Radioactivity is the result of spontaneous nuclide decay with the emission of radiation, i.e., α and β particles together normally in conjunction with electromagnetic waves in the form of γ-radiation and X-rays (12). In the International System the unit used to express radionuclide activity is the becquerel (Bq), equal to the activity of a quantity of a radionuclide spontaneously decaying at a rate of one nuclear transition per second. During decay, the so-called parent nuclide transitions to a different type of nucleus, known as the daughter, which may be a different chemical element or an isotope, depending on the type of decay. The energy emitted, either directly (α- or β-radiation) or indirectly (γ-radiation or X-rays) suffices to strip electrons off and thus ionising any atom in the path of radiation (12). A brief description of the above four types of radiation follows.

Alpha (α) radiation consists in α-particles containing 2 protons and 2 neutrons: i.e., it is identical to the nucleus of the He atom. Alpha particles are characterised by a relatively large mass and a positive elementary electrical charge of 2, an indication of low penetrating power. In biological tissues its path is no longer than a few tens of micrometres. Alpha radiation is ionising, generating more ions per unit length in the host matter than other types of radiation because the particles shed all their energy over a short distance. Alpha emitters induce primarily internal exposure via inhalation, ingestion or contact with the skin (12).

Beta (β) radiation comprises β-particles, either electrons with a negative elementary charge or, less commonly, positrons or particles with a positive elementary electrical charge equal to the electron’s negative charge. As an electron’s mass is about 8000 times smaller than α-particle mass, β-radiation has moderate penetrating power. Exposure to beta particles is more of an external and less of an internal radiation hazard than exposure to alpha particles. The external radiation attributable to beta particles is confined primarily to the epidermis or outer layers of skin, although it may also be harmful to the crystalline lens of the eye.

Gamma (γ) radiation consists in high-energy photons (electromagnetic waves) emitted by the nucleus. This type of radiation is only weakly ionising but has high penetrating power and can travel across hundreds of meters of air. Thick concrete or lead shielding is normally used as protection against γ-radiation, exposure to which primarily affects external body parts. Given the high penetrating power of gamma rays, the energy released in internal exposure is absorbed by a smaller volume of tissue than is impacted by alpha or beta radiation. Internal exposure to gamma radiation is therefore less of a health hazard than similar exposure to alpha or beta radiation.

X-rays are electromagnetic waves emitted from the atomic shell, normally carrying lower energy than γ-radiation.

Radiation can be classified from two perspectives: whether or not it is ionising and whether natural or artificial. Ionising radiation is electromagnetic or corpuscular. When interacting with matter it ionises its atoms, modifying the atomic structure of the host material and inducing chemical reactions. Non-ionising radiation, which lacks that capacity, generates electromagnetic fields and optical radiation (13).

Natural radiation is the radiation present in the air, the Earth’s crust, outer space, foodstuffs and even inside animal and human bodies. Artificial radiation is generated by human activity, such as in the wake of nuclear explosions or accidents, industrial activity or medical testing. All exist in the environment and impact human beings and all other species. Humans receive both natural and artificial radiation in the mean yearly proportions illustrated in Figure 1.

As the name infers, naturally occurring radioactive materials (NORM) are radioactive materials of natural origin whose potential as a hazard in their unaltered state may be enhanced by human technological manipulation. Pastor et al. (14) define NORM waste (in Spain) as material for which the generator envisages no use and which exhibits a natural nuclide concentration higher than stipulated for exemption and clearence "exemption and clearanceas per Spanish Ministry of Industry, Energy and Tourism (IET) order 1946/2013 (15) (Table 1) (14). Such waste must therefore be managed bearing in mind both its nature as industrial residue and the characteristics intrinsic to NORM materials.

NORM waste may be generated by a number of industries, and most significantly those listed in Table 2 (14). Its management, the responsibility of the generators, entails determining the most suitable type of processing based on its radiological characterisation.

Granite is a naturally radioactive material used in building or as a component in concretes. It has a high activity concentration of certain natural radioactive series, including uranium, thorium, actinium and potassium (16-18). A recent study conducted by the present authors (19) showed that granite aggregate particle size distribution affects activity concentration, with smaller particle sizes inducing higher activity. This matter is discussed in greater depth in a subsequent section.

Coal fly ash (FA) is a fine powder generated when coal is burnt to produce electricity, usually in coal-fired power plants. It consists primarily in non-combustible inorganic material but also contains some residual carbon sourced from partially non-combusted coal (54). Although fly ash particles are largely spherical, irregular or angular quartz or other grains are likewise often present in the ash. Coal fly ash is divided into two main groups: siliceous (V) and calcareous (W) in European legistation and Class C Fly Ashes in accordance with ASTM standard. V-type fly ash is pozzolanic whilst W-type ash generally exhibits hydraulic properties (21). V-type ash must have a reactive CaO content of less than and W-type greater than 10 %.

Both siliceous (type V in European legislation and type F further to ASTM) and calcareous (W) ash are used to manufacture CEM II ordinary Portland cement, CEM II composite portland cement, CEM IV pozzolanic cement and CEM V composite cement. Further to the data in Table 3, the proportion of fly ash in the 10 cement types defined in EN 197-1: 2011 (with these fly ashes): ranges from 6 % to 55 % and more specifically for CEM II/A-V from 6 % to 20 %; CEM II/B-V, 21 % to 35 %; CEM IV/A 11 % to 35 %; CEM IV/B 36 % to 55 %; CEM V/A 18 % to 30 % and CEM V/B 31 % to 49 %.

Siliceous fly ash with a low reactive CaO content is the type most widely used in cements and concretes. Due to ash pozzolanicity, early age mechanical strength is low in cements bearing a high content of the addition. The use of siliceous fly ash-additioned cements to manufacture concrete is nonetheless known to have beneficial effects, such as improved workability and significantly lower later age capillary porosity than the unadditioned material. That in turn induces higher mechanical strength and enhanced resistance to chlorides, sulfates and potential alkali-aggregate reactions (55).

The natural radioactivity of fly ash depends on the geological age of the original coal bed, the combustion process used and ash particle size; being of interest due to the accumulation of radioactivity - polonium 210 - in the smallest particles of fly ashes (20, 56, 57). US Geological Surveys show that coal containing phosphate minerals such as monazite or apatite have high radioactive ²³²Th concentrations, whereas coal containing both organic matter and mineral fractions have high levels of the ²³⁸U radioactive series (58). The respective radionuclides are retained during coal combustion and concentrate in the fly and bottom ash, more profusely in the former. Uranium and thorium concentrations may be up to ten-fold higher in bottom and fly ash than in burnt coal, while fly ash may contain even higher ²¹⁰Pb and ⁴⁰K concentrations. In modern power stations around 99 % of the fly ash generated is typically retained (90 % in some older plants) (59). The finest ash particles have been observed to bear ²¹⁰Pb contamination. Higher concentrations of that isotope are found in fly than in bottom ash because lead volatises in the combustion chamber (60, 61). ²²⁶Ra contamination has also been detected in the finer fractions, where the ²¹⁰Pb/²²⁶Ra ratio may exceed 3 compared to no more than 0.5 in bottom ash (20).

Detailed information on the radionuclides in fly ash can be found in (20) and (62, 63, 64). The uranium series (²²⁶Ra and ²³²Th) and ⁴⁰K concentrations (in Bq kg^-1) in several European countries graphed in Figure 2 vary widely and depend, as noted earlier, on the original coal, combustion process and ash particle size. Table 4 gives the concentration ranges for radionuclides in fly ash sourced from a series of widely differing coal-fired steam power plants. The data show that coal fly ash has a high radionuclide content and the highest radioactivity of all the SCMs listed in standard EN 191-1: 2011. Further to Directive 2013/59/EURATOM (51), ACI values (Equation [1]) should be calculated only for endproducts, not for their constituents such as fly ash. Nonetheless, based on its radionuclide concentration (Table 4), the ACI for that by-product would be on the order of 0.5 to 1.5 (65).

The iron and steel industry generates different types of by-products, most prominently blast furnace (iron) and steel (steel) mill slag. The former, when vitreous and finely ground, exhibits hydraulicity and hence is widely used in cement and concrete manufacture (76, 77). The latter is less frequent in construction in light of its high heavy metal content and radioactivity, along with its unbound CaO and MgO and dicalcium silicate (2CaO·SiO₂ or C₂S) contents that may pose expansion or soundness problems over time. Its poor physical properties (low polished stone value, PSV, and high aggregate abrasion value, AAV) further condition its use (22, 78, 79), which is restricted to non-structural works and road bases and sub-bases.

Blast furnace slag is the result of contact between the clay-like acid gangue from iron ore and the sulphur ash from (likewise acid) coke on the one hand and the lime and magnesium (both basic) from the more or less dolomitic limestone materials used as fluxes on the other. That combination of acid (SiO₂ and AI₂O₃) and basic (CaO and MgO) oxides at high (>1600 °C) temperatures yields slag. When the molten magma cools rapidly to ambient temperature via granulation or pelletising procedures, the resulting material is over 90 % amorphous. Such glassy, ground (GBFS) blast furnace slag is hydraulic, meaning that when mixed with water it can harden like cement (76). If it cools slowly, however, the result is a highly crystalline (crystallised slag), low reactivity material that can be included as a raw material in portland cement raw mixes (80) or used as a filler in road bases and sub-bases (81). The majority chemical components of blast furnace slag include SiO₂ (27-40 %), CaO (30-50 %), Al₂O₃ (5-25 %) and MgO (1-15 %). Mellite, a solid solution containing gehlenite (2CaO·Al₂O₃·SiO₂) and akermanite (2CaO·MgO·2SiO₂), is the majority phase in crystallised blast furnace slag. The chemical and mineralogical compositions of the two types of slag depend on the starting iron ore, the type of processing applied and the coke and fluxes used.

GBFS is an SCM used to manufacture ordinary CEM II/A-S and CEM/IIB-S cements, in which it accounts for 6 % to 35 % of the total weight; in cements CEM III/A, CEM III/B and CEM III/C the values are 36 % to 95 %; and in cements CEM V/A and CEM V/B 18 % to 50 %.

Steel slag, a by-product of steel manufacture, is formed in the reaction between fluxes such as calcium oxide and the non-metallic inorganic constituents of steel or scrap metal. Two types can be distinguished:

The origin of iron and steel slag radioactivity lies in the iron ore and scrap metal used and the sintering processes deployed. Depending on the origin, iron ore may contain different radionuclides and heavy metals, with uranium concentration in the 20 Bq kg^-1 to 30 Bq kg^-1 range (81). In blast furnace slag, the source of natural radionuclide content has been confirmed to be not only the raw materials but the manufacturing process (type of furnace, for instance) (18). Figure 3 graphs the radionuclide concentrations in the iron and steel mill slag used in cement manufacture, whilst Table 4 gives the range of ²²⁶Ra (uranium series), ²³²Th (thorium series) and ⁴⁰K concentrations in BFS and steel slag.

As the data in Figure 3 and Table 4 infer, blast furnace slag bears a much higher radionuclide content than steel slag, although the BFS values, with ACI generally <1, are lower than observed for coal fly ash (65).

Table 3 shows that in addition to type V and W fly ash and vitreous slag (S), waste and materials, such as silica fume, limestone, natural pozzolans and calcined shale are used as SCMs in the standardised ordinary cements listed in EN 197-1: 2011.

Silica fume (D) is a residue generated in electric arc furnaces producing silicon and ferro silicon metals. An inorganic product consisting in very fine spherical particles generated when coal reduces quartz, it is essentially (85 % to 90 %) amorphous SiO₂. With such a high amorphous SiO₂ content and large specific surface (around 20 000 m²·kg^-1), silica fume is highly pozzolanic. According to standard EN 197-1: 2011, CEM II/A-D cements may contain 6 % to 10 % silica fume. Blended Portland cements CEM II/A-M and CEM II/B-M as well as cements CEM IV/A and CEM IV/B may also bear up to 10 % of that type of waste.

Portland cement with silica fume (CEM II/A-D) is apt for manufacturing of prestressed concrete, concrete with reactive aggregates and shotcrete or fast-setting concrete. Silica fume pozzolanicity determines the high reactivity and early age strength development in those materials (83). Silica fume can also be used as a cement replacement or addition to prepare very high-strength, durable concretes (83, 84), although it must not account for more than 5 % to 10 % of the total cement weight. The presence of this mineral addition in concrete has induced the development of so-called ‘high performance’ concretes. The large specific surface in silica fume determines the need for variable amounts of superplasticising admixtures in the respective concretes to lower the water/cement ratio while ensuring suitable rheology as well as the production of very high density, high performance and durable concretes (85).

The concentration ranges of ²²⁶Ra, ²³²Th and ⁴⁰K (in Bq kg^-1) reported in the literature for silica fume (Table 4) are relatively low for the former two former and somewhat higher for ⁴⁰K. This SCM would not in any event appear to be a very significant source of natural radionuclides in the end cements and concretes.

Natural pozzolans are siliceous, alumino-siliceous (or a combination of the two) geological materials. The term was originally applied only to the volcanic tuff found in the area around Pozzoli, Italy, and used by the Romans to prepare hydraulic concretes. For several decades it has also been used, however, to designate natural materials able to react with the portlandite generated during the hydration at ambient temperature of the calcium silicates present in portland cement. The pozzolanic properties characterising the strong cohesive end products have determined the evolution of the term from its original geological meaning to its present technological connotations (86). Natural pozzolans, like aluminosiliceous fly ash and silica fume, exhibit the pozzolanic characteristics defined in standard EN 197-1: 2011, which describes two types of materials: natural pozzolans (P) and naturally occurring fired pozzolana (Q). Natural pozzolans are normally volcanic materials or sedimentary rocks with the chemical and mineral compositions specified, whilst naturally occurring fired pozzolans are thermally activated volcanic, clay, slate or sedimentary rocks.

These materials can be used to manufacture cements CEM II/A-P and CEM II/B-P, CEM II/A-Q and CEM II/B-Q with 6 % to 30 % pozzolan by cement weight, as well as cements CEM IV/A and B and CEM V/A and B. Content is defined in keeping with availability. The characteristics of cements with natural pozzolans are qualitatively similar to but more intense than observed in materials with greater pozzolanicity such as silica fume and type V fly ash. P-type pozzolans are also apt for the manufacture of alternative materials such as alkaline cements (87).

The concentration ranges of ²²⁶Ra, ²³²Th and ⁴⁰K (in Bq kg^-1) reported in the literature for natural and naturally occurring fired pozzolans used in cement manufacture are listed in Table 4, where metakaolin (MK) has been included as an example of the latter. Further to those data, the ²²⁶Ra content may be higher and more variable in MK than in non-fired natural pozzolans.

Limestone (L and LL) is another material used as an addition in ordinary cements. It must also meet a series of chemical and physical requirements to be apt for ordinary cement manufacture. Its calcium carbonate content (CaCO₃), calculated of terms of calcium oxide (CaO), must be at least 75 wt%, the clay content must be lower than 1.20 g/100 and the total organic carbon (TOC) must not exceed 0.20 wt% in subtype LL or 0.50 wt% in subtype L (21).

Limestone is present in cements CEM II/A-L and B-L at 6 % to 30 % by cement weight and in the same proportions in cements CEM II/A-LL and CEM II/B-LL. Limestone has no pozzolanic or hydraulic properties, but neither is it a wholly inert addition as believed until not long ago. Its presence in cement induces a series of very distinct chemical and physical characteristics explained by such non-inertness. Its most prominent effects include improved paste workability, which lowers water demand; absence of any impact on early or late age strength when limestone content is under 10 % by cement weight; and lesser expansion at all ages (88). In contrast, thaumasite formation-related durability issues have been described in cements with high limestone contents (89).

According to the concentration ranges of ²²⁶Ra, ²³²Th and ⁴⁰K (in Bq kg^-1) reported in the literature for limestone (Table 5), the material exhibits fairly low uranium and thorium series contents, but a much broader range of ⁴⁰K activity concentrations.

Table 5 also gives the ²²⁶Ra, ²³²Th and ⁴⁰K ranges (in Bq kg^-1) for other industrial waste/by-products such as red mud, biomass ash and waste glass which, while not standardised, can be used as portland or alkali-activated (geopolymer) cement additions.

Red mud is waste generated in Bayer process-mediated bauxite refining, in which ground bauxite reacts with sodium hydroxide at high temperatures and pressures. A highly alkaline (pH~10 to 12.5) slurry, it has a solids content of 15 wt% to 30 wt%. Its composition features fine silica, aluminium, iron, calcium and titanium oxide particles in proportions that differ depending on the bauxite ore, conditions governing aluminium extraction and quality control (18). Its alkalinity, toxic element content and natural radioactivity (bauxite waste ACI is consistently over 1: Equation [1]) limit its use in construction (20).

Burning plants, trees and seed waste sourced from a wide variety of species to produce electric power yields biomass ash, generally in the form of fly and bottom ash. The chemical, mineralogical and radiological composition of such ash is highly dependent on the material burnt and the process. Some types of plant ash, such as rice husk and bagasse, exhibit pozzolanicity and can be used in cement and concrete (95, 96) or alkaline cement manufacture (34).Others such as olive tree ash, however, are scantly applicable as SCMs. Some of the present authors recently assessed the viability of biomass ash as an alternative alkaline activator in geopolymer manufacture (43). The very high ⁴⁰K content observed, with ACI values consistently >1, (Table 5), must not be overlooked when considering its use.

Other types of waste such as glass hold promise for possible application in cement and concrete manufacture either as aggregates (97, 98) or as precursors and/or activators in geopolymers (32, 33, 38). These materials vary widely in terms of their composition, while sodium silicate-based glass is the most apt for use in geopolymers. Further to the radiological data in Table 5, this waste poses no natural radiation-related problems, for its uranium, thorium and potassium contents are all fairly low.

Gypsum and phosphogypsum have some similar characteristics but are conspicuously different chemically and radiologically. Gypsum, hydrated calcium sulfate (CaSO₄·2H₂O), is blended with clinker and SCMs in Portland cement manufacture. The most prominent of its various purposes is to react with the C₃A (3CaO·Al₂O₃) in the clinker and the water present in the early stages of cement hydration to form ettringite (3CaO·Al₂O₃)(SO₄)₃(OH)₁₂·31H₂O). That reaction constrains or impedes any direct reaction between C₃A and water, preventing flash setting by retarding some of the cement hydration reactions. Gypsum may also be found in calcium sulfate cement hemihydrate (CaSO₄·½H₂O) and anhydrite (anhydrous calcium sulfate, CaSO₄) or any combination of the two. Gypsum and anhydrite are present in nature and calcium sulfate may be generated as a by-product in certain industrial processes. The ceiling sulfate content in ordinary cements, expressed as % SO₃, is 3.5 % (in 32.5 and 42.5N cements) or 4.0 % (42.5R and 52.5 cements (21).

Phosphogypsum (PG), a phosphate industry by-product, is generated during the acid digestion of phosphate ore (20). The industry is known to be vital to the worldwide food supply, given the role of phosphate fertilisers in extensive farming. Phosphate minerals carry high contaminating contents of uranium, radium, polonium, thorium and lead isotopes.

The ²²⁶Ra, ²³²Th and ⁴⁰K concentrations (in Bq kg^-1) for gypsum and phosphogypsum used in construction are given in Table 6, which shows that as a rule the U, Th and K series values are fairly low.

Approximately 70 % of the phosphate ore mined is processed in an acid medium to produce phosphoric acid, during which procedure substantial quantities of phosphogypsum are generated. Depending on the raw ore used, PG may contain up to 60 times the radionuclide content found in the pre-processed rock (100). According to García-Díaz et al. (100) up to 80 % of the ²²⁶Ra, up to 86 % of the U and up to 70 % of the Th may concentrate in PG. The radionuclide content in this material must be determined because ²²⁶Ra generates radon gas (²²²Rn), which with a short half-life of just 3.5 d is very active, i.e., emits intense radiation that may harm bodily organs.

This industrial by-product is scarcely used in construction due to its high radioactivity, although research has been ongoing for some time on its use in place of mineral gypsum and for other applications. A review of possible applications in building material production (101) cites setting control, mineralisation in clinker preparation and gypsum product manufacture as potential uses. A recent study (102) addresses the use of phosphogypsum instead of gypsum in super-sulfated cement-based concrete manufacture. As noted, its use is conditioned by its high radon radioactivity and emissions. A few studies and patents suggest that phosphogypsum can be purified by eliminating heavy metals and ²²⁶Ra (104), while others (82) assess the use of alkaline cement matrices to immobilise the radon in phosphogypsum.

As the uranium, thorium and potassium concentrations in phosphogypsum of different origins graphed in Figure 4 show, the numbers vary widely. The ²²⁶Ra, ²³²Th and ⁴⁰K concentrations (Bq kg^-1) reported in the literature for this by-product and listed in Table 6 also attest to enormous variability. The specific concentrations of naturally occurring radionuclides in phosphogypsum depend on the origin of the phosphate ore and the chemical process used (20). In wet phosphoric acid production, the amount of radionuclide taken up by each fraction may vary with the technology. As a rule most of the uranium ultimately remains in the fertiliser, whilst radium is more evenly distributed among the (by-)products, with some possibly precipitating in the plant. Most of the polonium is eliminated with the phosphogypsum fraction (103). With such variability the ACI values range from 0.5 to 4.0, although most are >1 (20).

This section draws a distinction between the more conventional cements and concretes based on portland or standardised materials and alternative cements and concretes (as a rule with a low or nil clinker content) such as alkali-activated products or geopolymers. The radiological behaviour of all these cements and concretes is described in great detail in a recent paper (8).