Fillers in cementitious materials — Experience, recent advances and future potential

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Fillers in cementitious materials — Experience, recent advances and future potential Vanderley M. Johna,⁎, Bruno L. Daminelia,b, Marco Quattronea, Rafael G. Pileggia a b

University of São Paulo, Polytechnic School, Av. Prof. Almeida Prado, trav. II, 05508-070 Sao Paulo, SP, Brazil University of São Paulo, Institute of Architecture and Urbanism, Av. Trab. São-Carlense, 13566-590 São Carlos, SP, Brazil

A B S T R A C T The paper discusses the potential of fillers in CO2 mitigation in the cement industry. A historical overview of the use of fillers is presented as well as the limits of filler use given in cement standards. Globally, limestone filler currently represents only 7% of average worldwide cement composition. The limits of the current route to adding filler in cement by means of intergrinding are discussed. An innovative technology, that compensates binder dilution by a reduction of the water required for good rheological behavior, is presented; this allows clinker replacement rates of up to 70%. The theory that enables the design of such multimodal particle size distributions with high particle packing and low-water demand is presented, and examples of its application in concrete production are given. The efficiency in terms of CO2 mitigation is demonstrated by comparing concrete formulations designed with this innovative approach with a global benchmark of current technology. New filler minerals, as well as the effects of high-filler content on production processes and on durability are also discussed.

1. Introduction

2. Technology presentation

The traditional mitigation strategies for CO2 emissions in the cement industry are not sufficient to ensure the necessary mitigation in a scenario of increasing cement demand. Therefore, the adoption of expensive and environmentally risky carbon capture and storage (CCS) has been considered an unavoidable solution by cement industry leaders [1]. The limited availability of fly ash and blast-furnace slag, which will represent < 20% of global cement demand by 2050 [2], is an important component of this problem. The use of limestone filler as a partial replacement has been standardized since the 1980s [3,4]. The substitution of clinker reduces the CO2 emissions in cement production, almost proportionally to the replacement rate, because fillers do not require calcination. Limestone filler influences early hydration rates and reacts forming calcium carbosilicate and carbo-aluminate hydrates [5,6]. However, the major effect of limestone filler is a physical dilution of the binder. Due to that, it tends to decrease strength if water content remains unchanged. In consequence, the global average replacement rate of clinker by limestone filler has remained low at approximately 7% [7]. This paper presents a summary of the current knowledge regarding the use of fillers in cementitious materials, including a historical review and recent advances in cement production technology.

The technology consists of replacing clinker and other scarce reactive supplementary cementitious materials (SCM) by “inert” mineral fillers. There is no single, universal definition for fillers. In the context of this paper, fillers are defined as fine particulate materials that are inert or almost chemically inert when mixed with cement, produced by grinding with or without surface treatment. They are used as a partial replacement for clinker or other reactive SCMs. The definition of inert mineral fillers includes particles in the same size range as cement, as well as particles that are finer than cement. Our definition is broader than the usual one adopted by the cement industry, because it includes fillers made from other minerals beside calcium-rich limestone, which is currently the filler prescribed by cement standards. The mixing of fillers with cement is a dilution strategy that increases the volume and mass of the product by reducing the concentration of the reactive ingredients in the cement paste matrix. Fillers are used for economic, technical, and environmental reasons [7,8] in the cement industry as well as in other industries. For the cement industry, the use of fillers helps to reduce thermal energy and CO2 emissions and increases the expected productive life of limestone quarries, because bound CO2 becomes part of the commercial product and allows the use of low-grade limestone or other minerals.

⁎

Corresponding author. E-mail addresses: [email protected] (V.M. John), [email protected] (B.L. Damineli), [email protected] (M. Quattrone), [email protected] (R.G. Pileggi).

http://dx.doi.org/10.1016/j.cemconres.2017.09.013 Received 9 December 2016; Received in revised form 22 September 2017; Accepted 22 September 2017 0008-8846/ © 2017 Elsevier Ltd. All rights reserved.

Please cite this article as: John, V.M., Cement and Concrete Research (2017), https://doi.org/10.1016/j.cemconres.2017.09.013

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The global market for cement filler is estimated to be around 280 Mt per year in 2014 [7], the majority of which is produced by intergrinding with clinker and other SCMs. However, there are other markets for fillers in construction, including self-compacting concretes, fiber-cement production, asphalt pavements, and soil modification. State-of-the-art technology allows much higher levels of clinker substitution with fillers than intergrinding, by using separate grinding of filler, clinker and other supplementary cementitious materials. All fines are blended with optimized proportions, resulting in a multimodal (non-continuous or gap-graded) particle size distribution cement [9], and with a dispersant, a combination that allows for a steep reduction in water demand for adequate rheological behavior in the final product. The magnitude of water reduction achieved can be more than enough to compensate for the effects of binder dilution. Fig. 2. Influence of filler mineralogy and content (LF xx = limestone filler content, wt.%; GR xx = granite filler content, wt.%) on the relative compressive strength of standard 1:3 Ottawa mortar over 10 years. Cements had almost the same fineness. Data from Davis et al. [12], University California, Berkeley.

3. Market experiences with filler in cement 3.1. Beginning of the 20th century: first experiences The first recorded use of fillers as binder replacements was in the Elephant Butte and Arrowrock Dams (Fig. 1), built by the US Bureau of Reclamation between 1912 and 1916 [10]. For economic reasons, 45–48% of the cement was replaced with ground granite or sandstone that was excavated during construction. The materials were ground to pass the 850 μm sieve, then mixed with coarse cement, and interground to 90% passing the 75 μm sieve. The concrete produced had the same compressive strength at one year of age as the concrete that used the original coarse cement alone; this is because dilution was compensated by an increase in clinker fineness during the second grinding [6]. The downstream face and spillway channel of the Arrowrock Dam were repaired due to frost damage degradation in 1935 [11]. No major repair has been reported in the Elephant Butte Dam located in Texas [8]. These 100 years-old dams are still in use. Another landmark achievement was the very thorough 10-year-long investigation conducted in 1930s by a team from the Civil Engineering Department [12,13] of the University of California, Berkeley, on cements containing various fillers and artificial pozzolans, such as calcined clay and mixtures of both. Interground replacement in cement with 20% limestone filler and 45% granite filler (LF and GF respectively in Fig. 2) was part of the experiment. As Fig. 2 shows, despite the fineness of the cement and the water content being almost constant, a 20% filler replacement surpassed the original cement compressive strength in the long term, in both wet and dry curing. According to the authors, good results were obtained for blends of limestone filler and

various pozzolans, such as calcined Monterey Shale and siliceous clays, as well as pumicite [12]. The substitution of clinker with inert materials, or with slag and limestone, was considered ‘cement adulteration’ in the USA in the early 1900s [14], and it is still considered a problem today in the Eurasian region [15,16]. For Mayfield [14], this fact introduced an ethical bias to a technical discussion and most likely negatively influenced the public view of fillers in many cement markets. 3.2. Standardization period: 1980s–2016 The use of limestone as a replacement for clinker was most likely introduced into cement standards as an answer to the oil crisis [14] during the 1970s. Canada adopted a 5% limestone limit in normal Portland cement by 1983 (CAN3-A5-M83), and by 1988, the Europeans were already drafting proposals to allow 20% of filler in Portlandcomposite or Portland-filler cements [14]. Currently, several national and international standards include fillers, and the EN 197-1 cement composition standard allows up to 35% of limestone filler substitution in cements [17] (Fig. 3). Despite the maximum amount of limestone replacement being smaller than that allowed to fly ash and slag, limestone filler is 7% of global average of cement, a rate higher than slag (5%) or fly ash (4%) [7]. Morocco currently has the highest reported clinker substitution level with filler, with an average of above 20%. Recently, it is possible to observe a growing interest in the combination

Fig. 1. 100-year-old Arrowrock arch Dam, Idaho, USA, portrayed in 2014. Water height is 78 m. Source: US Bureau of Reclamation https://c2.staticflickr.com/8/7308/13977827009_ 07278638a7_b.jpg.

Fig. 3. Maximum limestone filler (wt.%) in standard cements. Figures represent the year of first publication of the standard [17,21–24].

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Fig. 5. Dilution effect on porosity and paste volume caused by the introduction of a completely inert filler when the water/solids ratio (weight) is kept constant. Density: Portland cement 3.1 g/cm3; filler 2.6 g/cm3; combined water: 0.24 g/g binder.

Fig. 4. Evolution of limestone filler content in cement for CSI WBCSD companies. Selected regions and worldwide. Data from [7].

Limestone filler partially reacts with aluminates, producing carboaluminate [25,27,28] and stabilizing ettringite [5,29]. Limestone filler can also react with aluminate from slags or pozzolans in a synergistic manner [29,30], allowing for greater porosity reduction. However, most of the filler is introduced into cement by intergrinding limestone with clinker, normally resulting in cements with a lower strength class [17,31] than the original pure clinker cement, despite the limited reaction of limestone filler. In Europe, CEM II/B Portland-limestone filler cements typically fall in the lowest-grade strength class (32.5 MPa), while CEM II/A (filler < 20%) is mostly of the 42.5 MPa class [32]. In Brazil, approximately 65% of cement producers offer the 32 MPa cement CPIIF strength class, with only 10% maximum limestone filler content; meanwhile, without the filler, the cement would be a 40 MPa strength class. This is a result of non-compensated dilution. Dilution can be partially compensated for by increasing fineness through grinding or by reducing water/cement ratio. However, cement testing standards such as EN 197-1 do not allow the adjustment of mixing water content to keep workability constant. Therefore, the standard does not incentivize market exploitation of water-reducing strategies to compensate for dilution. This can partially explain why filled cement usually provides better in-use performance than what is suggested by its standard classification [17,33]. From the point of view of plastic concrete, filler substitutions of 15% [31] or between 10 and 20% [17,34] do not significantly affect the compressive strength. Therefore, with current standards, increasing cement fineness is the only industrial solution available to compensate for dilution. Limestone is far easier to grind than clinker or slag; therefore, intergrinding will result in limestone particles much finer than binder particles [17,35]. This is inefficient because increasing the fineness of limestone filler will only slightly affect its mechanical strength [36]. The replacement of coarse clinker particles with inert fillers is much more effective [37]. The dilution of clinker or other SCMs by fillers beyond the limits where strength loss due to dilution can be compensated results in an increase of the environmental impact of cement-based materials. However, environmental impact measured at the gate of the cement producer will show net reduction of CO2 and energy. If we neglect eventual rheological improvements, it is possible to estimate (Fig. 6) that to make a 40 MPa concrete using a 32.5 MPa cement strength class, the concrete requires approximately 40% more cement than is needed when a 52.5 MPa cement is selected. The lower the concrete design strength, the less important the effect is. However, when a 52.5 MPa strength cement becomes a 32.5 MPa cement owing to dilution with 10% of filler, the environmental impact of cementitious materials increases by approximately 10%. But the CO2 footprint of cement producers is reduced by slightly < 10% because of the binder dilution.

of limestone with other SCMs, including calcined clay [18]. Almost all countries limit limestone filler to a restricted range of compositions. However, the use of non-limestone fillers is allowed under EN 197-1. This standard allows the addition of up to 5% of “minor mineral constituents,” including natural materials, which “… after appropriate preparation and because of their particle size distribution, improve the physical properties of cement. They can be inert or have slightly hydraulic, latent hydraulic, or pozzolanic properties.” The practice of adding fillers at the concrete mixing stage was most likely introduced to the market by the advances of self-compacting concrete technology in the 1990s [19] and now has been extended to other concretes [20] and may include fillers other than limestone. The actual average replacement rates are much lower than the maximum limits (Fig. 4), which clearly show the limitation of current technology and market segmentation. In Europe (the most technologically influential market), the rate shows a very slow growth, approximately 6.5%, which is close to the global average. The use of limestone in the USA is very low, 2.7% in 2013, despite the recent change in standards that allow up to 15% of limestone filler. The Indian limits for limestone filler are due the limited availability of clinker-grade limestone [24]. 3.3. Limitations of current filler technology: simple dilution of cements Considering that the filler is almost inert, the replacement of binders with a filler will result in dilution. This occurs because a binder is defined as a mineral that hydrates, such as clinker, slag, and pozzolans. A lower binder concentration implies a lower rate of increase of the solid volume fraction filled by hydration products. If the water-to-solids ratio used to test cement strength is constant, as it is in most standards including EN197-1, it causes an increase in the porosity of the system, which reduces its strength. This can be roughly estimated using a simplified Powers model by disregarding any influence of the filler in hydration (Fig. 5). Dilution can be compensated for by increasing clinker fineness within certain limits. Limestone fillers have densities in the range of 2.6–2.7 g/cm3; therefore, their substitution on a mass replacement basis also increases paste volume (Fig. 5) and the separation distance between aggregate particles. This partially explains the reported improvements in rheological behavior [17], which may allow a reduction of the water/cement ratio for constant workability, helping to compensate for the effects of dilution. At early stages, the dilution of clinker by filler is partially compensated for by an acceleration of the initial hydration rate [25,26] due to finer clinker particles and the nucleation effect. In addition, some fillers may react with cement phases. 3

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compose the suspensions. The equation requires an experimental determination of the empirical parameters for each set of combined materials. Essentially, the relation between the critical solid content and the packing of particles is not absolute but also dependent on the nature of solids and fluids. Therefore, this approach is insufficient since the maximization of packing does not ensure the minimization of water required for adequate rheological behavior. Usually the actual water content required to adequate rheological behavior is above to the quantity required for full binder hydration, and this excess determines the final porosity of the hardened cement paste [42]. The minimization of this excess water is the key strategy to improve strength and performance of low binder systems. Developing models capable of predicting the viscosity of suspensions from basic particle and liquid characteristics is still a challenge. Satisfactory results are achieved using the Funk and Dinger [43,44] model, which correlates the viscosity of suspensions with the interparticle separation (IPS) (Eq. (2)) that describes the space for particle motion in well-dispersed suspensions. The higher the IPS, the lower the shearing energy dissipation due to particle interactions, measured as viscosity.

Fig. 6. Estimation of the effect of cement strength class on cement consumption for concrete strengths varying between 10 and 50 MPa. Abrams curve CS = k · e2.634x is based on PCA [39], where k is a constant adjusted for compressive strength equal to cement class strength when the water/cement ratio is x = 0.5.

This effect is quite evident for industrial cement users; however, it is not perceived as relevant for self-help or small homebuilder companies that tend to use the same concrete formulations without considering the cement strength class. This may explain how Morocco, where most of the cement is sold in bags, can have 20% average filler content while the European Union only has 7%, despite having the same maximum standardized filler content [38]. However, in regions where the only available cement is a 32.5 MPa strength class with filler, professional users will be forced to use more cement than if they used a CEM I with 42.5 MPa class, and the environmental impact of the industry will increase despite the lower clinker factor. A similar effect may be expected when replacing filler with low-reactivity pozzolans or slags.

IPS =

(2)

where VSA = volumetric surface area (m2/cm3), calculated by the product of specific surface area SSA (m2/g) and solid density ρs (g/cm3), Vs = volumetric solid fraction in suspension, and Pof = pore fraction in the maximum packing condition calculated using the Westman and Hugill algorithm [43–45]. The packing porosity is one of the variables in the IPS equation, but it has a lower impact than the volumetric surface area (VSA). For concrete or mortars, a multiscale optimization of IPS can be performed by considering a fluid as (a) water to the paste, on a micro-scale, and (b) the paste (as a fluid) to aggregates, on a macro scale. Fig. 7 illustrates the IPS concept from the dry powder (left) to the distanced particles (right). The ideal combination for reducing the water demand in suspensions is achieved by reducing the packing porosity (water needed to fill the voids) and the surface area (water needed to cover the surfaces) because only the excess water will promote spacing among particles. Although the IPS model describes the spatial distribution of suspended particles which can be comprehended as sort of degree of freedom regarding mobility, the model is not absolute since it does not consider other physical aspects linked to the energy dissipation during collisions among particles of various sizes in movement (including particle blockage by neighbors) and the propensity for phase separation. Literature confirms this finding [46], where different cement-filler suspensions exhibit distinct tendencies for viscosity reduction due to IPS increase. To improve particle mobility, the ratio between the particle size and IPS should be less than one [46]. Another aspect to be considered is the energy dissipated by a particle when moving inside a liquid. This effect is described by the Stokes' law [47] (Eq. (3)) which calculates the distance d0 that a particle with diameter Dp and density ρs runs freely until stopping when projected horizontally at a speed v0 in a liquid with viscosity ηf.

4. New developments: high filler, low-mixing water, with high mobility mixtures Recent research developments show that it is possible to replace as much as 70% of clinker with filler without reducing mechanical strength. This can be achieved if dilution is compensated for by engineering a reduction in the amount of mixing water required to achieve the desirable rheological behavior. However, this is not a simple task because rheological behavior becomes increasingly difficult to control with reduced water content. 4.1. Packing and interparticle separation The plain strategy for achieving a reduction in water demand is to increase particle packing, which results in a lower volume of interparticle voids that needs to be filled with water. The packing maximization approach is not particularly innovative in the cement industry [40], at least for grading the aggregates, and guides researchers even today. The idea behind packing improvement is justified by a hypothetical increase in the critical solid content (φT) in the Krieger-Dougherty model [41], which describes the viscosity of suspension as a function of the volumetric solid content in suspension (Eq. (1)).

d 0 = v0 × Dp2 ×

−[η]1 φT

ø⎞ ηr = ⎜⎛1 − ⎟ φ T⎠ ⎝

2 ⎡1 1 ⎞⎤ x − ⎜⎛ ⎟ VSA ⎢ Vs 1 − Pof ⎠ ⎥ ⎝ ⎦ ⎣

(1)

ρs 18ηf

(3)

where d0 = distance travelled until the stopping (μm); v0 = launching particle velocity (μm/s); Dp = particle diameter (μm); ρs = solid density (g/cm3) and ηf = dynamic viscosity of the liquid (g/ μm·s). The ratio between the maximum distance (d0) that a particle launched at a certain velocity v0 may travel until immobility and the mean distance to other particles (IPS) results in a dimensionless parameter,

where ηr = relative viscosity, [η] = intrinsic viscosity, ϕ = solid content, and φT = critical solid content. Despite its accuracy in describing how viscosity is related to solid concentration, this equation is not predictive; it fails to explicitly consider the effects of the fundamental properties of particles (size, shape, surface area, density, etc.) and liquids (viscosity, density, etc.) that 4

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Fig. 7. Schematic representation of the IPS model. The volume of the liquid phase (water in cement base materials) increases from left to right.

particle diameter Dpi sub-suspension defined within its 10 times diameter interval, which allows the assessment of the effective natural interference (INTni) perceived by each particle diameter Dpi. The bulk interference (INTp) for multimodal suspensions under shearing can then be calculated using an algorithm that sums the individual contribution of each sub-suspension weighted by its volumetric contribution in suspension, thus converging to Eq. (5). As observed, this last equation also includes a constant H, which was proven to be related to the total volumetric solid content in suspension [46]. The physical meaning of the H constant involves the suspension capacity of energy dissipation in terms of heat during the experimental measure of viscosity.

named the dynamic interference (INTd), that estimates the probability of contacts among suspended particles in movement. The intrinsic characteristics of the suspensions can be identified through dividing INTd by v0, thus defining the natural interference (INTn) of the system by Eq. (4) [46]. This equation allows comparisons among different suspensions, independent of their shearing conditions.

INTn =

Dp2 IPS

×

ρs 18ηf

(4)

The mobility of particles with multimodal size distribution in suspensions also follows the known Farris concept [48,49]. According to this, a particle moving inside a suspension made of particles smaller than about 1/10 of its own diameter, behaves as if it was moving in a liquid with the same viscosity of a suspension. In other words, large particle kinetics is not affected by collisions with other particles that is more than one order of magnitude smaller. But when it collides with particles that are larger than ~ 1/10 of its own diameter, the particle is deviated. Therefore, a sub-set of particles of a given diameter can be modeled as flowing in a (sub) suspension, comprised of particles with diameters > 10 times smaller than those in the liquid phase. Fig. 8 displays such a concept by plotting the effective particle size distributions defined within the 10 times diameter size intervals around some exemplified diameters (125, 75, 45, 22.5, 11.25, 5.63, 2.37, 1.0, 0.5 and 0.25 μm) for some Portland cement samples [46]. The convergence between the predictive IPS model with the particle interaction concept of Farris arises by calculating the IPS for each

m

INTp = H ×

∑ x i × INTni,

(5)

i=1

where H = experimental dissipation parameter (function of the volumetric solid content), xi = volumetric content of particles with diameter Di, INTni = natural interference calculated for particle Di assuming the IPS is calculated inside its sub-suspension interval, and m = number of size diameters. Fig. 9 demonstrates [46] the accuracy of the Interference Algorithm in predicting actual viscosity estimated by Casson's Model from experimental results for several dispersed cement-filler-blended

3.0

ηc (Pa.s)

2.5 2.0 1.5 y = 1.0479x + 0.0699 R² = 0.9913

1.0 0.5 0.0 0

1

2

3

INTp

Fig. 8. Unitary particle distribution for some specific diameters (125, 75, 45, 22.5, 11.25, 5.63, 2.37, 1.0, 0.5 and 0.25 μm) of the tested cement considering the Farris 10 × diameter ratio for particle interactions [46].

Fig. 9. Linear relation between experimental results of viscosity (Casson's model) and the Interference (INTp) calculated accordingly Eq. (5) [46].

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(a)

(d)

(b)

(c)

Fig. 10. Distinct cement-filler particle size scenarios: (a) Clinker particles with nonoptimized PSD and high porosity; (b) clinker is partially replaced with dilution filler and ultrafine performance filler with dispersant and improved packing, which reduces porosity and water demand; (c) excess of ultrafine performance filler reduces packing and increases water demand; (d) excess of dilution filler and lack of clinker and performance filler results in high porosity and lower strengths; (e) without admixtures, agglomeration destroys packing and mobility, increasing water demand.

(e) like blast furnace slag and pozzolans, but also inert fillers. However, strength correlates well with the water/binder (w/b) ratio, which is a proxy for the non-combined water, which comprises the bulk of the paste pore volume. The values of the w/b ratio are higher than the usual water/cement ratios for ordinary concrete. A w/b ratio of approximately 0.5 can result in concrete strengths from 65 MPa up to 90 MPa, whereas a 20 MPa strength can be produced with a w/b ratio between 1 and 1.5. Data also show that concretes with different paste porosities (estimated by w/b ratio) can present the same strength. The w/c ratio merely describes the water concentration in the cement paste, and varies between 0.22 kg/kg and 0.5 kg/kg which has no correlation with porosity or strength. Water concentration in cement paste is lower than for typical concretes, even though the density of the filler is lower and its volume is higher than that of the clinker. A reduction of the mixing water content with no increase of the aggregate packing requires an increase of the amount of “cement” (fillers plus binders) to maintain the paste volume constant, a requirement for adequate rheological behavior. Therefore, the reduction of the clinker (or binder) fraction in the cement is higher than the actual CO2 footprint reduction of the concrete (Fig. 12). The binder intensity (bi) [60], defined as the amount (kg/m3) of reactive binder for each MPa of compressive strength at 28 days, can be much lower than the best global benchmark which is around 5 kg·m− 3·MPa− 1 for concretes above 50 MPa, and increases respecting the minimum cement content of 250 kg/m3, reaching −3 −1 12.5 kg·m ·MPa for 20 MPa concretes (Fig. 13a). The new technology, high filler content compensated by a reduction of mixing water demand by packing and dispersion, gives bi results as low as 2–4 kg·m− 3·MPa− 1 at values 50% lower than for current concrete technology; a bi of 5 kg·m− 3·MPa− 1 is obtained using current technology for concretes above 50 MPa. The resulting CO2 intensities (Fig. 13 b), ranging between 2 and 5 kg CO2·m− 3·MPa− 1, can be equivalent to those obtained by replacing clinker with other reactive SCMs which are considered CO2 neutral because they are wastes. The best formulations using pure clinker and high filler have a CO2 footprint equivalent to concretes produced using current technology, which use a clinker factor between 0.3 and 0.4 (Fig. 13c). However, not all formulations with high-filler perform equally well in comparison with the benchmark, particularly with respect to binder intensity. It is worth mentioning that an increase in aggregate packing can

suspensions of various mineral compositions, surface area, and particle size distribution, from basic particle and liquid characteristics. Therefore, the goal of minimizing the amount of water needed for the desired rheological behavior can be achieved by minimizing the interference in multimodal particle size packing. A successful approach to develop high-filler cements can be achieved by incorporating a combination of optimized clinker particle size distributions (to ensure hydration) and at least: (1) a dilution filler that has approximately the same particle size as the clinker; (2) an ultrafine performance filler that reduces the interparticle volume of pores; and (3) a dispersant that prevents particle agglomeration to disrupt mobility (Fig. 10). This approach allows for a reduction in water of 30–50% [50–53] in comparison to standard mixtures. The effect of packing with low interference can be appreciated considering that the usual water reduction with dispersants is between 10% and 15% [54]. However, when increasing packing density, other problems arise. Mixing energy needs to be controlled because good mixing is a precondition for dispersion. Another problem is that the compatibility between the binder and dispersant can interfere with the stability of dispersion over time, particularly at high temperatures, or can it lead to a dosage that retards hydration. 4.2. Practical application of filler substitution There is a growing number of studies that portray the application of high-filler content in concrete from various groups, including Sweden (KTH and CBI) [55,56], Germany (U Darmstadt [50,57], U T Karlsruhe [51,58], VDZ [59]), and Brazil (U São Paulo) [46,52,53]. The results show that it is possible to substitute up to 70–75% of the binder with a combination of engineering filler, which produces concrete with compressive strengths of up to 90 MPa. This is achievable if the dilution is compensated for by a sharp reduction of mixing water, from typical values between 170 and 200 L/m3 (liters of water per cubic meter of concrete) to 90–165 L/m3 range. The total binder content – clinker or other reactive SCMs – can be reduced to below 100 kg/m3. Some concretes with water content below 110 kg/m3 have slump exceeding 22 cm. As Fig. 11 shows, there is no correlation between strength and filler fraction or with water/cement (w/c) ratio, assuming, as usual, that cement may contain not only binders such as clinker and reactive SCMs 6

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100

Compressive strength (MPa)

Compressive Strength (MPa)

100

80

60

40

20

50 55

50

55

80 50 50 50 50 50 50 68 68 60 60 50 50 40 52 68 685240 50 5020 20 58 58 50 68 50 5050 5068 50 4165 64 62 65 62 41 50 64 65 50 65 34 34 65 65 71 69 7169 68 68 78 7865 65 67 67 75

60

40

20

0

75

0 20

30

40

50

60

70

80

0.0

0.2

Filler content (%)

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

water/binder and water/(binder + filler)

a

b

Fig. 11. (Left) Filler fraction in cement has little or no correlation with concrete compressive strength. (Right) Water/cement (binder + filler) ratio (in red) and water/binder ratio (in blue) versus compressive strength. Numbers represent filler fraction (%). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Data from [46,52,55,57–59] plus some of our own unpublished data.

reduce the amount of paste (binders, fillers, water and air) required to fill the space. The rheological behavior of the resulting concrete is not negatively affected, and an increase in such aggregate packing will further improve the environmental performance.

resulted in compressive strengths 51–144% higher than the original CEM I cement paste (Fig. 14a). Despite thermogravimetric results revealing the chemical reaction of fillers, particularly cristobalite and nepheline syenite at 28 days, the great majority of the variation in the strength between filler-containing formulations (Fig. 14b) could be explained by the simple variation in filler density. The filler influence on the initial volumetric water concentration is reflected in the porosity of the hardened paste. When replacing clinker with filler on a mass basis, which is typical in cement standards, filler density matters. It also matters for users because the lower the density of the filler, the larger is the resulting paste volume. Volume replacement will circumvent this problem (which is present in cements with high fractions of fly ash and slag), but it will add an additional step to the concrete and cement quality control process.

4.3. Additional filler minerals In general, industry standards limit filler additions to limestone (carbonate) materials within a specified range of chemical compositions. EN 197-1 requires a minimum of 75% of CaCO3, and the Brazilian standard, NBR 11578-1991, requires a stricter 85% minimum of CaCO3. This requirement seems to be primarily a reflection of the excellent performance of limestone as a filler. In addition, having limestone available at all cement plants makes its use economically convenient. However, any inorganic mineral product that has compatible strength and no deleterious chemical reaction with clinker can be used as a filler. The century-old USA experience with the Arrowrock and Elephant Butte Dams (see Section 3.1) shows that certain types of granite and sandstone can be used safely. There is additional literature supporting the use of quartz [25,26,55,61–63], dolomite [25,46], granite [12,46,52], cristobalite [52,55], nepheline syenite [52,55], and wollastonite [55]. Damineli [52] replaced 50% wt./wt. of CEM I with fillers of different mineralogy. The filler densities ranged from 2.35 (cristobalite) to 2.82 (dolomite). The water/binder (or clinker) ratio was a constant 0.5 (wt./wt.), which implies a 50% solid concentration (wt./wt.) increase when filler is added. The reduction of paste porosity due to the lower volumetric concentration of water in filler-containing formulations

4.4. High-MgO limestone and dolomite as fillers The use of carbonate filler with chemical composition that results in more than the maximum allowed MgO in the clinker by the standards, which is usually around 4–5% [64], is of interest because they are available in many cement plants. Even if their performance is inferior to pure limestone, it would be better than no filler at all. Dolomitic fillers have been used in self-compacting concrete formulations for at least 15 years [19,65,66]. They have been recommended by the European Federation of National Associations Representing Concrete (EFNARC) since 2002 [67]. The European specification for self-compacting concrete edited in 2005 by

80

300

75

Volume (dm³)

164

185

199

114

123

141

200

Water

150 225 100

269

Binder

270 50

0

75 20 MPa

64 56 56

40

50 Filler fracon

43

36

CO2 migaon

30

Filler+binder increase

20

225

145

90

0

60

330

265

220

Filler

Relave to reference (%)

65 250

0 0

65 40 MPa

0

20

50

40

70

Concrete strength (MPa)

70 MPa

a

b

Fig. 12. (a) Composition of cement paste with or without fillers for concretes with 20 MPa, 40 MPa, or 70 MPa 28-day compressive strengths. The figures represent the amount of each phase in kg/m3. (b) Comparison between formulations with and without filler, presenting filler fraction on fines, CO2 mitigation, and cement (filler + binder) content. Data from Proske et al. [57].

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Fig. 13. Comparison of high-filler, low-water-demand concrete formulations with a global benchmark set by Damineli et al. [60]. High-filler technologies are represented in darker colors. (a) Binder intensity; (b) CO2 intensity versus compressive strength; (c) clinker fraction versus CO2 intensity. The red lines identify the limits of the benchmark for conventional concretes. The green line in (b) identifies the inferior limits of CO2 footprint of concretes made with pure clinker. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

CEMBUREAU, EFNARC, ERMCO, BIBM, and EFCA [68] also recommends dolomitic fillers. In the USA, dolomitic fillers have been recommended since 2007 [69] by the Center for Advanced Cement Based Materials (ACBM). Many other organizations, such as the US Transportation Research Board [70] and the Portland Cement Association [71] recommend “limestone” fillers, which they define as a technical term that covers all mineral carbonates, including dolomite [72]. No reports of expansion problems have been detected in the literature. A recent, fairly comprehensive study conducted by the cement manufacturers in Heidelberg [73] shows that the chemical reaction between dolomitic filler and clinker mainly results in hydrotalcite, with no report of any expansion. Justnes [74] considers dolomite to be better than calcite. The so called “alkali‑carbonate” expansion has been lately associated with the presence of reactive quartz [75,76]. The accumulated practical experience and recent research results show that there is no technical justification to exclude dolomites and high-Mg limestone as fillers in cement by current standards.

(a)

4.5. Scope of application of high-filler cements Filler materials can be used as binder replacements in all markets, including mortar, prefabricated concrete components, fiber-cement (where they already have been in use in high amounts for a long time), and even to high strength structural and mass concretes. Eventually, demanding performance requirements for specific applications or aggressive environments will be more easily achieved with low-filler content materials or by barring some reactive minerals. Filled-cements can also be formulated to be used by non-professional builders. Aside from direct substitution during cement manufacturing, fillers can and often are used in the partial substitution of cement during the mixing process by industrial facilities when producing ready-mixed concrete, dry-mix mortar and concrete, and building products. This will certainly enable a more rapid increase in the average filler content in cementitious materials, even in an improbable scenario where cement standards remain unchanged. In this case, the filler may be supplied by other companies, including the largest multinational filler producers, and the cement sales will decrease accordingly. This also allows for a compromised solution, wherein the cement industry increases the average filler content to a limit that allows industrial users to add more fillers if they wish. This may limit the potential mitigation of the technology, as it will require much better communication regarding cement composition than exists today. Fillers can potentially be used with all new binders, including geopolymers, carbonation-hardening cements, and cement made with new clinkers such as BYF (belite-ye'elimite-ferrite) and CSA (calcium sulfoaluminate). Fillers can increase the market competitiveness of these new cements.

(b)

4.6. Robustness of the technology Thirty years of industrial experience in various countries has demonstrated the robustness of replacing up to 35 wt.% of clinker by interground limestone filler [31] with no dispersant. Robustness of the technology for higher amounts of filler – High Filler Low Water (HFLW) – that rely on dispersion to reduce water demand and compensate dilution, is yet to be demonstrated in various market and climatic conditions. Table 1 presents the main aspects that can become problems for robustness.

(c)

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Fig. 14. Influence of filler mineralogy on the compressive strength of cement pastes at 28 days (a), with water/binder ratio equal to 0.5. Filler density influence on volumetric water concentration and estimated hydrated paste porosity neglecting chemical reactivity (b). Data from [52].

The production of cements with controlled and stable particle size distribution by using a separate grinding process followed by drypowder mixing is not technologically a problem. Particle segregation during transportation of bulk volumes may be a problem. However, the dry-mix mortar industry has been able to formulate composites where segregation is not a problem despite the much larger range of particle size distributions. A challenge for robustness is both the compatibility between cement and admixture, and the time stability of its dispersing effects, especially at high temperatures. A low initial workability or a premature loss of workability will encourage the untrained user to add more water than specified, which will reduce the mechanical strength of the system. The presence of fines or clay contamination in aggregates in amounts that significantly increase the total surface and the demand for dispersant also introduces a risk of increasing water demand, which affects porosity, strength, and durability. This problem will be more relevant to the bagged cement market than for the bulk market, where aggregate quality control can be implemented. These may make bagged dry-mix concrete desirable because the compatibility of the cement-admixture and aggregate quality can be implemented by the producer.

Fig. 15. Fraction of cement used in reinforced concrete production. Data from [77,78] assuming cement consumption of 0.35 t/m3 and steel reinforcement density of 0.1 t/m3.

The replacement of clinker with fillers reduces the alkaline reserve of cementitious materials. The carbonation depth (dc) over time (t) can be described as dc = k · t1/2. The k (Eq. (6)) is directly proportional to mass transport properties or the permeability (p), which is related to the concrete porosity, paste volume, and IPS [79], and it is inversely proportional to the alkaline reserve (a). The alkaline reserve depends on the chemical composition and the amount of hydrated phases.

5. Durability of HFLW cementitious materials A major durability problem in cement-based materials is the corrosion of steel reinforcement. Carbonation and chloride exposure can cause reinforcement depassivation, allowing corrosion to initiate. Therefore, the major problem is confined to steel-reinforced concrete, which is estimated to consume only approximately 25% of world cement production and < 35% in countries with the highest consumption rate (Fig. 15). Therefore, the majority of cement used in construction does not need to protect steel.

k∝

p a

(6)

Therefore, by replacing binder with filler and holding all other variables constant, the carbonation depth will increase because a decreases. However, when a high binder replacement ratio with filler is

Table 1 Robustness of the technology in different scenarios of utilization. HFLW (High Filler Low Water) identifies formulations where filler dilution is compensated by packing and dispersion. < 35% identify the usual technology of interground filler. Is the technology suitable?

Unknown

Proved possible

Needs further development

Use in poor and remote regions For an illiterate worker Lack or poor control of aggregates Poor control of water content Possible to use without admixtures Hot climates Stability of workability at high temperatures High strength at early ages (precast) Sensitivity to common contamination

HFLW HFLW

< 35 < 35 < 35 < 35 < 35 < 35 < 35 < 35 HFLW < 35C

HFLW HFLW HFLW HFLW

HFLW

9

Not possible

HFLW HFLW HFLW

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compensated for by a reduction in mixing water, the concrete porosity is also reduced, which tends to reduce p and k. Consequently, the effects of filler on carbonation will depend on the balance between the variation of a and p. Data from VDZ [59] show that increasing the filler content from 17% to 50% (+ 290%) while reducing the water/(binder + filler) ratio from 0.5 to 0.3 (− 40%), while keeping the paste volume constant, results in a reduction in the mortar porosity of approximately 25%, and in an equivalent carbonation rate. Therefore, carbonation is not controlled solely by the filler content. Because it is possible to adjust binder and water content, i.e. the IPS [79] of aggregates, while maintaining strength almost constant (see Fig. 10 and item 4), it will be feasible to design concrete formulations that comply simultaneously not only with rheological and mechanical performance specifications but also with certain durability parameters such as carbonation resistance. It is also desirable to maximize carbonation as a strategy for “carbon capture and storage” (CCS) in all applications where steel protection is not an issue. This is the case with mortars and plasters and small unreinforced concrete blocks and pavers, which comprise a significant portion of the cement market in developing countries. This strategy also applies to indoor reinforced steel elements in dry environments. Models will be needed to allow for quantitative estimation of CO2 mitigation [80] by considering the actual formations to better estimate CO2 capture over time. The effect of filler replacement on chloride penetration resistance, as given by VDZ [59], shows a similar trend to carbonation phenomena. A 50% binder replacement with fillers requires a 30% reduction in water/(binder + filler) ratio to maintain the same chloride migration coefficient. This results in a 23% increase in the 28 day-compressive strength [81]. Steel corrosion is triggered when chloride concentration is greater than a certain critical chloride content, typically taken as 0.4% by mass of cement. Therefore, replacing binder with filler will reduce the maximum amount of chlorides needed to initiate corrosion. Chlorides may be present as contaminants in the raw materials (cast-in chlorides) or be transported from the environment. Coastal regions are rich in airborne salt, and roads and bridges in cold countries are subject to deicing salts [82,83]. Further, most reinforced concrete elements exposed to dry or indoor elements have a much lower corrosion risk than those exposed to outdoor elements. Carbonated reinforced concrete elements in dry environments (RH < 65%) protected from water have a corrosion rate that poses no risk to service life [84]. However, reinforcement corrosion rates in chloride-contaminated concrete can reach unacceptable levels at the same relative humidity [84]. Indoor concrete is exposed to much lower chloride from the shores [82]. These values are for ordinary concrete, but lowbinder, high-filler mixtures have a lower porosity and surface area, which will most likely will result in a lower equilibrium humidity and higher concrete resistivity, which may significantly reduce corrosion rates [85]. In addition, the majority of indoor concrete is usually finished with a paint coating or other materials, which reduces carbonation and chloride penetration rates. This is an aspect that is usually neglected. Therefore, while corrosion of steel reinforcement in concrete is an important problem, only a fraction of the total concrete and cement produced worldwide is used for protecting steel. In addition, concrete formulation can control its risk even with high filler contents. Freeze-thaw is relevant for concretes exposed to outdoor conditions in cold climates. Available data is limited, but suggest [81] that a reduction in water/cement (filler + binder) ratio from 0.5 to 0.45 is sufficient to compensate for 50% of filler. Further studies are necessary because the quality of limestone filler seems to affect the freeze-thaw resistance. The recent evidence presented confirms the conclusions of previous research [86,87] that service life is not a direct function of binder content or filler replacement rate. Further research is needed to make possible to engineer low-emission, advanced-performance concrete

Fig. 16. Evolution in the number of patents filled related to use of filler in cementitious materials. Search in Google Patents.

formulations that not only ensure the desired service life but also maximize carbon capture. This strategy can be applied to non-concrete protection solutions, such as paints or overlays or the use of corrosion resistant reinforcements such as galvanized steel rebar [88], which provide additional options to minimize environmental impacts. In the future, combining digital production technology and functionally graded material design concepts will potentially produce graded components. Therefore, it will be possible to produce a component with a low-binder, low-CO2 high-porosity core protected with a low-porosity external layer. 6. Stage of development and patent protection Cement produced by intergrinding clinker and limestone filler has been a standardized, market-proven technology for > 30 years. However, there are significant differences in maximum limits between countries, and the average replacement rate remains low. The new low-binder, high-filler method (which combines dispersants to reduce the water and compensate dilution) has already been tested in relevant environments such as ready-mix concrete and precast concrete. The addition of fillers during concrete mixing is already standardized in the USA [20,89]. This will have limited benefits because a minimum cement content remains in most of the standards, despite the abundant literature questioning its technical need [86,87] to ensure durability. The equivalent performance approach, described by De Schutter [90], introduces a possibility to overcome this limitation in some countries. A non-exhaustive search of Google Patents shows that the number of patents in the cement field has been growing and that they are mostly concentrated in the USA. There is currently a total of 20 patents (Fig. 16). The first patents related to the basic technology of packing and dispersion to allow flowability are > 20 years old. Authored by Ronin and Häggström [91–93], the patents seek to protect mechanochemically activated cements, a mixture between quartz, other SCMs, dispersants, and the clinker and gypsum associated with a specific process involving pre-grinding of fillers followed by an intergrinding of all materials. This patent does not allow for strict control of the particle size distribution in the individual phases as seen in the technology patent by LafargeHolcim (then Lafarge) [94] in 2007. The patent describes a multimodal composition of particles, including ultrafine fillers (below 1 μm), clinker, and other SCMs as binder, with total clinker consumptions between 25 and 160 kg/m3 [94]. Markets include a “binder premix” with Portland cement content between 5% and 35%. Other patent-owners include Roman Cement LLC. and Omya [95]. Omya is one of the largest specialty filler producers in the world, and has protected the concept of “functionalized filler” for use in cement. It appears that patents have quite a large overlap in terms of particle size distribution and technical concepts. Patents tend to ignore the 10

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secondary mineral in limestone, dolomite, and granite, as well as in SCMs such as metakaolin and calcined clays. It is not clear if anti-dust admixtures can be used to reduce occupational risk. Therefore, filler selection must include occupational health considerations.

influence of important aspects such as shape and mineralogy of the fillers and clinker, as well as efficiency of the superplasticizers. None of these patents seem to have been commercially explored to date, at least not on a large scale. In addition, we found no patent disputes on record in this subject. Experience with patents in other economic areas suggests that the primary goal for at least some of those patents was to block possible competitors [96] from the market.

8. Potential impacts on investment and production costs An increase in filler content will allow increasing cement production with lower or no investment in new kilns. In markets where kilns are already in operation, fillers will reduce kiln use and value, which may be an economic problem. Thermal energy consumption will decrease with filler content. In some markets, where alternative fuels are a source of revenue, a reduction of thermal energy demand can be problematic. On the other hand, the consumption of electricity, an expensive supply, will be increased due to finer and more sophisticated grinding and classification, as well as blending steps. The IEA GHG model cement plant [103] allowed us to roughly estimate that, without considering the cost of the dry dispersant admixture, additional silos, and blending equipment, filler production cost is approximately 37% of the total cost of pure Portland cement. If no special grinding equipment is required, which is not yet clear, CAPEX and operational costs are reduced by approximately the same value. Despite the uncertainty of our estimates, it is safe to assume that fillers do not increase cement production costs.

7. Potential of scalability 7.1. Raw materials Almost any inert natural rock can be used as filler, including mine tailings and other wastes (see Section 4.3) if they are free of deleterious minerals such as sulfates. Therefore, there is no limit on the raw materials that can be used to produce fillers, specifically because cement applications do not demand strict chemical composition, colour, or refractive index, common to the specialty filler market. However, filler mineralogy and particle shape may impose practical limitations on the maximum amount of substitution for some minerals [53]. Another issue is the world production of dispersants admixtures. The total dispersant content is very low, typically varying from 0.5 to 1% (wt./wt.) of total fines. Therefore, if dispersants become part of all cements produced, the admixture consumption will be approximately 25–50 Mt/year, which is a rather modest amount.

9. CO2 mitigation potential 7.2. Production process The technology will result in a substantial reduction of CO2 emissions and energy use over the life cycle of cement-based materials, at a low investment and operational costs, especially if compared with expected carbon capture and storage (CCS) costs. Filler production may require thermal energy for drying raw materials. Electrical energy demand will probably be 50–70% of the Portland cement — which is typically around 104 kWh/t for the CSI participants [104], roughly 10% of the total energy bill. CO2 emissions of filler will depend also on the electricity emission factor, which varies between 50 and 800 g/kWh. For the scenario from IEA GHG [103], with an electricity emission factor of 520 g CO2/kWh, the authors estimate an emission of 42 kg CO2/t of filler, without considering the extraction associated emissions. This value is consistent with the LCA reported data from the literature, ranging from 26 [105] to 75 [106] kg CO2/kg. The global warming potential of dispersants is reported to be 1.88 kg CO2eq/kg. However, the consumption will be as low as 5–10 kg/t of fines and the contribution to emissions will be low.

Filler production is an established technology, and companies supply equipment because there are other markets for fillers aside from the cement industry, including cosmetics, pharmaceuticals, hygiene products, paper, food, adhesives, plastics, sealants, and paints [97]. The global production of specialty fillers was approximately 100 Mt in 2012 [98]. Calcium carbonate fillers, ground and precipitated, are estimated as being from 66 [97] to 90% [98] of the total market. The raw materials used in fillers may require drying before processing. Most fillers are produced by milling, which is cheapest for products above 5 μm [99] in size. The ultra-fine filler class is perhaps a more challenging aspect of producing fillers on a large scale. Different from current intergrinding technology, multimodal distribution with high-filler formulated for low-water demand will require an individualized control of particle size distribution and specific surface area of each mineral component. Therefore, separate grinding, dedicated storage for each phase, and blending (dry mix) equipment will be mandatory. A dispersant must be applied at the grinding, blending or mixing steps. The dispersant must have a long shelf life and readily dissolve in water, becoming effective in a very short period to prevent water over-dosage. Filler production and blending can be completed at different sites, closer to market or filler sources, making logistics cheaper, and allowing for the exploitation of local raw materials and the easing of adaptation to local market particularities. However, this also has a potential to increase competition between producers.

10. Market penetration scenarios, barriers, and incentives Considering the current low limits for filler established by standards of countries like India, Brazil, the USA and Canada, and despite > 30 years of successful European experience with replacement ratios as high as 35%, it is obvious that there still exists considerable resistance in the market to fillers. This resistance includes some cement producers, despite the significant reduction in costs offered by fillers. Certainly, this resistance will be reduced by eventual increase of CO2 and energy related costs and the perspective of mandatory carbon capture and storage. Construction is a conservative, low-tech market, with highly regulated and standardized products. The market requirements for service life are also exceptionally long, which makes durability an intrinsically difficult research topic. Innovation in construction is slow and mostly introduced by construction suppliers [107]. However, cement is a standardized commodity and producers have limited experience in using product innovation to gain competitiveness. The environmental market is still small in the construction business, but is growing and fueling innovation [108]. Therefore, there is a

7.3. Health and safety The amount of inhalable and respirable low-solubility dust particles in air in the work environment is undesirable and already subject to strict regulations in many countries [100]. Therefore, the use of fillers composed by minerals with low solubility and with particles diameters that facilitate inhalation (< ~7 μm) may require workers to use additional personal protective equipment when manipulating cement or cement-based products. Examples of potentially problematic fillers includes crystalline silica (quartz, tridymite, and cristobalite), which is classified by IARC [101] as carcinogenic to humans (Group 1) and also causes silicosis [102] when inhaled. Quartz may be present as a 11

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market for low‑carbon cement solutions with proven technical performance and durability. The lack of reference standards for high-filler cement can be overcome by performance-based technical approval. However, standardization is a pre-requisite to large-scale market penetration. It can also be a time-consuming activity, especially if part of the technical community remains skeptical. The acceleration of the standardization process will require substantial investments in research and development. These investments will be capable of delivering scientific evidences that will be validated by the worldwide community of experts. Demonstration projects and technical approvals may also help to demonstrate the validity of the technology. A progressive increase of the maximum filler content may help to accumulate practical experience and public confidence, reducing the resistance to necessary changes. Market segmentation of cement products or the introduction of fillers in industrialized cement-based products may allow for higher filler substitution in markets where it poses no durability problems. Because fillers are mostly inert materials, they potentially carry the perception of being a low-quality product for the average consumer. In markets [15,16] where low-quality counterfeit cement made with fillers are still a problem, this perception will be even stronger. One option for the industry is to develop products that work better and have better environmental performances than ordinary cement, to shift the perception from a low quality to a cost/benefit analysis. A clearer communication of the cement composition and its technical and environmental performances is a precondition for gaining trust from consumers. Aggregate quality (especially regarding clay contamination [109]) and variable fines content affect dispersant efficiency. Therefore, it may also be a barrier, especially in developing countries. Industrialized products, including dry-mix concrete and mortar, can be a solution particularly in urbanized areas. If the cement industry fails to raise the average filler content in the cement, other companies will certainly capture the filler market, making it possible for industries producing cementitious materials to replace binder with filler during the mixing process. This trend is already perceptible in Brazil as well in the USA, which have adequate standards in place [20,89]. Therefore, it is necessary to acknowledge that the increase in filler content will proceed at a relatively slow pace. However, the pace will accelerate as science and technology advances and environmental pressure, CO2, and energy cost increase. There is also a learning curve from both industry and users, which takes time. Considering that (a) > 70% of cement is not used in steel-reinforced concrete, an application that is durability-critical; (b) it has been proven that replacing up to 70% of binder with filler is technically possible; (c) the fact that in up to 10% of interground filler, the dilution effect is usually compensated for by grinding; and (d) that in some markets such as Morocco the market already operates with a 20% filler replacement using simple interground dilution technology, with the new technology a global average of 30% filler replacement cement is considered possible to be reached in the year 2050.

• • • • • • • • •

12. Conclusions A newly developed technology allows for up to 70% of binders to be substituted by inert fillers, which compensates for the binder dilution effect. This technology has the potential to raise the worldwide average filler content in cement from the current 7% to 30% by the year 2050. This technology has the potential to diminish CO2 emissions in the cement industry at a low cost, because fillers require thermal energy only if raw-material drying is needed and the dispersant is used in low amounts. Packing optimization can be achieved simultaneously with lowwater demand for adequate rheological behavior by using a combination of sufficient IPS with low interparticle interference. This concept enables the formulation to be engineered to allow for the control of binder content and paste porosity, independently of the mechanical strength. It also has a potential to control durability in different scenarios and even to optimize the amount of CO2 captured during the use phase. Consequently, it will allow cementitious material formulations to be engineered to have low‑carbon and advanced performance, simultaneously. The data show that there is no technical reason to limit cement fillers to low-Mg high-purity limestone. Dolomites and other minerals have a potential to be explored by the industry, preserving valuable clinker-grade limestone quarries and facilitating logistics. The robustness of dispersants regarding the variability of cement and aggregate quality, as well as its stability at high environmental temperatures are critical aspects that need to be addressed. The degree of the success of this technology in the market, and its future contribution to CO2 mitigation, will depend on the advancement of scientific knowledge and degree of success of industry's technological development. An internationally coordinated research and development effort will reduce the costs and speed up its development.

11. Further research priorities The success of the high-filler, low-binder, low-water technology will depend on extensive research and development efforts, because the sharp reduction in binder content and material porosity may have unforeseen implications. An internationally coordinated research effort will reduce the costs and speed up the development. Urgent needs include:

Acknowledgments This research has financial support from The São Paulo Research Foundation (FAPESP), Grant no 2016/05278-5 and InterCement. MQ work was partially supported by FAPESP (grant no. 2012/15195-9) and CNPq (grant no. 136635/2016-4). VMJ and RGP acknowledge the support of CNPq (310705/2014-2 and 308716/2016-7). The authors wish to acknowledge with thanks the comments of Ellis Gartner and Karen Scrivener, as well as all participants of the UNEP/SBCI working group.

• Long-term •

contaminated concrete at various relevant environments, including dry indoor environments; Models to design mixes with the desirable rheological behavior as well as better and affordable tools to measure different aspects of rheology in cementitious suspensions; Dry-based dispersants, with higher robustness to cement and aggregate variability, with longer stability time in high temperatures of the tropical developing world; Performance of high-filler, low-water systems on the do-it-yourself market; Mixing science and equipment suitable for systems with high concentration of solids; Mechanical behavior over the long term, including adhesion to steel, fracture energy, fatigue, shrinkage, creep of the clinker-filler, and fire resistance of structures considering different fillers; Filler processing technologies and new filler mineralogy; Better processing and quality methods for aggregates; Models to estimate and optimize carbon-capture of cement-based materials; Integrated model to allow for the design optimization of concretes with low-emission and advanced performance over the entire service life, including rheological behavior, strength, durability, and CO2 capture.

durability, including aspects such as carbonation, chloride diffusion, abrasion resistance, alkali-silica reaction and frost-action and the risk of chemical reaction of various filler minerals with environmental contamination; Effect on corrosion kinetics of carbonated and chloride 12

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