Greek supplementary cementing materials and their incorporation in concrete

Abstract

Sustainable development of the building sector can be achieved by a significant incorporation of cementitious and pozzolanic by-products, such as fly ash and slag, as well as some natural pozzolanic materials (supplementary cementing materials––SCM). Millions of tons of SCM, especially in Greece, are dumped due to overproduction or non-conformity with the existing standards. In this work, various types of SCM produced in Greece are investigated for a potential use in concrete. Their behavior as regards strength and durability is approached by a practical efficiency factor (k-value). The work is further focused on fly ashes, as they constitute the vast and more active majority of Greek SCM. The effectiveness of the Greek fly ashes in concrete improvement is widely proved through strength and durability measurements at laboratory and pilot-plant scale. However, they contain high amounts of free lime and sulphates, which may cause durability problems. In addition, they exhibit inadequate fineness and composition variations. Thus, an appropriate treatment is required and proposed by the present study in order to eliminate or reduce any harmful factors. Finally, a mix design strategy ensuring optimum strength, durability and ecological profile is proposed.

Introduction

Supplementary cementing materials (SCM) may be divided into natural materials and artificial ones. To the former belong true pozzolanas and volcanic tuffs. To the second category belong siliceous by-products, such as fly ashes, condensed silica fume and metallurgical slags (blast furnace slag, steel slag, non-ferrous slags). As is well known [1], [2], [3], [4], [5], [6], these materials can be used in concrete production either as blended cement constituents or as separate concrete admixture.

In Table 1 the category, location, industry, production rates, way of disposal, etc., for all SCM produced in Greece are given. In the main category of natural materials about 700,000 t/year are quarried and the vast majority is used in the cement industry as additions in the production of pozzolanic cements. These materials are mainly volcanic deposits located in Cyclades Islands and Macedonia. Some other materials, such as diatomeous earth, are being investigated for use in cement production. In the other main category of industrial by-products about 12,000,000 t/year are produced and only the 13% is used, mainly in the cement industry, leaving a vast amount of 10,500,000 t/year unexploited. The main part of these materials is fly ash produced by Public Power Corporation (PPC). The total production of fly ash is 9,500,000 t/year and only about 10% is used; the rest is placed back to the lignite quarries. About 77% of this fly ash amount is produced in various plants in Macedonia (Agios Dimitrios, Kardia, Ptolemais, Amintaio: Fly ash Ptolemais) and the rest in Peloponnese (fly ash Megalopolis). The second in the importance, is a nickel slag produced by LARKO SA in Larymna, Central Greece at a rate of 1,500,000 t/year. A 42% of this slag is used mainly by the cement industry. Various other slags (steel and pig iron slags) are produced at much lower rates. Finally, red mud, a by-product with potential pozzolanic properties is produced by the aluminium industry (Aluminium of the Greece, Central Greece) at a rate of 650,000 t/year and this remains totally unexploited.

A typical composition of the main Greek SCM in terms of oxide analysis is given in Table 2. As regards Greek fly ashes, they exhibit high contents in calcium due to lignite form of the burnt coal. However, they may be divided into two sub-categories, according to the total calcium content:

• Fly ash Megalopolis. The CaO content in these ashes varies between 5% and 15% and thus they may be characterized as cementitious and pozzolanic mineral admixtures (category II, according to RILEM classification [3]) or as normal pozzolans (category IV). They could also be characterized as Type F according to ASTM C 618 [5], as the sum of the oxides SiO₂ + Al₂O₃ + Fe₂O₃ is greater than 70%.

• Fly ash Ptolemais. The CaO content in these fly ashes of Northern Greece is always greater than 20% and thus they can be characterized as cementitious mineral admixtures (category I, according to RILEM classification [3]). They could also be characterized as Type C according to ASTM C 618 [5], as the sum of the oxides SiO₂ + Al₂O₃ + Fe₂O₃ is between 50% and 70%.

The most serious problem in the use of these fly ashes, especially of fly ash Ptolemais, is the high contents in sulphur, which may introduce expansion problems in a concrete made by these materials. Another potential problem is the high content in free lime (1/4–1/3 of the total CaO), which also may create problems of expansion at early ages. Both types of ashes contain titanium, phosphorous, unburnt carbon, organic materials and other trace elements. Another indicative component is that both ashes are coarse with a retained amount in the 45 μm sieve between 50% and 60%, and thus they do not comply with the European Standards (<35% [7]). Their density varies between 2300 and 2600 kg/m³. Finally, the large variation in the quality of these ashes (chemical and physical characteristics) introduces skepticism in their efficient use.

The Greek fly ashes have been used since 1980 in the production of pozzolanic cement, and their efficiency has been proved in practice. Since there are no regulations for their use in concrete and other related applications, their exploitation is limited only in cement production, in spite of the accumulated knowledge and the successful applications in practice [8], [9], [10], [11]. The only application until today was the construction of a dam (Platanovrissi, Drama, Macedonia) by PPC and AEGEK (a construction company) using the method of roller compacted concrete (RCC). Fly ash Ptolemais was the main cementitious material (80%), which was selected, homogenized and treated (by grinding with partial hydration) in order to fulfill special requirements [12], [13].

The rest of the materials presented in Table 2 (slags and red mud) can be characterized as mineral admixtures of low reactivity (category V, according to RILEM classification), as they are slowly-cooled by-products. They consist essentially of crystalline materials, and relatively small amounts of non-crystalline matter. These materials are extremely coarse and must be pulverized to very fine particle size in order to develop satisfactory cementitious or pozzolanic activity.

Based on the above classification for the Greek SCM, these can be used in the same construction applications as all the corresponding materials which belong to the same category, e.g. as a separate constituent in concrete (for common buildings, foundations, dams, tunnels, reservoirs, bridges, highway pavements, silos, stadiums, etc.), prefabricated and prestressed concrete, in mortars, as soil stabilizers, etc. [5], [6], [14], [15], [16], [17], [18]. However, the suitability of the Greek SCM has to be proved by experiments, their effect on durability has to be studied in detail, and, finally, an appropriate mix design strategy has to be proposed; all these are exactly the main objectives of the present work.