hydrated cements and geopolymers

Construction and Building Materials 101 (2015) 1105–1112

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Radiological characterization of anhydrous/hydrated cements and geopolymers F. Puertas a,⇑, M.M. Alonso a, M. Torres-Carrasco a, P. Rivilla a, C. Gasco b, L. Yagüe b, J.A. Suárez b, N. Navarro b a b

Eduardo Torroja Institute for Construction Sciences (IETcc-CSIC), Madrid, Spain CIEMAT, Madrid, Spain

h i g h l i g h t s
Activity concentration in hydrated blended and non-blended cements is first time studied.
Radioactivity in alkaline cements or geopolymers is also first time investigated.
Radioactivity of blended cements with industrial wastes may be optimised controlling their final dosage.

a r t i c l e

i n f o

Article history: Received 14 May 2015 Received in revised form 24 August 2015 Accepted 15 October 2015

Keywords: Cements Industrial wastes Alkaline cements NORMs Radioactivity Index

a b s t r a c t The aim of this study has been to determine the activity concentrations of 40K and radionuclides from the 238 U and 232Th decay series in commercial cements, some industrial wastes and geopolymers (also called alkaline cements) and to calculate their activity concentration levels after the cement hydration/activation at 28 days to ascertain the effect of these processes on their final activity concentrations. The highest activity concentration levels of radionuclides from uranium and thorium series have been obtained in the fly ashes, slags and calcium aluminate cement. When a blend among industrial wastes and cement is realised, the radionuclides activity concentration are calculated on the grounds of their percentages in the blend material and tested by its experimental measurement. Once the hydrated/activated cements and geopolymers have been obtained, a proportional decrease to the percentage of added water is observed in the radionuclides activity concentration for the hydrated cements and no-correlation in the case of 40K in geopolymers pointing to an increase of this isotope in the sodium compounds used for its activation. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction Construction and more specifically the Portland cement industries are excellent targets for the reuse and valorisation of waste and by-products from a wide variety of industries. Given the adverse effects inherent in Portland cement manufacture (high thermal and electrical energy demand, need to quarry large quantities of limestone and clay and the emission of greenhouse gases, especially CO2), the study and development of more eco-efficient cements based on the reuse of waste of varying origin is a priority line of research and technological innovation in the pursuit of industry sustainability. A broad range of types of waste can be valorised for use in eco-efficient cements.

⇑ Corresponding author. E-mail address: [email protected] (F. Puertas). http://dx.doi.org/10.1016/j.conbuildmat.2015.10.074 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

A number of procedures are in place to obtain more ecoefficient cement. Perhaps one of the most widespread practices entails the use of secondary cementitious materials (SCMs) to replace part or nearly all of the clinker in Portland cement. Of the 27 types of ordinary cement listed in European standard EN 197-1:2011 [1], 26 contain some manner of mineral addition: limestone, blast furnace slag, siliceous or calcareous fly ash, sintered schist or silica fume. Most of these additions are industrial by-products or naturally occurring radioactive materials (NORMs). Industrial waste and by-products are used not only as clinker substitutes, but may also be added during clinkerisation itself, partially or totally replacing the prime materials in the raw mix (limestone in particular) or the fuel. Very different types of waste or by-products can be used as partial raw mix replacements, including crystallised blast furnace slag [2] or waste from the manufacture of clay-based products [3] or aluminium recycling [4]. In

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Spain in 2011 only 2.8 million tonnes of wastes (8.5% of total raw materials), primarily slag, steam power plant ash, iron slag, paper mill sludge, casting sand and sugar refining foam, were used as an alternative raw material in Portland cement production. Efforts are also being made to use alternative fuels in OPC manufacture: in countries such as Netherlands, Austria, Germany and Norway, these fuels account for over 60% of the total. The sources vary widely in nature, including shredded tyres, solvents, water treatment plant sludge and used oil. In Spain the replacement rate is around 23% [5], some of which consists of NORMs. Another avenue for manufacturing eco-efficient cements is the development of new materials wholly different from ordinary Portland cement. Thanks to their excellent properties, alkali-activated cements (also known as geopolymers), are among the most prominent of these new materials [6–10]. These cements are defined as the binders resulting from the chemical interaction between highly alkaline solutions and natural (clay) or artificial (industrial waste or by-products) aluminosilicates with a high or low Ca content. Alkaline activation calls for two basic components: preferably amorphous or vitreous aluminosilicates and an alkaline activator. The aluminosilicates may be natural products such as metakaolin or industrial by-products such as blast furnace slag or aluminosiliceous fly ash. The alkaline solutions able to interact with aluminosilicates to generate such new binders include alkaline metal or alkaline-earth hydroxides (ROH, X(OH)2), weak acid salts (R2CO3, R2S, RF), strong acid salts (Na2SO4, CaSO42H2O) and R2O (n)SiO2-type siliceous salts known as waterglass (where R is an alkaline ion such as N, K or Li). From the standpoint of end product strength and other properties, the most effective of these activators are NaOH, Na2CO3 and sodium silicate hydrate (waterglass). Other industrial by-products are presently also being studied for use as possible alkaline activators. Patents have been awarded for the use of industrial waste or by-products such as ash from rice husks, silica fume and urban and industrial vitreous waste as potential alkaline activators to replace the family of substances known as waterglass [11–13]. Here also, the main components of these cements are NORMs. The foregoing is indicative of the high potential for reuse and valorisation of industrial waste and by-products in the manufacture of cement and other construction materials. To be apt for such purposes, the waste must exhibit certain chemical, physical and microstructural characteristics that favour their reactivity and behaviour. In addition, as NORMs, their natural radioactivity must also be determined. These are among the primary objectives addressed in the research conducted under COST Action TU1301 NORM4BUILDING [14]. The ultimate objective is to obtain cements containing industrial waste (such as fly ash, slag, silica fume and glass) with minimal radioactivity and optimal chemical and physical properties. For this purpose a radiological characterization of raw and processed materials have to be performed, in order to establish potential limitations for their use as building materials. Depending on the material, the concentration of natural radionuclides (mainly 226Ra, 232Th and 40K) ranges from 1 to 4000 Bq kg1 [15,16]. The most recent legislation in Europe about safety standards for protection against the dangers arising from exposure to ionising radiation is the European Directive 2013/59 Euratom [17]. This specifies that building materials are in the scope of this regulation but also with the existing standards about construction products. The indoor gamma radiation emitted from these has to be established introducing requirements on the recycling of residues from industries processing naturally-occurring radioactive materials. The quantity adopted in this Article 75, as a reference level for indoor external exposure due to gamma radiation emitted by building materials, is 1 mSv per year. An activity

concentration index ‘‘I” is defined in the Annex VIII – calculated with Formula (1) – to accomplish this level.

I ¼ ðC 226Ra =300 þ C 232Th =200 þ C 40K =3000Þ

ð1Þ

where: C226Ra, C232Th, C40K are the activity concentrations of 226Ra, 232Th and 40K in building materials expressed in Bq kg1. This mentioned index is a conservative screening tool for identifies materials that may cause the reference level laid down in Article 75(1) to be exceeded. The calculation of dose needs to take into account other factors such as density, thickness of the material as factors relating to the type of building and intended use of the material (bulk or superficial). The index relates to the gamma radiation dose, ‘‘in excess” of typical outdoor exposure, in a building constructed from a specified building material. The index applies to the building material, not to its constituents except when those constituents are building materials themselves and are separately assessed as such. For application of the index to such constituents, in particular residues from industries processing naturallyoccurring radioactive material recycled into building materials, an appropriate partitioning factor needs to be applied. It has to be highlighted that the activity concentration index should be considered as screening tool for identifying materials which might be of concern: ‘‘The use of industrial by-products containing natural radionuclides in building materials which could result in activity concentration indices exceeding the values specified in these recommendations should be justified on a case by case basis by Member States”. When industrial by-products containing natural radionuclides are used in building materials is recommended separate controls on such by-products at the point of entry into the building industry. Some previous reflections about the significance of this index were done in the guidance (1999) [18] to harmonise the controls of European States to allow movement of building products. Its purpose was – as the European directive – to limit the radiation exposure due to materials with enhanced or elevated levels of natural radionuclides. The dose criterion was determined depending on the use of the construction material: bulk or located in some parts of the building (basement, roof, walls, etc.). A material used in bulk amounts with an index I 6 0.5 means a dose of 0.3 mSv a1 and with an index I 6 1 a dose of 1 mSv a1. In superficial and other materials with restricted use: tiles, boards, etc., an index I 6 2 means a dose of 0.3 mSv a1 and I 6 6 a dose of 1 mSv a1. Calpena [19] belonging to the European Alara Network for NORM, in its 6th workshop showed and explained the new regulatory framework European regulation for marketing of NORM residues and building materials. Several authors have studied the natural radioactivity of cements and their constituents [20–26]. Misdaq et al. [22] showed that the radon emission coefficient and production rate in building materials depend on their porosity. Turhan et al. [23] found no significant radiological hazards in the Turkish cement used in building construction. Kovler et al. [25] have demonstrated that despite higher 226Ra content in fly ashes (more than 3 times to OPC), the radon-exhalation rate is significantly lower in fly ashes than in OPC. In other study [26] the same author appointed that although radon exhalation rate in concretes containing fly ashes can be slightly higher than those of reference concrete, radon emanation coefficient is lower. In a systematic study of natural construction materials, Piedecausa et al. [27,28] revealed the need to establish criteria for monitoring such waste and analysed risk rates by country. Nonetheless, no thorough data have been published comparing the radioactive content in hydrated paste to the

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radioactivity of its component materials. Nor has the literature addressed the radioactive rates and content in alkali-activated cements or geopolymers, materials of great promise in the twenty-first century. The original radioactive content in several types of cement (ordinary Portland, calcium aluminate cements and alkaliactivated cements) and their constituents were determined in the present study. The activity concentration after cement hydration/activation was also analysed to ascertain the effect of these processes on final radioactivity.

Table 2 Sample preparation. Hydration and activation conditions.

2. Experimental work

Table 1 gives the XRF-determined chemical composition of the materials used in this study. In addition to the commercial cements listed, other binders were prepared by blending CEM I 52.5R cement and mineral additions (limestone, slag, fly ash and silica fume) as follows: CEM CEM CEM CEM

Solution

L/S ratio

SiO2/Na2O

OPC W-OPC CAC OPC + 10% SF OPC + 50% S OPC + 50% FA OPC + 50% L Wg-AAS Glass-AAS N/15Wg-AAFA

Water Water Water Water Water Water Water Sodium silicate NaOH/Na2CO3 + glass waste NaOH 10 M + 15% wt sodium silicate NaOH 10 M + glass waste

0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3

– – – – – – – 0.86 0.86 0.19

0.3

0.11

Glass-AAFA

2.1. Materials

50% 50% 50% 90%

Sample

I + 50% I + 50% I + 50% I + 10%

slag (OPC + 50% S), fly ash (OPC + 50% FA), limestone (OPC + 50% L), silica fume (OPC + 10% SF).

The preparation of these blended cements is previously described [29]. 2.2. Cement hydration and alkaline activation Table 2 lists the cements and hydration or activation conditions used in this study. After hydration, the Portland cement and CAC samples were stored at 98 ± 2% RH and 22 ± 2 °C. Cement pastes were also prepared by conventional alkaline activating slag (AAS) and fly ash (AAFA) as well as with glass waste as an alternative alkaline activator [11]. The AAS pastes were stored under the aforementioned conditions, whereas the AAFA pastes were cured at 85 ± 2 °C for the first 20 h and subsequently in a climatic chamber (RH = 98 ± 2%; T = 22 ± 2 °C). The samples were treated with acetone and ethanol after 28 days to detain hydration/activation [30]. 2.3. Gamma spectrometry analysis Radiological characterization of the samples was carried out by gamma spectrometry. Radionuclides occurring in natural decay series headed by 238U and Th as well as 40K were determined in the samples using a system constituted by three High-purity Germanium detectors. One of the detectors is coaxial Type p, and the other two, BEGe Type (Broad Energy Germanium detectors). The detectors have an active surface from 26 to 38 cm2 and 2 keV resolution for 60Co and intrinsic efficiencies from 30% to 100%. They are protected with 15 cm iron shield for avoiding background radiation (Fig. 1).

232

The gamma spectroscopy system connects the detectors to an electronic chain supported by Software Genie-2000, developed by CANBERRA that allows the calibration, acquisition, visualisation, analysis and storage of the obtained gamma spectra. Mathematical efficiency calibrations of the detectors in the energy range 45 keV to 7 MeV were carried out with LabSOCS software, developed by CANBERRA and based on detector characterization by MCNP Monte Carlo modeling code. 226 Ra and 232Th determination was based on the detection of emissions of their daughter nuclides, 214Pb and 228Ac respectively, since their own gamma emitting lines are not of sufficient intensity (232Th at 63.8 keV) or present significant interferences with other naturally occurring radionuclides (186.1 keV from 226Ra and 185.7 keV from 235U). In order to ensure secular equilibrium between 226Ra and 232 Th and their progenies, measurement containers (plastic cylinders 75.4 mm of diameter, 31 mm height), were hermetically sealed to avoid 222Rn losses and stored for at least 25 days before their measurement. 40K was directly measured by its emission at 1460.8 keV. Counting times of 60,000 s were selected for all the samples. Table 3 summarizes the energy and emission probabilities of the gamma lines used for the radiological characterization of the samples. The detection limits (DL) entered in the software was calculated as described in international standard ISO-11929 [31]. 2.4. Activity concentration index The activity concentration index normally used to characterise building materials poses certain practical problems. One is that 226Ra and 232Th are determined indirectly by measuring their daughters in the natural radioactive series, assuming that they are in equilibrium. This is the procedure of choice because these radionuclides are more readily identified in the spectrum or their emission probability is higher at the peak studied. 226Ra reaches equilibrium after 20–30 days, forming 214 Bi and 214Pb, providing the container holding the sample can be sufficiently sealed and no 222Rn is lost through its walls. In addition to 40K, 214Pb (226Ra) and 228 Ac (232Th) were selected here for calculating the index in the cements and potentially recyclable materials, blended cements containing both and the end product, i.e., hydrated cement containing some of the recyclables.

Table 1 Chemical and physical composition of commercial cements, mineral additions and glass waste (% wt). OPC

White-OPC (W-OPC)

CAC

Limestone (L)

Slag (S)

Fly ash (FA)

Silica fume (SF)

Glass waste (WGlass) (<45 lm)

LoI SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5 SO3 SiO2 reactive Blaine (m2/kg) BET (m2/g)

2.3 20.5 5.4 2.1 0.02 3.7 57.0 0.6 1.4 0.2 0.1 6.4 – 501.7 1.2

1.8 19.0 3.3 0.3 1.7 1.80 68.0 1.3 0.4 0.0 0.1 3.6 – – –

0.2 3.3 44.9 15.0 – 0.8 33.5 0.3 – 1.5 – – – 342 –

43.6 0.3 0.04 0.1 0.01 0.9 54.6 0.4 – 0.01 0.1 – – – 4.4

2.7 35.3 13.6 0.4 – 4.1 41.0 0.01 – – – – – 325 –

6.8 46.3 31.0 4.5 0.1 1.3 4.9 0.3 1.3 1.5 1.0 1.0 36.4 – 2.7

4.0 94.3 0.2 0.1 0.02 0.2 0.5 0.1 0.4 – 0.05 0.1 92.7 – 20.3

– 70.7 2.0 0.5 – 1.2 11.8 11.7 1.1 0.1 0.04 – – – –

Dv (lm) 10 50 90

1.2 7.1 22.5

2.1 15.5 37.4

2.0 13.7 49.1

0.8 3.6 35.1

0.9 13.1 38.4

1.8 13.8 59.3

0.3 10.5 –

– – –

LoI: Loss on ignition; CAC: Calcium aluminate cement.

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Fig. 1. Gamma spectrometry system.

Table 3 Peak energy values used and probability of emission. Radionuclide

Energy (keV)

Probability of emission (%)

40

1460.82 277.37 583.19 860.53 238.63 300.09 351.93 911.20 968.96 63.30

10.55 6.60 85.00 12.40 43.60 3.18 35.60 26.20 15.90 3.75

K 208 Tl

212

Pb

214

Pb Ac

228

234

Th

The samples were split in two aliquots for raw materials and blends and measured by gamma spectrometry. The activity concentration index has been calculated in each aliquot and an average value has been determined by each type of material. Entire samples were analysed in case of hydrated cements. The activity concentration index I was calculated from the following equation:

I ¼ ðC 214Pb =300 þ C 228Ac =200 þ C 40K =3000Þ

ð2Þ

Its uncertainty was computed as:

uðIÞ ¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1=300Þ2 u2 ðC 214Pb Þ þ ð1=200Þ2 u2 ðC 228Ac Þ þ ð1=3000Þ2 u2 ðC 40K Þ

ð3Þ

where: I = activity concentration index; C214Pb = 214Pb activity concentration; C228Ac = 228Ac activity concentration; C40K = 40K activity concentration; u(C214Pb) = uncertainty of 214Pb activity concentration; u(C228Ac) = uncertainty of 228Ac activity concentration; u(C40K) = uncertainty of 40 K activity concentration. The mean index uncertainty was calculated as:

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uðIÞmean ¼ 1=2 u2 ðIÞ1 þ u2 ðIÞ2

ð4Þ

where: u(I)mean = uncertainty of index mean, u2(I)1 = uncertainty of the index of the first measurement, u2(I)2 = uncertainty of the index of the second measurement.

3. Results and discussion 3.1. Radioactivity The findings for the anhydrous cements, mineral additions, blended cements containing mineral additions and hydrated/ activated cements are given in Tables 4–6. The aims pursued in the study of these building materials were as follows. (i) The first was the determination of the radioactivity of natural gamma emitters, focusing on the radionuclides before and after 226Ra in the uranium series, namely 234Th and 214Pb, respectively. Equal activity concentration values for all radioactive members of the decay chain denote secular equilibrium. In the 232Th series, 228Ac, 212Pb and 208Tl

were determined. Due to radioactive decay, 208Tl constituted just 36% of the activity of its parent radionuclide. As 40K is not a member of any series, its activity was quantified separately. (ii) The second was the quantification of the activity concentration index not only for the anhydrous cements, but also for the (natural or recycled) mineral additions and the cements containing them, as well as the alkali-activated and hydrated cements. This index is used to determine radioactivity in building materials (concrete, clay brick, sand-lime brick, natural building stones, cement, tiles) as well as to optimise radioactivity in by-products and hydrated cement containing recycled waste. (iii) The third was the optimisation of the chemical, physical and radioactive characteristics of end or intermediate products. Natural radioactivity content was defined in the materials analysed in this study as lowest, intermediate or highest for each radioactive series and for 40K. The highest radionuclide content in the uranium series was found in coal combustion (fly ash) and metallurgical industry (blast furnace slag) by-products. The 226Ra content (i.e., the 214Pb or daughter level) ranged from 126 Bq kg1 (214Pb) in fly ash to 151 Bq kg1 (214Pb) in slag. The lowest values for 238U decay chain natural radionuclide content were found in glass waste, anhydrous OPC, limestone and silica fume. The activity concentration for the two considered radionuclides ranged from <2.9 Bq kg1 (234Th) to 1.4 Bq kg1 (214Pb). The intermediate values, 64.9 Bq kg1 (234Th) to 57.1 Bq kg1 (214Pb), were observed in anhydrous white OPC and CAC (see Table 4). The findings for the 232Th series followed a similar pattern, except for CAC, which exhibited the highest values, similar to the concentrations observed for fly ash: 130 Bq kg1 (228Ac) to 134 Bq kg1 (212Pb). The lowest activity concentration values, ranging from <0.6 Bq kg1 (212Pb) to 19.0 (212Pb) Bq kg1 were found for glass waste, limestone and silica fume, as in the 238U decay chain. In slag, radioactivity was lower in the thorium than in the uranium series, while intermediate concentrations were observed for slag and anhydrous OPC (see Table 4). 40 K is a natural potassium radioisotope present in every other components of the terrestrial crust. Here the highest values were found in glass waste, ranging from 513 to 873 Bq kg1 (40K). The lowest were recorded for white OPC, CAC, silica fume, slag and limestone, at <2.5–100 Bq kg1 (40K) in silica fume. The intermediate values were observed for OPC and fly ash, at up to 321 Bq kg1 (40K) in the latter. The radioactivity data in blended cements containing supplementary cementitious materials (SCMs) are given in Table 5. These data show that suitable mineral additions can reduce radioactivity in the by-product where the activity concentration in the starting material is known. Although here radioactivity was measured in the blends (cement + silica fume, cement + slag,

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Table 4 Activity concentration of natural radionuclides in cements and mineral additions (some of them are industrial wastes) (in Bq kg1) (uncertainty, k = 2) and activity concentration index (uncertainty, k = 2). Series

238

Material

234

OPC OPC Mean value

19.6 ± 1.9 20 ± 2.2 19.8 ± 1.5

19.30 ± 0.78 19.16 ± 0.75 19.23 ± 0.54

20.1 ± 1.0 17.8 ± 1.1 18.95 ± 0.74

19.16 ± 0.29 19.10 ± 0.58 19.13 ± 0.32

5.50 ± 0.37 6.02 ± 0.37 5.76 ± 0.26

237.5 ± 7.1 238.3 ± 7.5 237.9 ± 5.2

0.244 ± 0.0063 0.2325 ± 0.0064 0.2382 ± 0.0045

W-OPC W-OPC Mean value

54.2 ± 3.2 62.8 ± 5.5 58.5 ± 3.2

56.0 ± 1.2 58.3 ± 1.1 57.15 ± 0.81

4.77 ± 0.71 4.85 ± 0.77 4.81 ± 0.52

4.05 ± 0.52 4.68 ± 0.32 4.36 ± 0.31

1.27 ± 0.08 1.45 ± 0.24 1.36 ± 0.13

84.6 ± 5.1 84.7 ± 4.6 84.6 ± 3.4

0.2386 ± 0.0056 0.2468 ± 0.0055 0.2427 ± 0.0039

FA FA Mean value

131 ± 10 129 ± 10 130 ± 7.1

128.5 ± 1.8 126.2 ± 1.9 127.4 ± 1.3

129.6 ± 1.9 131.0 ± 2.4 130.3 ± 1.5

133.4 ± 2.0 134.2 ± 1.6 133.8 ± 1.3

40.01 ± 0.71 42.66 ± 0.90 41.33 ± 0.57

311.5 ± 7.3 321.4 ± 9.3 316.4 ± 5.9

1.18 ± 0.011 1.183 ± 0.014 1.1815 ± 0.0089

CAC CAC Mean value

75 ± 8.5 72.5 ± 9 73.8 ± 6.2

65.4 ± 1.4 64.4 ± 1.3 64.9 ± 0.96

130.4 ± 2.1 132.2 ± 2.3 131.2 ± 1.6

136.4 ± 1.8 138.4 ± 1.5 137.4 ± 1.2

41.68 ± 0.83 43.25 ± 0.84 42.42 ± 0.59

18.7 ± 3.6 64 ± 11 41.4 ± 5.8

0.876 ± 0.012 0.897 ± 0.013 0.8865 ± 0.0088

SF SF Mean value

<3.4 <2.9 –

1.72 ± 0.19 1.09 ± 0.37 1.4 ± 0.21

<1.6 <1.4 –

0.85 ± 0.34 0.79 ± 0.09 0.82 ± 0.18

0.52 ± 0.20 <0.54 –

99.8 ± 5.2 91.8 ± 4.6 95.8 ± 3.5

<0.047 <0.041 <0.044

S S Mean value

149.9 ± 6.5 163 ± 12 156.4 ± 6.8

150.9 ± 2.2 143.4 ± 1.6 147.2 ± 1.4

45.6 ± 1.5 45.80 ± 0.84 45.7 ± 0.86

42.5 ± 2.3 43.21 ± 0.66 42.9 ± 1.2

14.25 ± 0.53 15.17 ± 0.30 14.71 ± 0.30

75.7 ± 4.7 76.8 ± 2.8 76.3 ± 2.7

0.757 ± 0.011 0.7326 ± 0.007 0.7448 ± 0.0065

WGlass WGlass Mean value

12 ± 1.5 10.7 ± 1.7 11.4 ± 1.1

10.38 ± 0.27 7.07 ± 0.27 8.73 ± 0.19

5.98 ± 0.28 5.68 ± 0.35 5.83 ± 0.22

6.72 ± 0.16 5.84 ± 0.18 6.28 ± 0.12

1.940 ± 0.090 1.79 ± 0.12 1.867 ± 0.075

239.8 ± 7.4 213.8 ± 4.9 226.8 ± 4.4

0.1444 ± 0.003 0.1232 ± 0.0026 0.1338 ± 0.0020

L L Mean value

16.1 ± 3.4 18.8 ± 3.5 17.5 ± 2.4

17.82 ± 0.72 16.13 ± 0.70 16.98 ± 0.5

<1.4 <1.4 –

<0.43 <0.66 –

<0.43 <0.41 –

6.5 ± 2.2 <2.5 –

<0.068 <0.061 <0.064

U 214

Th

228

Pb

Ac

232

Th

212

Pb

Index 208

40

Tl

K

Table 5 Activity concentration of natural radionuclides in blended cement in (Bq kg1) (uncertainty, k = 2) and activity concentration index (uncertainty, k = 2). Series

238

Material (Blend)

234

232

U 228

Index Tl

OPC + 10% SF OPC + 10% SF Mean value

17.1 ± 2.2 18.3 ± 2.1 17.7 ± 1.5

17.28 ± 0.83 17.7 ± 1.1 17.49 ± 0.69

15.98 ± 0.50 17.4 ± 1.0 16.69 ± 0.56

17.60 ± 0.34 19.22 ± 0.85 18.41 ± 0.46

5.23 ± 0.33 6.31 ± 0.45 5.77 ± 0.28

229.8 ± 7.5 231.5 ± 7.7 230.6 ± 5.4

0.2141 ± 0.0045 0.2232 ± 0.0068 0.2186 ± 0.0041

OPC + 50% S OPC + 50% S Mean value

128 ± 7.1 87 ± 11 107.5 ± 6.5

81.8 ± 1.0 85.9 ± 1.5 83.85 ± 0.90

30.69 ± 0.69 32.1 ± 1.3 31.4 ± 0.74

31.37 ± 0.53 33.44 ± 0.84 32.40 ± 0.50

10.44 ± 0.23 10.99 ± 0.49 10.72 ± 0.27

156.2 ± 3.9 160.4 ± 6.2 158.3 ± 3.7

0.4781 ± 0.0050 0.5003 ± 0.0085 0.4892 ± 0.0049

OPC + 50% FA OPC + 50% FA Mean value

74.4 ± 8.1 82.4 ± 7.3 78.4 ± 5.5

70.39 ± 0.93 75.5 ± 1.4 72.90 ± 0.80

68.6 ± 1.0 75.2 ± 1.7 71.90 ± 0.99

68.90 ± 0.88 80.5 ± 1.1 74.70 ± 0.70

23.23 ± 0.39 25.56 ± 0.66 24.4 ± 0.38

268.7 ± 5.7 286.6 ± 8.3 277.6 ± 5.0

0.6670 ± 0.0063 0.723 ± 0.010 0.6951 ± 0.0060

OPC + 50% L OPC + 50% L Mean value

18.4 ± 1.8 25.9 ± 4.5 22.2 ± 2.4

18.97 ± 0.66 16.85 ± 0.26 17.91 ± 0.35

9.80 ± 0.31 8.96 ± 0.24 9.38 ± 0.20

10.31 ± 0.61 8.99 ± 0.15 9.65 ± 0.31

2.99 ± 0.09 3.01 ± 0.08 2.997 ± 0.062

120.1 ± 5.2 119.6 ± 2.4 119.8 ± 2.9

0.1523 ± 0.0032 0.1408 ± 0.0017 0.1465 ± 0.0018

Pb

212

Th 40

Th

214

Ac

208

Pb

K

Table 6 Activity concentration in blended and non-blended cements after hydration or alkaline activation (in Bq kg1) (uncertainty, k = 2). 238

Material

234

Raw

OPC W-OPC CAC

Blend

OPC + 10% OPC + 50% OPC + 50% OPC + 50%

Geopolymers

Wg-AAS Glass-AAS N/15Wg-AAFA Glass-AAFA

SF S FA L

U Th

214

Pb

228

Ac

232

Th

212

Pb

Index 208

Tl

40

K

16.3 ± 5.3 47.6 ± 3.4 60.0 ± 3.8

7.86 ± 0.60 26.96 ± 0.73 28.6 ± 1

12.2 ± 0.57 3.52 ± 0.55 86.7 ± 2.1

13.83 ± 0.60 3.42 ± 0.16 98.9 ± 1.8

4.3 ± 7.8 1.080 ± 0.091 29.19 ± 0.79

166.7 ± 6.7 59.0 ± 3.2 17.2 ± 3.4

0.1428 ± 0.0041 0.1271 ± 0.0038 0.535 ± 0.011

15.4 ± 4.4 71.2 ± 5.9 66.7 ± 6.7 15.7 ± 3.1

6.97 ± 0.59 41.5 ± 1.1 49.1 ± 1.3 9.35 ± 0.33

12.02 ± 0.98 23.0 ± 1.1 55.1 ± 1.7 6.55 ± 0.78

13.31 ± 0.58 22.76 ± 0.69 62.2 ± 1.1 7.09 ± 0.48

3.87 ± 0.31 7.49 ± 0.38 18.48 ± 0.64 2.30 ± 0.30

165.1 ± 6.8 114.1 ± 5.4 211.6 ± 8.0 86.8 ± 4.8

0.1384 ± 0.0057 0.2914 ± 0.0069 0.5097 ± 0.0099 0.0928 ± 0.0044

91.5 ± 5.6 94.4 ± 6.7 56.4 ± 5.7 57.4 ± 3.2

48.7 ± 1.1 54.5 ± 1.4 36.44 ± 0.97 37.9 ± 1.1

22.84 ± 0.71 23.78 ± 0.81 67.8 ± 1.4 62.2 ± 1.2

23.3 ± 0.69 24.99 ± 0.83 75.1 ± 1.8 75.1 ± 1.2

7.7 ± 0.39 8.04 ± 0.41 21.57 ± 0.59 22.62 ± 0.63

77.0 ± 5.0 89.2 ± 4.8 578 ± 15 550 ± 14

0.3022 ± 0.0054 0.3303 ± 0.0064 0.6531 ± 0.0092 0.6207 ± 0.0084

cement + fly ash and cement + limestone) by gamma spectrometry, it could have been deduced from the percentage of each constituent and the respective percentage of the radionuclide found with the original data and the empirical values in Table 4.

A number of observations are in order respecting the 28-day solid hydrated/activated cements and cements with mineral additions (Table 6) compared to the activity concentration of blended and non-blended cements (Tables 4 and 5). The activity

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1.40

Acvity concentraon index

1.20 1.00 0.80 0.60 0.40 0.20

0.00

Acvity concentraon index in raw materials and blends (blends in green colour) Fig. 2. Activity concentration index for materials (red line = reference level in EU; purple line = detection limit). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

1.00 45

0.90

35

0.70 0.60

25

0.50 0.40

% Index Decreasing

Acvity

concentraon index

0.80

15

0.30 0.20

5

0.10 0.00

-5

Acvity concentraon index(

hydrated and • non hydrated sample)

Fig. 3. Activity concentration index before and after hydration. Dotted line = percentage rise or decline in activity concentration index after hydration.

concentration was lower per unit of mass in the hydrated than in the anhydrous unblended and blended cements because the concentration of the materials themselves was lower due to the presence of hydration water (30–40%). The radionuclide activity concentration in alkaline cement pastes (Wg-AAS, Glass-AAS, N/15Wg-AAFA and Glass-AAFA) (see Table 6) was calculated on the grounds of the percentage of slag or fly ash in the anhydrous geopolymers and in the hydrated end product. 40K concentration rises in such activated materials because the potassium impurities often present in the NaOH activator raise the 40K potassium content in the end product.

A recent paper on natural radioactivity in building materials in the European Union [32] lists the materials of radiological significance in a number of European countries. In Spain 423 such materials were identified, 317 of which were summarised in the article. The mean results for brick were 54 Bq kg1 (226Ra), 68 Bq kg1 (232Th) and 569 Bq kg1 (40K); for concrete, 30 Bq kg1 (226Ra), 32 Bq kg1 (232Th) and 204 Bq kg1 (40K); and for cement 61 Bq kg1 (226Ra), 40 Bq kg1 (232Th) and 305 Bq kg1 (40K). The means and ranges for the 23 European Union countries were: for brick, 47(1-148) Bq kg1 (226Ra), 48(2-164) Bq kg1 (232Th), 598 (12-1169) Bq kg1 (40K); for concrete, 60(1-1300) Bq kg1 (226Ra),

F. Puertas et al. / Construction and Building Materials 101 (2015) 1105–1112

35(1-152) Bq kg1 (232Th) and 392(7-1450) Bq kg1 (40K); and for cement 45(4–422) Bq kg1 (226Ra), 31(3–266) Bq kg1 (232Th), 216(4–846) Bq kg1 (40K). While the values for cements are comparable to the present findings, new cement blends carrying waste materials from different industries pose further challenges. Obtaining the radiological characteristics of these new by-products is a NORM4BUILDING objective. 3.2. Index Figs. 2 and 3 show the activity concentration indexes of the various groups of materials studied. As all the values for the first group (Table 4), with the exception of fly ash, are less than 1, they would be EU legislation-compliant. Generally speaking, additions lower the radioactive content of the original materials or can be mixed to meet the index requirement. Lastly, the chemical reactions taking place during cement hydration may alter the radioactive content by dilution, reducing concentration activity by unit of mass. In geopolymers, however, the index is higher due to the use of NaOH, with the concomitant rise in the activity concentration of 40 K in the cement paste. Valorising industrial waste to obtain new materials to replace ordinary cement or its traditional additions would be environmentally beneficial. The caveat is that their radioactivity must be quantified to ensure that the population is not exposed to doses that exceed the limits laid down in the legislation. 4. Conclusions Further to the present findings, the uranium series radionuclide concentration in the materials studied is, in descending order: Fly ash > anhydrous CAC > slag > anhydrous OPC  anhydrous W-OPC > L  SF  glass waste Radioactivity in cements containing mineral additions (SCM) mirrors the activity concentration in the component materials. Activity concentration is lower per unit of mass in hydrated cements than in their anhydrous components because the concentration of the materials themselves is lower in the former. The extent of the decline closely parallels the proportion of hydration water (30–40%). The radionuclide activity concentration in alkaline cement pastes (Wg-AAS, Glass-AAS, N/15Wg-AAFA and Glass-AAFA) is calculated on the grounds of the percentage of slag or fly ash in the geopolymers and in the hydrated end product. 40K concentration rises in such activated materials due to the use of NaOH, which often contains potassium impurities that raise the 40K potassium content in the end product. The activity concentration index for building materials can be deduced from the radioactivity of cements and mineral additions. The radioactive composition of industrial by-products used as additions can be optimised by controlling their dosage in the resulting cement. The activity concentration index of the hydrated or activated end product by unit of mass is slightly lower than in the anhydrous material because of the presence of water. The chemical and physical properties of optimised cements obtained in this process would have to be tested to ensure their suitability for civil engineering or building construction. Acknowledgement This study was realized under the EU’s COST Action TU1301 NORM4BUILDING.

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