an objective of cement producers owing to technical, economical and ecological considerations. Among the technical considerations, the gain of initial strength, the control of bleeding in concretes with low cement content and less sensitivity to the cessation of moist curing can be pointed out [1]. PLC manufacture reduces environmental impact because it requires less energy consumption, fewer natural resources, the CO2 emissions are reduced per ton of cement, and its combination with other supplementary cementing materials can further improve these objectives [2]. Finally, economical benefits can be obtained from less energy required to produce a ton of cement with the same strength class and reduced cost of investment for a given quantity of cement production [3]. In Latin America, cement consumption is expected to grow by 50% in the next ten years. Cement plants generally use limestone as raw material, subsequently the emission of CO2 can be reduced introducing limestone filler in all cements and this reduction can be further improved by combining it with others supplementary cementing materials (such as fly ash, slag or natural pozzolan) to produce a ternary cement. The addition of limestone filler to clinker is used to supply the fine particles in the granulometric curve of cement without any increase in water demand, thereby improves the cement packing and blocking the capillary pores by obstruction. It also interferes during hydration of C3A forming carboaluminates and delays the ettringitemonosulfoaluminate transformation. It also constitutes nucleation sites for calcium hydroxide crystals at early hydration ages [4,5], accelerating the hydration of clinker particles, especially the C3S [6,7], and consequently improves early strength. However, it does not possess pozzolanic properties and consequently it does not produce C-S-H [8]. An associated effect of limestone addition is the reduction of clinker, which is

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commonly known as dilution. A literature review indicates that limestone filler produces an acceleration of hydration and the dispersion of clinker particles [4,9]. Finally, the performance of PLC depends on the amount of limestone replacement, the quality of limestone, the composition and reactivity of the clinker [10,11], and the intergrinding process [12-14]. As concerns durability, the more important risks of using PLC are associated with sulfate resistance (especially thaumasite formation) and chloride-ion diffusion [15]. Both problems can be aggravated when the strength criteria prevails over the durability criteria because these cements can produce a high early strength concrete with a high w/c. During the 1990s, the use of cement made with portland cement and two supplementary materials, also called ternary or composite cements, has increased because it presents more advantages than some binary cements [16-21]. Ternary blended cements containing combinations of fly ash-silica fume or slag-silica fume are common in practice [22]. There is general agreement that the principal hydration products formed when slag is mixed with portland cement and water is essentially C-S-H similar to the compound produced by the hydration of calcium silicates of portland cement [22]. The rate of hydration of slag is initially lower than the corresponding cement containing only portland cement. Thereafter, portland cement containing blast-furnace slag typically shows a reduction of strength at early ages (7 to 20 days) and similar or greater strength at later ages [22]. Mostly, slag reduces permeability and ionic diffusion of chloride regardless of composition and replacement level [23]. Currently, the addition of slag is being investigated to reduce the damage caused by sulfate attack in concrete containing limestone filler [24]. From the above description, it can be inferred that the combination of limestone filler and slag in a ternary blended cement can help to formulate a cement with an appropriate development of strength, because limestone filler contributes to the early strength and the slag increases the long-term strength. This paper presents a summary of data from different projects carried out in the research program on the performance of PLC and its combination with ground granulated blast furnace slag and their effect on the strength and durability of concrete. EXPERIMENTAL This investigation consists of two phases: the objective of phase I is to determine the performance of concrete containing commercial PLC and the phase II objective is to determine the improvement of these cements with the addition of ground granulated blast furnace slag. Materials In phase I, a normal portland cement (C0) and two portland limestone cements (C10 and C20) obtained from the same portland clinker by an intergrinding process in the cement plant were used. The clinker contained 65% of C3S and 5.3% of C3A, and the limestone content by mass was 0, 9.3 and 18.1% according to the data supplied by

434 Irassar et al. the cement producer. The cements have the same strength class (44.1, 49.2 and 47.6 MPa at 28 days) leading to a larger specific surface area of PLCs (317, 372 and 420 m2/kg for C0, C10 and C20, respectively). To investigate the addition of slag (phase II), a normal portland cement (C0a) and two portland limestone cements (C12 and C18) with the same strength class (45, 39 and 39 MPa at 28 days) were used. Their Blaine specific surface area was 321, 380 and 383 m2/kg and the limestone content was 0, 12 and 18% for C0a, C12 and C18, respectively. The blending material was a ground granulated blast furnace slag, which contained over 90% of glass in its composition. Its chemical modulus (CaO+MgO+Al2O3/SiO2) was 1.8 and it was ground at a Blaine fineness of 458 m2/kg. The coarse aggregate was crushed granite stone with a maximum size of 19 mm for mechanical test specimens and 12.5 mm for chloride penetration test specimens. The fine aggregate was natural sand with a fineness modulus of 2.35. In phase I, two admixtures were used: a commercial water reducer (WR) for concrete with w/cm of 0.40 and a high range water reducer (HRWR) based on sulphonated melamine formaldehyde containing 22 percent of active ingredient in aqueous solution in concrete with w/cm of 0.34. Mixture proportions, casting and curing For phase I, nine concrete mixtures (w/cm = 0.34, 0.4 and 0.5) were made to determine the mechanical properties of PLC and nine mixtures (w/cm = 0.4, 0.5 and 0.6) to study the chloride penetration. These mixtures are described in Tables 1 and 2. For phase II, seven mixtures were made. The cementitious materials content was 350 kg/m3 and the w/cm 0.5. A binary mixture containing slag (C20S) was achieved by a 20% replacement, by weight, of slag for C0a cement. Ternary mixtures (C12F10S, C12F20S, C18F20S) were made replacing 10 and 20% of slag by C12 and C18 cements. The mixture proportions and properties of fresh concrete are given in Table 3. Specimens were cast according to a specific standard, covered with a plastic sheet and left in a laboratory environment for 24 hours. Subsequently, specimens were demolded and immersed in water saturated with lime until they reached test age. To study the influence of early curing in phase I, specimens of concrete with w/cm of 0.50 were also cured under air and wet regimes. For the wet curing regime, specimens were immersed in water saturated with lime for six days and then air cured in laboratory environment, while for the air curing regimen, specimens were directly exposed to the laboratory environment after being removed from the molds. Test procedures Mechanical properties: Compressive strength and static modulus of elasticity were determined on 100x200 mm cylinders in accordance with ASTM C 39 and C 469, respectively. Tensile strength was determined on 150x300 mm cylinders using the ASTM C 496 splitting test. The modulus of rupture was obtained using a similar procedure to ASTM C 78. In all cases, the average of three test values is reported.

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Hydration degree and water absorption: In phase I, fragments of concrete specimens were used to determine the water absorption and the amount of non-evaporable water to estimate the hydration degree (α). For water absorption determination, saturated fragments were oven dried at 105 ± 5ºC until a constant weight was achieved. Finally, this was calculated according to ASTM C 642. During phase II, water absorption was also determined using ASTM C 642. The non-evaporable water was obtained according to the procedure proposed by Powers [25]. The hydration degree was estimated assuming that the filler is a chemically non-active admixture and that the water needed for the full hydration of cement used is 0.23. Sorptivity: Three specimens (100x120x150 mm) were used to determine the water capillary sorptivity after 7, 28 and 90 days in mixtures of phase II. In this test, the specimen is dried in oven at 105 ± 5ºC for a 24 hours period. After temperature stabilization, specimen surfaces were coated with an epoxi-painting layer delimiting the absorption surface (100x100mm) and preventing the axial surface absorption. Subsequently, the specimens were exposed to water on one of plane ends by placing them in a plastic tank as shown in Fig. 1. The fluid level was maintained at constant level throughout the experiment. At 1, 5, 15 and 30 minutes and 1, 2, 4, 6, 12, 24 and 48 hours, the mass of specimens were measured using a balance. The sorptivity was obtained by the volume of water absorbed per unit wetted area against the root of the time (hour1/2). Chloride ion penetration: Concrete prisms (100x150x530 mm) were cured in lime saturated water for six days. After curing, the surfaces of the specimens were epoxycoated with exception of one surface perpendicular to molding and then immersed in water for 24 hours to saturate the specimens. Subsequently, concrete prisms were immersed in 3% NaCl solution. After 45, 180 and 365 days, a 70 mm thick slice was sawed from the top of each prism and the specimens were freshly epoxy-coated and immersed in solution again for future determination. Concrete powdered samples were obtained from each slice by drilling at 10-mm increments at least 60 mm deep. Chloride contents were obtained by the acid soluble determination technique according to ASTM C 1152. Finally, the surface concentration (Cs) and the apparent diffusion coefficient (Da) were determined from the standard solution of Fick’s second law of diffusion. Sulfate resistance: Mortar bars (285x25x25 mm) were cast according to ASTM C 1012 (sand to cementitious material ratio of 2.75 and w/cm of 0.485). Specimens were stored in a moist cabinet for 24 hours, and then removed from the mold and cured in saturated lime-water until 30±3 MPa compressive strength was achieved. At this age, mortar bars were stored in individual plastic tanks (mortar bars volume to sulfate-solution volume 1:4) containing Na2SO4 (0.352 M) at constant pH (7 ± 1). Complete details on sulfate performance of PLC have been previously published [26]. Complementarily, XRD analyses to determine the compounds formed during the attack were carried out on powder samples of mortar using a Philips X’Pert diffractometer equipped with a graphite monochromator.

436 Irassar et al.

RESULTS AND DISCUSSION Slump test: From Tables 1, 2 and 3, it can be observed that slump of PLC concretes was approximately the same for a given w/cm while the water content in the mixture remains constant. This behavior occurs in spite of the increase of specific surface of PLCs in both program phases. Previous studies carried out on mortars also demonstrated that the addition (5 to 20%) of extremely fine limestone filler (specific surface Blaine =1020 m2/kg) does not increase the water demand significantly [12,27]. All concretes with PLC also showed an adequate workability, good finish, and they were more cohesive than plain concrete. However, concretes with PLCs require an increase of admixture dosage to achieve a given slump. This is attributed principally to the large specific surface of the PLCs. For phase II, the addition of slag to both PLCs also improved the characteristic of fresh concrete due to the increased volume of paste and its cohesiveness. Concrete bleeding: Fig. 2 shows that the increase of filler content in cement reduces drastically the bleeding rate and bleeding capacity of fresh concrete with a w/cm of 0.5. Reduction of bleeding capacity is due to the large specific surface of PLCs and to the blocking of capillary pores by the filler particles obstructing the water movement through the fresh concrete [28]. The increase of specific surface does not directly relate to an increase of fineness of clinker particles. Limestone has a higher grindibility degree than clinker and consequently limestone particles occupy the fine fraction ( 150 > 150

++ Water reducer (WR) admixture

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Table 3: Concrete mixtures used in phase II. Materials, kg/m3

Mixture

Cement type

C0a

C0a

175

350

---

840

1000

85

C12

C12

175

350

---

834

1000

115

C18 C20S

C18 C0a

175 175

350 280

--70

831 831

1000 1000

80 100

C12F10S

C12

175

315

35

830

1000

120

C12F20S C18F20S

C12 C18

175

280

70

827

1000

105

175

280

70

825

1000

105

Water Cement

Slag

Fine Coarse aggregate aggregate

Slump, mm

Table 4: Sorptivity coefficient for phase II concretes.

Mixture

Cement type

C0a

Sorptivity coefficient, g/cm2 * h1/2 7 days

28 days

90 days

C0a

0.18

0.10

0.08

C12

C12

0.13

0.09

0.06

C18 C20S

C18 C0a

0.20 0.21

0.14 0.10

0.12 0.06

C12F10S

C12

0.15

0.11

0.06

C12F20S C18F20S

C12 C18

0.19

0.11

0.05

0.20

0.13

0.06

Table 5: Best-fit diffusion coefficient and surface concentration for concretes studied after 360 days immersed in 3% NaCl solution.

w/cm

0.4

0.5

0.6

Cement C0 C10 C20 C0 C10 C20 C0 C10 C20

Best-fit values Cs (% of concrete Da x 10-12 m2/s weight) 5.0 0.12 11.2 0.13 10.5 0.14 6.9 0.15 20.3 0.12 23.8 0.15 25.7 0.15 21.6 0.18 41.4 0.25

446 Irassar et al.

Fig. 1: Schema of sorptivity test

Fig. 2: Bleeding of concrete with w/cm =0.50.

Fig. 4: Compressive strength of concrete used in phase II: a) binary mixtures and b) ternary mixtures.

Fig. 3: Relative compressive strength for both portland limestone cements used.

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448 Irassar et al.

Fig. 5: Influence of initial curing on compressive strength development of concrete with w/cm of 0.5.

Fig. 6: Compressive and tensile strength relationship.

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Fig. 7: Evolution of modulus of rupture: a) binary mixtures and b) ternary mixtures.

Fig. 8: Evolution of modulus of elasticity in phase I concretes.

Fig. 9: Hydration degree of clinker fraction in phase I concretes.

450 Irassar et al.

Fig. 10: Water absorption in phase I concretes.

Fig. 11: Water absorption of concrete used in phase II: a) binary mixtures and b) ternary mixtures.

Fig. 12: (a) Expansion of mortars containing 0, 10 and 20% of limestone filler in 5% Na2SO4 solution. (b) XRD pattern of mortar samples at 360 days.