Natural Pozzolans

Chapter 2 Natural pozzolans

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2.1 GENERAL

The term pozzolan originates from the ancient Roman town of Puteoli (Pozzuoli), the glassy pyroclastic rocks of which had long been known to react with lime, inMaterials powdered form, under moist conditions to attain bind- ing characteristics. Today, however, pozzolan has a much wider definition that embodies all those natural and artificial inorganic materials that, in pulverised state and in the presence of water, react with calcium hydroxide to form compounds of cementitious value. This broad definition embraces many different materials in terms of origin, composition and structure. A precise classification of pozzolans is almost impossible, as the broad definition includes so many different materials.- However, pozzolanic materials can be divided into two major groups asTaylor natural pozzolans and artificial pozzolans. The basic difference between the two groups is that the former do not need any treatment except grinding whereas the latter are either obtained by the heat treatment of materials with no or very little pozzolanicity or as industrial by-products which experienced high temperatures and then sudden cooling during the industrial processes from which theyand are obtained. A fairly comprehensive classification of pozzolans which was proposed by F. Massazza (1974) and updated with small modifications later (Massazza, 1988) is given in Figure 2.1. Various artificial pozzolans will be discussed later in the relevantFrancis chap- ters. This chapter will be devoted to natural pozzolans. Natural pozzolans can be subdivided into two categories as materials of volcanic origin (pyroclastic) and materials of sedimentary origin (clastic). The first category includes materials formed by the quenching of molten magma when it is projected to the atmosphere upon explosive volcanic eruptions. The explosive eruption has two consequences: (1) The gases, originally dissolved in the magma are released by the sudden decrease of pressure. This causes a microporous structure in the resulting material. (2) Rapid cooling of the molten magma particles when contacted with the atmosphere results in quenching, which is responsible for the glassy state of the solidified material. Pozzolanic materials of volcanic origin may be

5 6 Cement and Concrete Mineral Admixtures

Italian pozzolans, Incoherent rocks Santorin earth Pyroclastic rocks (volcanic) Coherent rocksZeolitised materials Trass, tuff olans zz Materials of organic po Diatomaceous earth origin Clastic rocks (sedimentary) Materials of simple Clays CopyrightedNatural deposition Hybrid rocks (mixed)

Burnt clays Thermally activated materials olans Burnt shales zz po Fly ash tificial Ar By-productMaterialss Silica fume

Rice husk ash

found in loose (incoherent) or compacted- (coherent) forms in nature. The latter results from the post-depositional processesTaylor such as weathering, com- paction, cementation and hardening of the originally loose material. These processes may change the original structure into clayey or zeolitic charac- ter. Transformation into clayey structure reduces the pozzolanicity whereas zeolitisation improves it (Massazza, 1988). The second category of natural pozzolans includes andclays and diatomaceous earth. Clays have very limited pozzolanic reactivity unless they are thermally treated. Diatomaceous earth, which is a sedimentary rock, con- sists basically of the fossilised remains of diatoms (a type of algae).Francis It has an amorphous siliceous structure but may contain crystalline phases upto Downloaded by [Taylor and Francis/CRC Press] at 07:59 11 August 2016 30%, by mass (Aruntaş et al., 1998). Scanning electron microscopy images of several natural pozzolans are given in Figure 2.2.

2.2 CHEMICAL COMPOSITION

The chemical compositions of natural pozzolans vary widely with silica

(SiO2) prevailing over the other constituents. Alumina (Al2O3) has the second highest mass and then comes iron oxide (Fe2O3). In most of the stan- dards related to the use of natural pozzolans in cement and concrete, the Natural pozzolans 7

(a) (b)

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Materials

Figure 2.2 SEM images of various natural pozzolans: (a) volcanic ash. (Courtesy of Ilker Tekin.); (b) trass. (Courtesy of TÇMB R&D Institute, Ankara, Turkey.); (c) raw perlite. (Courtesy of TÇMB R&D Institute, Ankara, Turkey.) and (d) diatomite. (Reprinted with permission from Aruntaş, H.Y. 1996. Usability of diatomites as pozzolans in cementitious systems,- PhD thesis, Gazi University Institute of Natural and Applied Sciences [In Turkish].)Taylor sum of these three oxides is limited to a minimum (70%, by mass) in order for the pozzolan to be a suitable one, besides several other requirements which will be discussed in detail in Chapter 13. Examples for the chemical compositions of various incoherentand and coherent pyroclastic pozzolanic materials and diatomaceous earths are given in Table 2.1. The chemical compositions of pyroclastic materials depend on the composition of the ejected magma. Diatomaceous earth is very rich in silica since it is formed from the siliceous skeletons of diatoms. Francis Downloaded by [Taylor and Francis/CRC Press] at 07:59 11 August 2016

2.3 MINERALOGICAL COMPOSITION

Similar to the variability in chemical compositions, many different minerals may be present in natural pozzolans. However, the fundamental constituent is always a glassy phase of siliceous nature which can be dis- tinguished by the halo corresponding to about 23° 2θ (Cu Kα) that is close to the main peaks of various crystalline forms of silica in the x-ray diffractograms. Glass content of natural pozzolans of volcanic origin generally varies from 50% to 97% and the rest are mostly clay minerals, quartz 8 Cement and Concrete Mineral Admixtures 9.11 8.29 LOI 11.10 3 SO 2 O TiO 2 OK 2 Francis 0.61 0.21 0.21 and CaO MgO Na 3 O 2 Fe 3

O Taylor 2

Al - 2 SiO 85.97 2.30 1.84 54.68 17.70 3.82 3.6688.32 0.95 3.47 3.43 0.48 6.38 0.42 0.26 0.17 0.28 0.18 5.84 57.74 12.14 3.84 4.8484.24 0.55 4.75 0.64 0.91 2.50 0.94 0.73 0.26 0.30 0.15 11.36 0.25 0.09 8.47 71.65 11.77 0.81 0.88 0.52 1.80 3.44 0.34 9.04 48.52 17.49 7.80 7.84 1.41 5.20 3.10 2.02 0.29 1.75 76.57 9.99 0.96 0.51 0.03 – 5.58 – 0.04 5.23

Materials Reference Mielenz et al. (1950) Mielenz et al. Massazza (1988) (1998) Arunta ş et al. Johansson and Andersen (1990) and Johansson 75.6 8.62 6.72 1.10 1.34 0.43 1.42 1.38 2.15 ğ du (1996) Erdo (1995) Tonak Kasai et al. (1992) Kasai et al. ğ du (1996) Erdo Meral (2004) Takemoto and Uchikawa (1980) and Uchikawa Takemoto 71.77 11.46 1.14 1.10 0.54 1.53 2.55 0.14 – 6.50 Copyrighted Chemical compositions of various natural pozzolans (%)

earth)

Clastic (diatomaceous Pyroclastic (coherent)Pyroclastic Ludwig and Schwiete (1963) 52.12 18.29 5.81 4.94 1.20 1.48 5.06 Pyroclastic (incoherent) (incoherent) Pyroclastic Costa and Massazza (1974) 53.08 17.89 4.29 9.05 1.23 3.08 7.61 0.31 0.65 3.05 Table 2.1 Table Type Downloaded by [Taylor and Francis/CRC Press] at 07:59 11 August 2016 2016 August 11 07:59 at Press] Francis/CRC and [Taylor by Downloaded Natural pozzolans 9

Table 2.2 Minerals in various natural pozzolans Type Reference Mineral phases Pyroclastic Costa and Massazza (1974) Glass, feldspars, quartz, olivine, clay (incoherent) minerals Mehta (1981) Glass, quartz, anorthite, labradorite Erdoğdu (1996) Glass, augite, albite, hornblende CopyrightedMielenz et al. (1950) Glass, calcite, quartz, feldspar, sanidine, montmorillonite Pyroclastic Ludwig and Schwiete (1963) Glass, quartz, feldspar (coherent) Erdoğdu et al. (1999) Glass, quartz, calcite, albite, sanidine Erdoğdu (1996) Glass, clinoptilolite, quartz, sanidine, illite Clastic Aruntaş et al. (1998) Glass, quartz, feldspar, smectite (diatomaceous Takemoto and Uchikawa Opal, quartz, cristobalite earth) (1980)

and feldspars. GlassyMaterials phase in diatomaceous earths may be as low as 25% and some may be almost totally glassy. The remainder is composed of clay minerals, quartz and feldspars. Minerals in some natural pozzolans are listed in Table 2.2 and examples of the x-ray diffractograms are shown in Figure 2.3.

(a) s+a - a: Anorthite m s+a Taylors: Sanidine q: Quartz m: Muscovite p: Plagioclase

m+s+a s′: Smectite

s and q

s+a a s a m+s+a q m+s+a a q q a Glassy phase Francis Downloaded by [Taylor and Francis/CRC Press] at 07:59 11 August 2016 (b) ′ s p s′

Glassy phase 10 20 30 40 50 60 2θ

Figure 2.3 X-ray diffractograms of two natural pozzolans: (a) trass. (Courtesy of TÇMB R&D Institute, Ankara, Turkey.) and (b) diatomaceous earth. (With permission from Aruntaş, H.Y. 1996. Usability of diatomites as pozzolans in cementitious systems, PhD thesis, Gazi University Institute of Natural and Applied Sciences [In Turkish].) 10 Cement and Concrete Mineral Admixtures

2.4 FINENESS

Natural pozzolans are used in cementitious systems after they are finely ground to a desired fineness which generally lies between 350 and 600 m2/kg (Blaine specific surface). When they are used as a component in blended cements, they are usually interground with PC clinker. Copyrighted

Materials

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