CEM I 42.5 R Alternative Cement for Durability

Introduction

The ready-mixed concrete sector in Istanbul predominantly prefers CEM I 42.5 R cement. Among widely used manufacturers such as Akçansa, Nuh, Traçim, Çimentaş, Aslan and Limak, the 28-day standard compressive strength of the cement in question ranges between 55-62 MPa, positioning it well above the 42.5 MPa characteristic limit and Anatolian producer averages. This high strength level enables, on one hand, the use of high proportions of mineral additives such as ground granulated blast furnace slag (GGBFS) or fly ash (FA) in concrete production, and on the other hand, allows the total binder dosage in the concrete composition to remain quite low. With good limestone aggregates from Ömerli, Cebeci, Gebze, and Çerkeşli quarries, quality natural sand, appropriate aggregate gradation and carefully selected chemical additives, C25 can be produced with 215-230 kg/m³ binder, C30 with 230-250 kg/m³ binder, C35 with 240-260 kg/m³ binder, and C40 with 260-280 kg/m³ binder at the plant at S3 consistency and delivered at S4 consistency on site through "redose admixture" application. Given that concrete compositions can contain GGBFS of 50-100 kg/m³, cement dosage rarely exceeds 200 kg/m³. For example, C25 can be produced with 160+60 kg/m³ cement+GGBFS, and C40 with 200+80 kg/m³ cement+GGBFS.

Durability Issue

While the primary priority of the ready-mixed concrete sector is compressive strength, durability and long economic life are also important and have a place in standards. TS EN 206 and TS 13515 concrete standards classify and group environmental effects in terms of durability. XO: No risk of corrosion or harmful effect. XC: Corrosion induced by carbonation. XD: Corrosion induced by chlorides other than sea water. XS: Corrosion induced by chlorides from sea water. XF: Freeze/thaw effect. XA: Chemical attack on concrete. XM: Mechanical wear effect. XW: Concrete deterioration due to alkali silica reaction. Even if there are no other harmful effects, "reinforced concrete," due to the reinforcement it contains, is at risk from "corrosion induced by carbonation" as a result of oxygen, carbon dioxide and moisture in the air, and the risk is classified as in Table 1 of TS 13515.

Table 1 - Corrosion induced by carbonation

The mildest of the exposure classes in Table 1, and the least harmful, is XC1, and reinforced concrete containing reinforcement must meet at least the XC1 class requirements. This provision has also entered the revised Earthquake Code and will be mandatory from 1 January 2019 onwards. On the other hand, Istanbul is a coastal city surrounded by the Black Sea to the north and the Marmara Sea to the south, with the Istanbul Strait passing through it, and soon the Kanal Istanbul will pass through it. It is at risk from corrosion induced by chlorides from sea water. (Table 2).

Table 2 - Corrosion induced by chlorides from sea water

The mildest class in Table 2 is XS1, and as can be understood from its definition, a significant portion of structures in Istanbul have concrete on or near the shoreline and are exposed to the effects of salt-laden air and must meet the requirements of this class. The requirements sought by the standard for these environmental exposure classes are summarized in Table 3.

Table 3 - Recommended class values for concrete composition

As seen from Table 3, to meet the mandatory and mildest environmental exposure class, XC1: - Either at least 250 kg/m³ cement; - Or at least 240 kg/m³ cement and 12.5 kg/m³ GGBFS (with cement equivalent factor k=0.8) - Or at least 240 kg/m³ cement and 25 kg/m³ FA (with cement equivalent factor k=0.4) appear to be mandatory. However, from the examples given in the Introduction section, it is known that these C20, C25 and C30 class concretes can be and are produced with lower cement dosages. Therefore, durability and 50-year economic life conditions are not being met.

Solution Alternatives

The first and easiest solution that comes to mind is to continue using the current CEM I cement and mineral additives, but to increase the cement dosage to the level required by the mandatory environmental exposure class XC1 at 240 kg/m³ (or 270 kg/m³ if XS1 is required). This solution will increase both the cost per cubic meter of concrete and its strength compared to the current composition. The cost increase will put pressure on the ready-mixed concrete producer, which is already excessively competitive and unprofitable. If cement and mineral additives are not to be changed, ways to partially offset the increase in strength and cost can be sought in changing aggregates and chemical additives: - A transition can be made to a crushed rock quarry with lower performance and cost. - A transition can be made to a natural sand source with lower performance and cost. - A transition can be made to a chemical admixture with lower performance and cost; usage rate can be reduced. Ready-mixed concrete producers will engage in this type of optimization search that they are accustomed to from public projects with technical specifications containing minimum cement dosage and maximum W/C ratio, and will produce partially successful solutions. For example, a plant using Cebeci limestone will transport Cendere sandstone; will substitute Sinekli natural sand instead of Arnavutköy; will switch from superplasticizer admixture to mini-super. It is not mandatory for the minimum 250 kg/m³ cement that the standard requires for the XC1 environmental exposure class to be CEM I 42.5 R. The standard allows the cement to be CEM II A or B, CEM III A or B, with some limitations CEM IV or CEM V. Let us call this cement, which is expected to be lower strength and logically cheaper than CEM I 42.5 R, CEM *. CEM * can be used directly in place of CEM I in rough concretes, screeds and similar applications without issue. CEM * can also be easily used alone in lower strength classes. As the class strength increases, it would probably be a more appropriate approach to use it in blended form with CEM I 42.5 R both to optimize early and final strength and to optimize cost per cubic meter of concrete. In blended use, CEM * cement can also be evaluated as if substituted for GGBFS.

Table 4 - Alternative CEM * cement concretes

The CEM * cement in Table 4 is approximately TRY 20/tonne (approximately 8%) cheaper than CEM I 42.5 R. Using 340 kg/m³ instead of 320 kg/m³ CEM I 42.5 R with a ~6% increase can provide equivalent strength at 28 days; while the 1-day strength is lower, it is at a usable level and appears to be substitutable. From Table 4 it can also be seen that CEM * cement can be substituted for GGBFS: using 100 kg/m³ CEM * instead of 100 kg/m³ GGBFS positively affected strengths. CEM * cement is probably somewhat more expensive than GGBFS, which will have a slightly negative impact on cost; moreover, it will not help with the hydration heat problem in mass concretes. But it will provide a solution to the durability conditions required by standards and codes.

Conclusion

It is a positive development that durability in reinforced concrete has become a property that is given equal importance to strength and is actively sought and monitored. The ready-mixed concrete sector will quickly adapt to this transition, and cement producers will implement alternative production of CEM II, CEM IV, CEM V to their CEM I 42.5 R cements or will diversify existing ones. Over time, optimal solutions will emerge.   Prof. Dr. Erbil Öztekin Chryso Technical Consultant