Rethinking cement standards: Opportunities for a better future

Cement and Concrete Research 124 (2019) 105832

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Rethinking cement standards: Opportunities for a better future ⁎

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Vanderley M. John , Marco Quattrone, Pedro C.R.A. Abrão, Fábio A. Cardoso National Institute on Advanced Eco-efficient Cement-based Technologies, Brazil Department of Construction Engineering, Escola Politécnica, University of São Paulo, 05508-070 São Paulo, Brazil

A B S T R A C T

This paper discusses possible changes in cement standards to cope with megatrends: climate change and industry 4.0. Most standards are prescriptive, defining cements by composition of SCMs, hence the number of types increase, and composition limits widen. Cements are classified by compressive strength of fixed water/ cement mortars. This pragmatic approach ignores physical effects, that results in variable mixing water demand. It hinders the development of cements with low water demand, products of optimized particle engineering and dispersants, which allows formulate cements with lower clinker fraction. Also, most standard tests are difficult to automate to generate large datasets crucial to train artificial intelligence. Performance-based standards are an alternative, but simple and progressive approach is recommended to ease transition. Cement types should not be solely defined by composition but rather classified by performance characteristics including durability and environmental. Combined water fraction, cwf, may be a good parameter to replace strength class.

1. Introduction Standards are ubiquitous in the industrial society. They provide obvious benefits in consumer protection and simplify business, helping to create large markets, being particularly useful for commodities like cement [1]. They help society to manage risks. They increase productivity replacing an in-depth technical description of the desirable cement with quotation of the standard number and a cement type. In his keynote to the XII ICCC in 2007 Hooton [2] registered the 1995 ASTM Workshop “Cement and Concrete Standards Of The Future” – an event which dare imagine a better future (now our past, 2010) for cement industry. In the report from the workshop, Geoffrey Frohnsdorff and James Clifton [3] stated that cement standards should improve considering “computing and information technology, high-performance concrete (HPC) and material science in general, international trade agreements, and the needs for sustainable technologies.” Nowadays, 23 years later, this message is even more contemporary. Very few professionals in academy or cement industry will disagree about the urgency of change to increase the chances of a more sustainable human society. The main challenge is related to the mitigation of anthropogenic CO2 emissions, of which the cement value chain has an ever-growing share [4]. The large amount of natural resources used – about 1/3 of total consumption – and waste generated by the cement industry are also being recognized as an emerging problem that will require the development of a new circular economy model [5]. If the amount of knowledge and practical experience accumulated in 1995 was considerable beyond the one registered in the cement standards, this gap certainly has grown during the last 25 years. Modern

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digital electronics have allowed large improvements on existing devices, not only in terms of better accuracy and resolution, but also in speed, reduced size and reduced initial and operational costs. Today, electronics control most of the cement plants allowing real time automatic adjustment of the process. New knowledge and experimental techniques that enable measurements and design at the nanoscale have been introduced. The general advance of knowledge allows us to use sophisticated science-based models to better understand hydration. The existing thermodynamics model of hydrated solids is now a mature tool, supported by a database that has been continuously improved and enriched [6], and allows forecasting the produced mineral phases and their chemical composition. Coupled models of hydration, mass transfer and chemical materials degradation are being developed. If we succeed to coordinate the research efforts, in the future it may become possible to simulate, at low-cost and within a short period of time, the long-term behaviour of almost any mixture between clinker Portland and any potential supplementary cementitious materials. The research community has also grown and established much stronger ties with the industry and succeeded to establish networks, such as Nanocem, elevating to another level the quality of research and work. Average citations of papers published on CCR had grown twelve times, from < 0.5 to 6 and the total number of citations of paper's journal grown by a factor of seven between 1999 and 2017 [7], exemplifying at least one dimension of this growth. The information technology, that impressed the authors in 1995, is exponentially growing. To put the advance in perspective, in 1995 the ASTM president informed the audience that organization had an e-mail address. This was the year that Microsoft launched Windows 95 and an

Corresponding author at: Department of Construction Engineering, Escola Politécnica, University of São Paulo, 05508-070 São Paulo, Brazil. E-mail address: [email protected] (V.M. John).

https://doi.org/10.1016/j.cemconres.2019.105832 Received 25 March 2019; Received in revised form 25 July 2019; Accepted 25 July 2019 0008-8846/ © 2019 Elsevier Ltd. All rights reserved.

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cement-based industry. Nowadays a team of building designers usually define strength, durability and other aspects as a function of the most critical situation in the component or in the building. However, it is already possible to implement a much more optimized design, varying strength, mass transport or any other property of interest inside a single component, using existing models to functionally grade materials [20–22]. Digital production platform will then automatically produce a component with a variable formulation, matching the required specification at every point as it has been already demonstrated [21]. A beam or column will therefore be built by a range of formulations locally optimized to fulfil global performance requirements with minimum life-cycle environmental loads and costs. More sophisticated compositions can be developed, varying mix proportions, actual chemical composition of cement, admixtures or even applying protective surface coatings. The digital production platform can provide more adequate conditions for the successful employment of functionally graded materials creating finetuned hybrid products. Designers may be free to create shapes without the limits of mould, changing not only the way of construction, but also the way construction will be conceived [23]. Design may be defined either by architects' imagination or by mathematically-based topology optimization concepts [24,25] that consider localized demands and can be combined with functional gradients of properties to optimize materials performance. Mass customization can be also applied to the cement plant. It is possible that a future cement plant will be equipped with multiple silos (or multi-chamber silo), allowing separate grinding and storage of clinker, gypsum and SCMs, including inert fillers and chemical admixtures. A robotized dry blending system fed by the silos might be able to deliver custom-made cements for different markets or large consumers without requiring any human intervention and error. The potential gains embedded in this scenario seem to be sizable not only in terms of technical performance, but also in cost and environment. This solution will require some CAPEX but the need of knowledge development appears to be the greatest barrier. The current cement types based on composition may not be the best options to suit the needs of such advanced market. Apart from that, the 3D printing technology will probably benefit from cements different than those available today. For example, 3D printing requirements in terms of setting time and rheological behaviour are rather different than those of today's cementitious materials [26]. Therefore, stricter control of chemical reactions and rheological behaviour over time is required [23]. In a full digital supply value chain, digital machines and omnipresent sensors must generate large amounts of data to allow the formation of reliable AI models. In a cement plant, an AI model integrating all digital data generated from plant machines and quality control may help minimize the risks associated to the long-time between production and the actual compressive strength and performance test results, which a colleague has described as being equivalent to “driving a car combining the previous experiences with a video feed of the drive delayed by 1 min”. If AI identifies the risk of non-compliance on clinker that may impact strength development, for example, it will immediately fix the production process. Moreover, even a faulty clinker may be used if the information arrives on time to reduce the filler content before grinding or blending stages. Another option is to electronically inform industrial clients to adjust cement dosage or even to allocate it to markets in which its performance is still suitable. The gains in terms of economy, efficiency and environmental impact are evident. But, certainly, this will be a long transition. However, standard tests of the cement industry somehow defy the digital revolution: Vicat's needle is 201 years old being originally developed to hydraulic lime; Le Chatelier soundness is younger, from 1870; compressive strength was developed in 1851 [27] and the 1:~3 mortar was already in use around 1900 in the USA [28]. These tests are not digital, are intensive in labour, time consuming, and have not been

excellent desktop computer had 8 Mb of RAM, 1Gb hard disk and the best internet (or BBS) connection was 28.8 Kbps modem over a copper telephone line. Global processing power has been growing exponentially and computing and data storage cost decreasing [8,9]. We are living the “digitalization of everything” era [9] and entering in a new industrial revolution [10]. These technologies are multi-purpose and are going to impact all industries [9,11], including the construction sector [12–15] and, therefore, the cement industry. The 4th industrial revolution or industry 4.0 includes the digitalization of the value chain – from design to industrial floor –, not only by robots and 3D printing machines but also by ubiquitously present advanced smart sensors embedded in the Internet of Things (IoT). In fields which enough data are available, the power and speed of artificial intelligent (AI) algorithms are already visible, as it is on the fast progress of self-driving cars and digital assistants. Only when those digital devices begin to generate enough data - big data [16]- it will be possible to develop and feed selfimproving AI algorithms. AI will enable on-the-fly finetuning of processes, controlling and progressively optimizing day-to-day industrial operations, ensuring better performance with tremendous potential of positive impacts on economy and environment. The 4.0 industry will ease the adaptation of cement to actual client needs and probably will create new cements tailored to suit digital (3D printing) production processes. It seems that Portland cement will probably remain the dominant low-cost mineral binder in the foreseeable future [4] because it is produced with a variety of natural resources available worldwide that are capable to cope with the enormous and increasing demand of building and construction materials. However, in a business-as-usual scenario, cement materials industry can certainly loose competitivity due to a combination of (a) growing costs related to environmental mitigation, such as installing and operating Carbon Capture and Storage (CCS) technologies, environmental taxation and new regulations, which are expected to grow in parallel to tangible effects of global warming; (b) advent of innovative lightweight construction solutions produced in more robotized environments. This paper discusses possible impacts of two global megatrends: (1) the environmental sustainability and (2) digitalization of production – the industry 4.0 - in the cement industry. It also explores possible course of action in terms of changing a few key aspects of cement standards and material testing methods, not only to cope with the challenges, but also to better explore opportunities. The discussion is limited to aspects that, in our opinion, seem to be more directly affected by the megatrends; thus, key performance aspects such as those related to durability are not encompassed. This work is mostly based on literature review, but it does include a few new original ideas and data. We have no intention of making predictions. Rather than presenting definitive answers, the aim is to encourage discussion and to incentive research and innovation, which certainly will influence the future, hopefully for the better. 2. Digitalization and the cement industry An all-digital supply chain driven by low-cost almost unlimited computing power will probably change cement-based materials industry in unknown ways. For the sake of the argument, we dare to present a possible scenario. Projects will be expressed in standardized digital building information models (BIM) with unprecedent levels of detail and seamlessly digitally transferred to the production. Robotic machines will mass produce custom-made designs, allowing to reach large scale production without full standardization. The mass customization construction is already a reality in the prefabricated housing market of Japan, dominated by lightweight steel and wooden frame panelised construction [17,18]. This concept is slowly being expanded to other regions [19]. Mass customization construction can similarly be applied to the 2

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4. Cement standards

significantly improved by digitalization. If the multitude of machine sensors can generate thousands or millions of measurements per second and robotized analytics dozen measurement per hour, these tests cannot be easily and cheaply scaled up to generate a stream of data large enough to allow creating and validating the AI models. The near future will be a period when it will no longer be possible to avoid changing those tests, among other things.

Cement standards are among the oldest standards in many countries. Cement Committee of ASTM is C1, which was inaugurated in 1902 [40]; the Brazilian cement standard EB 1 was the first standard specification published in the country, in 1935 [41]. All these standards were prescriptive, meaning they specify, among other aspects, the composition of Portland cement. With the progressive introduction of supplementary cementitious materials – blast furnace slag, fly ashes or other pozzolans and, in due time, limestone fillers - the number of standardized cements has grown, including an increasing variety of composite cements due to new combination of SCMs. The SCMs' compositions are also more or less clearly defined in standards. On one side, the prescriptive specification of the cement's ingredients has the advantage of simplifying the entire process, including quality control. On the other side, introducing innovation in cement composition demands modification in prescriptive standards. This modification is time-consuming, requires consensus and delays the effective introduction of new products in the market; this is conflicting with the concept and rhythm of 4.0 industry. The EN 197-1 had 27 cement products in 2000. In 2006 the CEN/TC 51 committee started to discuss 10 new composite cements, with clinker factor varying from 35 to 64% [42] using the traditional constituents (limestone, granulated blast furnace slag, fly ash and natural pozzolans) and finally, in 2019, the prEN 197-1:2019 was issued. The latest proposed version of February 2019 included provision for the LC3-type cements [42] combining calcined clay pozzolans and limestone fillers, allowing the number of cement type notations increased up to 39, being 27 types of CEM II. The large number of cement types is certainly confusing. Allowing a broad range of mixtures in a single cement type, the increasing number of SCMs in a same cement, and the differences on reactivity and fineness of each SCM, are rendering progressively clients to estimate the actual composition of the reactive fraction of cement and forecast the expected performance in terms of heat of hydration, CO2 footprint, resistance to alkali-aggregates reaction, carbonation, etc. The classification is somehow losing its capacity to describe the technical performance. Despite the great deal of experience accumulated internationally and the impressive advance of knowledge on blended Portland cement, there are significant differences among countries. Some differences are related to local availability of raw materials and other technical reasons. Other differences are harder to justify. For example, despite the overwhelming success and solid technical data, there are still large differences on the maximum amount of limestone filler [4]: it took decades to introduce it in ASTM standards [2]. Another example is the use of calcined clay as pozzolan, which is limited in many countries, despite the successful use in the USA of about 100 years ago [43], the continuous commercial production in Brazil for nearly five decades and recent developments of LC3 formulation. Globally, these arbitrary standards limitations may imply in unnecessary higher clinker factor, and higher CO2 emissions, a loss for the environment and for society. Besides the large experience with these constituents, good understanding of their chemical reaction with clinker and water and the availability of thermodynamic models that allow forecasting the mineralogical composition, understanding the different effects on performance associated to the combination of SCMs and on in-use performance is becoming difficult. This is an area that certainly requires further research [44].

3. Environmental mitigation Environmental problems require new supplementary cementitious materials, since available blast furnace slag and fly ash are insufficient to supply the cement demand [4]. Apart from fillers – with plural, since limestone is by no means the only option [29,30] – there is no other source of traditional secondary SCMs as large as those ones. On the other side, there are myriad local industrial wastes that can be explored as SCMs and artificial pozzolans as calcined clays [31,32] and others. The diversity of raw materials for cement will grow even more, because local availability and the potential benefits of blending various raw materials that may complement each other [33,34], something that is already a fact. Since industry is already accepting inert materials as cement constituents, it seems inevitable to start accepting SCMs with low degree of reactivity. All considered, by today's standard definition, the number of cement types would increase continuously, or compositional types would be defined loosely and, therefore, become less and less capable to describe the actual cement composition and inform the user of its expected performance. Another requirement is the increase of resource use efficiency, reducing simultaneously the environmental impact [35]. Increasing the strength to reduce the amount of materials is certainly an option with limits related to aspects of acoustical performance of buildings, buckling, etc. that digital production might help overcoming. Another necessary strategy is the reduction of the binder (clinker and other chemically reactive materials) content required to produce cementitious materials; this can be better achieved by a multiscale approach. In the millimetre-scale, particle size distribution of aggregates must be optimized considering the actual particles' shape to ensure flowability with minimum paste volume. On the micrometre scale, optimizing particle size distribution and morphology of fines, plus ensuring dispersion to achieve the optimal packing density to make a paste with reduced amount of water and adequate rheological behaviour [29,36]. This water reduction will allow replacing the binder fraction by filler, within the limits allowed by durability and other requirements. There is evidence, proved in relevant industrial application, that it is possible to replace up to 70% of binder by filler [29,37–39]. Because it compensates dilution by reducing the amount of mixing water, it can be performed without extra-grinding of the binders, even at the mortar and/or concrete plants, but only if the producer has the technical skills and the location has the required supply chain, which reduces dramatically the scalability of the technology and its mitigation potential. Therefore, in the best of interest of humanity, binder dilution should be performed by cement producers. Another dimension is the need of a more circular economy of cement-based materials, which implies to increase recycling rates of construction and demolition wastes. This will probably demand innovative technologies to improve the separation of cement paste from aggregates. Cement industry, which so far has been little active in this subject, will probably have to deal with its low-carbon waste, and incorporating the recycled cement paste in the cement production process can become the best available option in many places. The forecast is of a competitive future; hence the cement industry must be flexible and capable to seize the best available raw material. In other words, cement industry must make easy the introduction of innovative products.

4.1. The limits imposed by current prescriptive standardization Apart from specialized cements, such as sulphate resistant, current standards classify cements based on the content and combination of a limit set of maximum eight SCMs according to the prEN 197-1:2019. Because the traditional reactive SCMs, slag and fly ash are practically 3

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generic terms how a cement must behave when exposed to certain conditions of use, and what is reasonable to be expected over its service-life in order to fulfil the needs of its various users. The role of performance-based standards in facilitating the introduction of innovation is recognized in the literature [57,58] by various organizations such as the European Conference of Ministers of Transport [59], the OECD [60], particularly to overcome environmental problems. The potential of the application of performance-based standards and codes to allow innovation in construction is known for long time. In 1972 the RILEM president, Dr. Wright [61], reported that the proposition of performance-based construction dated back to the 1930's. France was the pioneer on adopting a national performancebased technical approval, “Agrément” system for innovative building technology around 1950, issued by the CSTB [62]. Dr. Frederick Lea, author of the famous Lea's Cement Chemistry Handbook reedited since 1935, was the first to present a comprehensive view of performance concept applied to buildings in 1962 during a CIB conference.1 In 1982, the ISO 6241 Performance standards in building – Principles for their preparation and factors to be considered - was published under the leadership of Gérard Blachère, one of the pioneers of performance thinking in construction [63]. In 1989 the first version of the Construction Products Directive institutionalized the performance concept in the European Union. For construction products and systems, performance-based standards have been a reality for > 30 years. Apart from European Union, other countries have adopted the same scheme. It includes Brazil (National System of Technical Approvals), Canada (Canadian Construction Materials Centre), South Africa (Board of Agrément South Africa), Australia (CSIRO), Israel (Building Systems Evaluation & Approval), Japan (Center for Better Living), USA (ICC), Russian Federation (FCC). It means that, in most countries, the construction sector, particularly the building sector which consumes most of materials, is already familiar with the performance concept and its application. The technical approval system has succeeded because it reduces the inherent risk related to innovation [64], increasing the acceptance of approved new technologies among private and commercial users, but also policy makers [65]. It also reduces the risk from the producers' point of view by giving a validated model to submit new technologies. However, currently the technical approvals are usually issued for product that has defined constituents and composition, an aspect that can be overcame by better data able to link composition of reactive phases as well as reaction kinetics to various aspects of performance. The potential of performance standards in cement has been acknowledge for almost 40 years. Frohnsdorff et al. [66] in 1983 stated: “it is generally agreed that performance standards should at least be available as an option”. Twelve years later, 1995, in the Workshop “Cement and Concrete Standards for the Future”, Bryant Mather, ASTM C1 leader at that time stated “performance tests are necessary so that cements and concretes with novel compositions can compete and innovative materials or systems can be evaluated.” [67]. The final report included the view of Frohnsdorff & Clifton in 1995 that predicted cement and concrete products standards for 2010: “Performance-based standards will be commonplace and will co-exist with prescriptive standards; they will facilitate evaluation of new materials and materials to be used under unusual conditions.” In the last 30 years, some progress has been observed in the adoption of performance-based standardization for cement and performance-based specification for concrete. In 1992, ASTM C1157 was published, a performance-based cement standard meanwhile the prescriptive ones were maintained. However, its market penetration was low and, four years later, cements produced accordingly to this

exhausted in most regions [4], the cement industry needs to start exploring a great variety of locally existing materials in order to mitigate CO2 emissions within the budget. Naturally, this requires changes in the cement standards to accommodate these local materials. The number of possible future SCMs is much larger than the 5–10 currently allowed by most standards. Except for inert filler minerals, these materials will not be available globally in large amounts as well as slag, fly ash and natural pozzolans. They will be relevant only in a few parts of the world, where availability and logistics favour. Individually their contribution to mitigation may be considered small. But, adding up the small individual contributions from the use of many SCMs, the total mitigation may become relevant. Today's standards usually specify fillers to be limestone only, which is actually slightly reactive [45], specially combined with aluminium rich additional SCMs such as calcined clay [46]. Most of standards go further, requiring clinker grade limestone. However, science evidences and practical experience in some markets show that almost any inert or quasi-inert material can be filler [29] and that even the limitation of chemical composition, such as maximum MgO content in limestone is not necessary. The number of minerals that can be explored as fillers, some of them presenting a useful low-degree of chemical reactivity [47,48] is unlimited in practical terms and may include mining residues. It is not possible to include all of them in prescriptive standards. However, requirements related to aspects such as occupational health and environmental contamination as well as long-term stability need to be addressed. There are also a variety of Si-Al-Ca-Mg minerals which can present various degrees of chemical reactivity when mixed with clinker Portland, including pozzolanic reaction consuming portlandite. It seems reasonable to explore all of them, including SCMs with rather low reactivity such as the already accepted case of some fly ashes [49]. The metallurgical industry is certainly an interesting source, and literature reports potential utilization of slags such as ferro‑manganese [50], stainless steel [51], ladle slag [52,53], electric arc-furnace steel slags both ordinary [53] and one optimized for cement production [54], copper slag [55]. Some of these slags – manganese, nickel, EAF steel slag, are already in use by the Brazilian cement industry as “pozzolanic materials”, because the Brazilian standard does not specify the source of pozzolans. Ashes from a large variety of biomass are silicon-based and, when properly produced, can be used as pozzolans [56]. None of these potentially new SCMs isolated can solve the CO2 problem. Some of them may not perform well in special markets due to leaching of chemical species that can contaminate water. The summation of all contributions of Si-Al-Ca-Mg residues, combined with a variety of fillers are yet to be estimated, but it will probably be far from insignificant. The problem is that including all these (new) minerals, and all possible combinations of SCMs rising from local availability, in the cement specification is not practical. The collective effort required to change prescriptive standard specifications to include a single new SCM is large. Considering the variety, it may become a never-ending task, probably making the effort not justified. Market players that have no interest or access to a SCM will not be keen to support. In a voluntary and consensus-based standardization process, a market player can take active measures to block the inclusion of a given SCM on cement specification to prevent a competitor from gaining a competitive advantage. In addition, keeping the current way of classifying cements will imply in a large growing number of cements types, which may generate confusion and no benefits for the cement users. From this point of view, a new approach to standardization seems unavoidable. 4.2. Performance-based standard: an old proposal Performance based standards specify goals to be reached or functions to be delivered, leaving the means to reach them to the market. In other words, the composition will not prescribe but rather specify in

1 It is worth noting that P.C. Hewlett, the current lead author of the book was for long time director of the British Board of Agreement (BBA) in charge of performance evaluation in the UK.

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fulfilment of purpose and reduce risk of failures [85]. Various applications of cement – reinforced concrete and rendering mortar, for example – have different requirements and the standard must encompass all of those. A “pure” cement performance standard, as advocated by Frohnsdorff and Skanly [66], must include provisions for mineral cements whatever its chemical and mineralogical composition, when used in any conceivable application and environment and various applications, ensuring minimum service life. This will require a very complex, time consuming and expensive evaluation. Despite the advance of knowledge, it will require a great effort of the research community. Simplification is, therefore, advisable.

standard were not accepted by the ACI 318 and 301 concrete codes neither by the ready-mix concrete [68]. In 2000, only 6 out 35 departments of transportation from the USA did accept cements based in the performance-based standard, but 32 of them approved blended cements accordingly to the ASTM C595, a prescriptive specification [69]. Authors identify several reasons for that, including: (a) the lack of minimum values on relevant requirements; (b) the complexity of the specification which offering too many options - five options for specifying compressive strength, which combined with the 6 cements types, resulted in 1730 possible cement varieties; (c) the minimum strength required was lower than the minimum in the prescriptive ASTM C150 and C595 and did not include 28 days requirements. Between 2005 and 2012, cement with up to 15% of interground limestone was commercialized under the C1157 standard reaching in 2012 < 0.5 Mt. per year – little more the 0.5% of the total production. Market acceptance started to growth after the Portland limestone cement was introduced in the prescriptive ASTM C595 in 2012, reaching about 1% of the cement market by 2016 [70]. The peculiarity of the North-American cement market to blended cements, which represents < 2% of the market [71] certainly have a role in the acceptance of the performance standard. Almost 20 years ago, the EN 206-1 (Concrete Specification performance production and conformity) introduced a flexibility on concrete formulation by accepting concretes that do not comply with prescriptive specifications but have equivalent performance to a reference concrete that complies with the prescriptive standard. The demonstration of equivalent performance or, more precisely, equivalent durability can be done as a European Technical Approval Document or through national standard [72]. At least Portugal [73], Belgium [74,75] and UK [76] had put standards in place. According to VDZ [77], in Germany a technical approval scheme was already in place in 2005. At least the Belgium standard applies the rule for non-standardized cements [74]. In 2015, both FIB [78] and ACI [79] issued reports in the subject and, in 2016, RILEM published a more comprehensive report on PerformanceBased specification [44]. Literature is becoming more and more abundant [75,80–83]. Nevertheless, almost all cements worldwide still are probably produced under prescriptive standards. The building industry has already accepted the performance approach in construction in a significant part of the world. Therefore, cement industry's clients probably are not a barrier to performancebased cement in the markets where performance-based concept is already established for construction products. More than that, performance evaluation of cements is already present in some markets. At the same time, the prescriptive standardization regime is generating a growing number of new cement types which is becoming difficult to understand and forecast the expected performance. The growing number of cement types combined with the accumulation of knowledge is eroding the confidence on traditional prescriptive criteria, such as water/cement ratio [74,79] and minimum cement content [84] in ensuring service life. As a result, countries with similar environment are adopting divergent criteria regarding the acceptability of cement types for a given application [78] among others. It is not a surprise that the introduction of an increasing variety of cement compositions, which has been described [64] as “no fundamental innovations… but many variations”, are simple combinations of known materials and used for almost a century, resulted in technical inconsistencies on complex subject such as durability. The reason is inherent to the consensus-based elaboration process of prescriptive standards, which is done most of times without throughout assessment [1], fuelled by the confidence and familiarity resulting from years of continuous use [1], with an empirical approach that still prevails in construction [64]. One reason for the fact that industry is still working on prescriptive cement compositions is probably the complexity of the pure performance approach [1,66,69], whose difficulties cannot be underestimated. Pure performance specification must be very comprehensive to measure in detail all properties and performance indicators to ensure

4.3. Ideas for a simplified performance approach A simplification without increasing risk can be achieved limiting the scope for systems based on clinker Portland with reactive SCMs from SiAl-Ca-Mg system. For mostly inert mineral materials - fillers - requirements must include dimensional stability and low water solubility. For these systems, reliable thermodynamic simulations [6] that allow estimating some of the long-term performances are required; however, in many cases, we fail in forecasting aspects related to durability and environmental interaction. Limiting the approach to ordinary applications – reinforced concrete, precast components, mortars – will further simplify without increasing the risk. In-use performance requirements can also be considered by allowing technical approval for limited markets, which further simplify durability studies. As soon as the knowledge and experience accumulate, the scope could be safely widened. Brazilian cement standard NBR 16697:2018 introduced a large flexibility in terms of pozzolana content (6–50%) and composition (SiO2 + Al2O3 + Fe2O3 > 50%, SO3 < 5%, total alkali content < 1,5% Na2O eq., and loss on ignition < 6%) as defined by NBR 12653:2014. Additionally, a Brazilian standard (ABNT NBR 5752:2014) for determining the performance index of pozzolanic materials states that a standard mortar formulated with 25% of Portland cement substituted by the pozzolan must deliver 75% of the 28 days compressive strength of the reference mortar made with the same Portland cement. Water is fixed, but to overcome workability problems, superplasticizer can be added in the pozzolanic mortar to achieve the same flow-table consistence index of the reference mortar (ABNT NBR 5752:2014). Although still based on broadly defined chemical composition, the Brazilian cement standard NBR 16697:2018 specification offers a very large degree of freedom for the market in terms of origin, chemical and mineralogical compositions. There are no requirements of chemical pozzolanic activity, neither requirements of long-term dimensional stability of all other phases eventually present (e.g. magnesium oxide or allotropic transformations). The minimum relative strength can be reached without significant chemical reaction, as simple side effect of porosity reduction resulting from the lower density product combined with fixed water/cement test and finer grinding of clinker. This flexibility allowed industry to explore new SCMs as ferro‑manganese and nickel slags “pozzolans”, in an unprecedent innovative activity. Despite the lack of detailed information regarding cement actual composition and actual Portlandite consumption, which may affect durability in specific environments, so far, no systematic problems associated to this standard have been detected in the Brazilian market. Probably, this outcome is the combination of good performance of those cements in the dominant application and environment, with a side effect of technically careful adoption of the solution by cement producers aiming to protect their market reputation. The fact that Brazilian regulation does not limit leaching of dangerous chemical species can also play a role. Therefore, an excessively complex standard can inhibit innovation, whereas a very flexible standard may increase the risks for the society. A performance-based standard must balance caution with the need of innovation [64]. Requiring evidence of a minimum level of technology readiness as a 5

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Fig. 1. - Life cycle of a cement that must be covered by a performance-based standard. By rigour, the raw materials used, the chemical composition of the cement will be outside of the scope of a pure performance-based standard. The physical characteristics of cement powder will probably not be prescribed either but will be an additional information provided.

condition. They should be described by their basic properties (e.g. alkaline reserve, gas and water permeability, rheological curves for typical water/solid ratio). Resistance to chloride diffusion is relevant only for steel reinforced applications with risk of corrosion. It should not be relevant for mortar and unreinforced concrete. It may become a special type of cement, like the sulphate resistant. Carbonation is usually seen as a problem due to the depassivation of steel reinforcement and the consequent induced corrosion. However, for applications such as mortars, unreinforced concrete, concrete in indoor dry environments [89], carbonation acts a CO2 capture strategy. It not only can improve the microstructure of cementitious materials, but, most importantly, it may also reduce cement industry carbon footprint [90]. Of course, most of the reinforced concrete applications need to be protected against carbonation, thus, cements designed to accelerate carbonation could be a special class of low CO2 footprint materials, exploring carbon capture and becoming attractive in many markets. Minimum initial setting time is a simple method to establish the rheological behaviour variation over time, which is a performance requirement for fresh cement paste. But the 3D printing and shotcrete [91] applications require shorter setting times than ordinary concretes or mortars. More than that, each digital production route – from dry cast to 3D printing by extrusion – will benefit from cements with particular rheological behaviour over time, demanding greater control of rheological behaviour than the one employed for today's technology. A digitalized cement plant and supply chain can become flexible enough to supply tailor-made cements and concretes to suit client's demand at minimum cost and environmental impact. This will require substantial advance of the knowledge and give rise to knowledge-based competition in the cement industry. By the other side, it may help industry to remain competitive.

precondition for the technical approval application [64] can also make the system simpler and robust. Technology readiness level allows understand the limits of pure laboratory research, making evident further developments such as demonstration of good performance in relevant environments integrated in the whole functional system [86]. Performance requirements must include all life cycle of the product, from storage to the end of service life (Fig. 1). This encompasses frequently neglected aspects such the shelf life of the stored cement and post-use “waste” impact. However, the major focus is the performance of the cement paste, both in fresh and hardened states, perhaps assuming aggregates chemically inert. In the fresh state, cement paste must provide a workable and stable (against phase separation) suspension, ideally with a minimum amount of water. In this stage, requirements may consider the time-resolved rheological behaviour – today's limited to setting time - stability of the suspension regarding phase separation under dynamic or static conditions (bleeding, aggregate settlement, particle segregation, etc.), reaction kinetics, heat of hydration, chemical shrinkage, etc. The hardened cement paste must provide adequate mechanical properties – a result of the dissolutionprecipitation of hydrated phases increasing the solids volume – and dimensional stability, something strongly related to the microstructure, particularly its pore structure. Durability requirements will depend on the markets and the selected environments, because not all cements must perform well in all possible environments. For hydraulic cement, water resistance of the hydrates is mandatory. It also should include requirements concerning the environmental impacts in cradle to cradle basis. The top agenda includes footprints of CO2, energy, water, waste and natural resources use. Another relevant topic is the risk of contamination due to leaching of chemical species during the use and postuse phases [52,87,88]. An appropriate assessment method, with acceptable repeatability and reproducibility is needed for each requirement, which is not simple to be achieved without extensive research, as RILEM workgroup report clearly demonstrates [44].

4.5. Performance assessment The application of performance-based standards is not straightforward, requires deep and comprehensive knowledge of materials and its interaction with possible environments. In building products, performance assessment is a domain of specialized organizations, that in Europe are united in the European Organization for Technical Assessment (EOTA) which issues Technical Approval for innovative products without standards, based in clearly stated methodology for each product class (a.k.a European Assessment Document). The German Institute for Building and Technology provides this service for the industry at least since 2005 [77], with several special cement products being approved until today,2 including some containing new SCMs, such as the Durabilo3 from Lafarge-Holcim. Still, the composition of those cement is clearly described.

4.4. Performance-based cement classification criteria A classification of cements based on their raw-materials composition is not coherent with performance-based approach. A performancebased cement classification can be multidimensional, emphasizing relevant differences for the market's point of view, both professional as well as retail. For retail market, classification accordingly to application – e.g. masonry cements – probably will be the best option. For the professional market, with bulk cement, a more sophisticated approach may help cement industry maintain and even increase competitiveness, because it will allow industry to select the cheapest composition that fulfil users needs. Nevertheless, experience related to ASTM 1157 [69] recommends to keeping the classification system easily understandable by professionals. Performance requirements related to aspects such as durability, mechanical performance, environmental or rheological behaviour are relevant and different for each application and environmental

2 For a list of technical approvals https://www.dibt.de/fileadmin/ verzeichnisse/NAT_n/vSVA_3.htm. 3 https://www.holcimpartner.de/produkte/cement/durabilo-4.

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where admixtures dominate. Over the decades cement composition and fineness have changed. Construction industry demanded shorter production cycles, requiring a cement with faster strength growth rate and higher final strengths. By the other side, environmental pressures caused an increase of SCMs content, reducing chemical reactivity and strength gain speed. To answer both demands simultaneously, cement surface area became higher [92]. Increasing fineness becomes a tool to partially compensate the dilution by SCMs: the less reactive is the SCM and the higher its fraction, the finer the cement tends to be. Additionally, due to the replacement of clinker by SCMs the cement density has been reduced, increasing the volumetric concentration of solids in a fixed 1:3 weight bases mix proportion mortar. As a result of fineness and lower density, without packing optimization and dispersion the water demand for an acceptable rheological behaviour does increase. In this scenario, the fixed water/cement is becoming a problem: for many contemporary cements the ~0.5 water/ cement is not enough to produce a workable mortar. In these cases, the mortar is dry, and proper casting following the standard procedure becomes difficult, often impossible. Consequently, the test variability increases due to higher frequency of large and random defects from inadequate workability. It has been found that sometimes variability increases to the point of making the test statistically inadequate [93]. When the amount of mixing water is adequate to ensure good workability, a coefficient of variation from 5% [94] to 7% [95] is expected in interlaboratory programs. On the other hand, for SCMs whose particle features (particle size distribution, surface area, morphology) have positive effects in flowability, which would allow the reduction of mixing water could even be reduced to provide adequate workability, as for fly ash (round particles) and some metallurgical slags (smooth vitreous surface) [96,97]. In both opposite cases, the fixed water/cement ratio is impairing an adequate evaluation of the cements: either causing an imperfect production of test specimens or penalising the strength values of cements with lower mixing water demand. To circumvent the moulding problems associated with fixed water content, it is not uncommon to technicians to add dispersant admixture, something that generally is not specified in standard test protocols. ABNT NBR 5752:2014 for assessing the performance of pozzolanic materials with Portland cement is an exception as mentioned in Section 4.3. Despite being generally non-compliant with nowadays standards, this practice controls variability and may help to detect eventual incompatibility between cement and dispersants [98]. Fig. 2 exemplifies the mentioned problems of a fixed water/cement showing the final spread and appearance of mortars after flow-table test: images (a) OPC, (b) 50FA are Brazilian commercial cements, and (c) LC3 – lab mixed cement using OPC, calcined clay and limestone filler. These mortars had no dispersant and, therefore, comply with Brazilian standard with fixed w/c of 0.48. Images (d) OPC_dis and (e) 50FA_dis are of mortars with w/c of 0.48 and the amount of dispersant to guarantee the full dispersion, as determined by rotational rheometry [96,97]. The dry and non-cohesive behaviour of standard mortars with OPC and LC3 is evident – more intensely for the latter –, which is not favourable for a good moulding. For the composite cement with fly ash (50FA), however, this amount of water provided a fairly workable mortar with a cohesive and continuous paste. Contrastingly, the w/c value of 0.48 became too high when the suspensions were effectively dispersed (images d, e, f), resulting in extremely segregated mortars due to the very low paste viscosity. Mortars with such behaviour are not suitable for proper moulding either, since a very heterogeneous microstructure tends to form, higher concentration of sand at the bottom, a progressively thinner paste at higher layers, and bleeding at the top. Optimum test conditions will require the water content to be adjusted in order to achieve good rheological behaviour as it is in practical applications, since proper workability is essential to ensure quality and productivity. The compressive strength obtained with fixed mixing water is an

The digital cement value chain scenario, as described in the previous item, probably can be better explored if the industry embraces the performance concept: an accurate quantitative description how the cement will perform over time before setting and after moulded will be needed. This will require that researchers from industry and academy collaboratively develop performance models that could be translated in common set of relevant performance indicators for cement. 5. Ideas for cement standardized tests for the future Digitalization will make possible and desirable to reinvent most of the test methods currently used for all materials, including cement, in order to reduce cost and labour, making viable the increase of test frequency. Discussing all the already existing opportunities, many of which in use at research labs, is not possible. Therefore, few tests we consider important as cement performance indicators and particularly crucial to scale up tests' frequency for developing an AI-based cement industry will be the focus of the discussion. 5.1. Mechanical performance Worldwide, cements of all compositions, are classified mainly by composition and 28 days minimum compressive strength4 with a fixed water/cement ratio. Compressive strength is not a performance characteristic of a cement, since the actual strength depends on the amount of water in excess to the needed for chemical reaction; i.e. the water amount added to provide acceptable workability. Cement compressive strength test performed with constant water/cement ratio as done today, is the prime indicator of the cement's chemical reactivity and binder capacity. Despite today's importance, compressive strength requirement was not universal in the past. For a long time, tensile strength was far more common requirement. In the USA compressive strength became mandatory for all cements only in 1953 [28]. Its importance for markets derives from the fact that stronger means better for most users, since stronger construction is generally associated to a safer construction. Technical users generally expect that selecting a cement with higher strength class will reduce the amount of cement needed in a product formulation, therefore, a savings opportunity. Today's typical standard tests are based on a mortar with mix proportions of 1 part of cement to around 3 parts of sand, being the mixing water constant around 0.5. It is 0.5 in European EN 197; 0.48 in Brazilian NBR 7215. In the USA ASTM C109 specifies 0.46 for air-entrained cements, 0.485 for Portland cement and for blended cements the value is defined by flowability. The amount of mixing water has been changed over time in almost all countries. The ASTM Portland cement specification used the normal paste consistency until 1934, fixed to 0.53 between 1934 and 1944 and defined by flowability 100–115% from 1944 to 1970 [28]. From 1937 to 1978 the Brazilian standard established strength with constant flowability; from 1978 it was fixed to 0.48, despite that the standard consistency test remained in the standards as a non-mandatory test. In Belgium, repeated changes on mixing water have also been reported [92]. 5.1.1. Constant w/c is becoming a problem It is understandable the decision of the standardization bodies in keeping the mixing water amount constant. Adjusting mixing water for workability is time consuming and introduces another variable in the system: small changes due to experimental variation in water demand have large consequences on compressive strength. More than that, the water demand with no dispersant admixture has limited correlation with actual industrial water demand in concretes, a major market 4

In the USA minimum strength is present but do not defines a cement class. 7

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Fig. 2. – Images, after flow-table test, of Brazilian standard cement mortars prepared with fixed w/c (0.48): (a) OPC, similar to CEM 1, (b) 50FA, composite cement with 50% of fly ash and (c) LC3, Portland limestone calcined clay cement; (d) OPC_dis and (e) 50FA_dis with optimized content of dispersing admixture and fixed w/c of 0.48. The w/c of 0.48 was adequate only for 50FA without admixture, whereas for all the other cements, this value was inadequate: OPC and LC3 mortars were dry and with no cohesion; OPC_dis, 50FA_dis and LC3_dis segregated intensely, since 0.48 was too high when the mortars were dispersed. Images (a), (b), (d) and (e) from [96,97].

particles and dispersion can improve ITZ and bond strength, even with 30%vol reduction of clinker content [105]. One possible drawback of the technology based on dilution by filler is that significant reduction of mixing water demand implies in lower paste volume for the same amount of cement. Considering the need of minimum cement paste required to provide enough paste to maintain aggregates apart from each other ensuring workability, the amount of cement powder may increase in some practical applications. In this cases, aggregate packing optimization can decrease the paste volume demand. Nevertheless, even in the cases that an increase of cement is required to compensate water reduction, filler dilution substantially reduces cementitious materials CO2 footprint [29] and is one of the most promising low-cost, worldwide-available strategy for emissions mitigation [4]. Hence, it is evident that both industry and society will benefit from cement standards that consider mixing water demand as a performance criterion of primary importance and test cement strength considering its actual water demand.

indicator of chemical reactivity but is becoming a poor estimator of the cement consumption. For instance, the increase of water demand above standardized 0.5 can cause a ~15–20% strength reduction in OPC blended with diatomite and ~8% for pumice [99]. For the client, such blended cements will mean higher cement demand in comparison with reference OPC cement with the same strength class. The use of fixed amount of water can result in the cement being classified with a higher standardized strength than the one justified by its predictable in-practice performance. This approach penalises cements engineered for low-water demand, which can be attained by improved particle features (particle size and morphology) and/or using dispersing admixtures. With a fixed w/c in the strength test, these cements are classified at lower strength class than they perform, making them less appealing for the clients and inhibiting the industry to adopt this strategy to reduce clinker factor with fillers and dispersants. 5.1.2. Environmental implications The major consequence of compressive strength determined from mortars with fixed mixing water is that it prevents industry to exploit the mitigation potential of low-water demand cements, which are based on the employment of dispersant admixtures or a combination of dispersion, optimized physical characteristics (size distribution and morphology) of particles and dilution by fillers. The use of admixtures is allowed in standards such as EN 197-1 2012 which limits to a maximum 1% of the mass. The decrease in water demand provided by dispersants makes possible to reduce up to 20% the binder content for the same strength, which has obvious benefits [100]. However, without adjusting the mixing water to the actual demand during application, the benefit is not evident if the cement strength class is defined by a fixed water/cement. Apart from that, in a fully dispersed cement, the w/c range (0.48–0.5) of the standards definitely produces mortars very prone to segregation (Fig. 2, images d and e) to be properly tested [96,97]. The fixed water on the cement strength determination also hinders the cement industry to explore the technology in which dilution by filler is compensated by reduction of mixing water, achieved by packing optimized to flowability and full dispersion. It allows reducing water by 50% making possible substitution levels up to 70% of binder by inert filler [29,37]. This strategy can reduce permeability, provide adequate carbonation rate and higher dimensional stability when compared with reference Portland cements [101–104]. Interestingly, recent experimental results also indicate that the combination of adequate fine filler

5.1.3. 4.0 cement industry and compressive strength Cement performance optimization by AI (artificial intelligence) requires massive amounts of data, both from process and about the actual performance of the cement product. Mechanical performance is a relevant performance indicator and a unique indicator of the chemical activity of the cement. Therefore, it seems to be crucial to properly training AI based system-optimization. However, the compressive strength testing are carried out currently with low frequency, typically in a daily basis or even, as required by the EN 196-1 and other cement standards, only twice a week [106]. These testing frequencies are not enough to provide the amount of data to develop a capable AI. And, because test is carried on “average” sample composed from sub-samples collected between tests, the effect of an eventual problems in a specific moment of production may be compensated in a sample with mostly good results. This makes difficult to correlate almost instantaneous process measurements with actual performance, making AI training difficult. Furthermore, brittle materials tend to require a considerable number of specimens to provide statistically reliable results of mechanical behaviour. One reason for the low frequency of testing is that, for process quality control, it gives answers too late, between 2 and 28 days after the event. Speedier results will be definitively important. Additionally, testing for compressive strength has not been fully automated and 8

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note a decrease of the correlation coefficient (R2 = 0.91); probably the lack of standardized method to determine the combined water has a large impact on the cwf. The typical scatter observed in Fig. 3 (a) CS vs. cwf curve is about ± 6 MPa in the worst case. This is a large value, but if we compare with interlaboratory coordinated by ATIHL [94], the compressive strength range for a single cement, with standardized test that has been performed for long time is ± 5 MPa. A more detailed analysis (not shown) of Fig. 3 (a) indicates that each cement formulation has a different optimum trendline. If we take one single class of cement tested in our lab using the same methodology, for example variations around LC3 formulations (filler + calcined clay) or limestone filler only, considering combined water between 7 and 91 days, R2 results around 0.94–0.98 and the data are typically scatter around the regression line ± 3 MPa, being the differences of the regression line coefficient significant. Apart of some model deficiencies, the major source of error is the determination of combined water, which maximum value for OPC varies between 22% up to 32% of the ignited mass [112], as discussed in item Section 5.2.2. The results seem promising and we are preparing a larger study on it. Because porosity and mass transfer properties are correlated with strength, they will also be correlated to cwf. Therefore, it is also expected good correlation between cwf and crucial durability-related performance indicators.

requires substantial specialized laboratory work. As discussed before, including the water demand in the standard is mandatory, which will imply in additional work. Therefore, it is reasonable to think that digitalization of the cement industry can benefit from a different testing protocol. 5.2. Combined water fraction–a possible cement classification criterion Compressive strength is not a characteristic of cement: changing the water/cement ratio is possible to produce a range of strengths. A few years ago, in a discussion with the deceased Dr. Ellis Gartner, we concluded that a SCM has almost always a double effect on cement: (a) it changes chemical reactivity; (b) its physical effect influences the mixing water demand for a flowable paste and cementitious product. The natural conclusion was that a simple indicator combining both dimensions would be useful, since we found the gel-space ratio difficult to work especially in a multi-composite cement. After some tests, the combined water (or bound water) fraction index (cwf) emerged:

cwf =

wc wm

where wc is the combined water or bound water at any age and wm is the amount of mixing water. The term combined water in this work refers to the water chemically combined by the binder during hydration, which was also defined in [107]. From data obtained in our laboratory, the determination of combined water followed these steps: (i) pastes with w/c ratio of 0.5 were mixed with a high-energy rotational mixer, moulded and stored at 23 °C and 90–100% relative humidity (then kept under water after 24 h). At the age of interest, slices (2 mm-thick) were removed and then underwent the hydration stoppage protocol with isopropanol and diethyl ether. After hydration stoppage, ground samples were subjected to thermogravimetric test (STA 409PC/PG, NETZSCH) up to 1000 °C (10 °C/min) in Nitrogen protective atmosphere. Further details regarding this methodology can be found elsewhere [108,109]. The quantification of the combined water (Cw) is expressed as follows [110]:

5.2.1. Practical use of cwf concept The concept seems to be helpful as starting point for the design of complex cement mixtures (Fig. 4) and their strength characterization considering both synergic chemical reactivity with possible influence on water demand. Because it introduces the mixing water demand in the equation of cement development, it facilitates the industry to better explore any locally available SCM. For example, it makes easier estimating the reduction of mixing water demand needed to compensate the low reactivity of SCMs in order to keep the performance. It also eases to estimate the variation of clinker factor to keeping under control the cement performance when raw materials or process change, e.g. unexpected changes in clinker, SCMs or sulphate reactivities. These possibilities provided by cwf combine nicely with a digitalized cement plant, where each cement component can be ground separately, and the process optimized by a sophisticated AI tool. It also allows estimate cement compressive strength at w/c = 0.5 (or other) from results obtained from other w/c, provided there is enough water for hydration. It is possible to classify cements using cwf at 28 days, a classification that will be correlated to the cement's strength classes, providing conditions for an easy transition between the two systems. Using the limited amount of data, we have already (Fig. 4) estimated the 28 days combined water: it resulted in 0.14, 0.18, 0.22 respectively for cement classes for 32.5, 42.5 and 52.5 as tested with the fixed w/c of 0.5. Such classification of cement may be complemented by the actual combined water data at relevant age, making the cement formulation strategy clear for the technical user. It also evidences the potential benefits from the development of binders capable to combine with more water than the today's clinker Portland and the feasibility of “super-cement” class equivalent to 72.5 – which will be justified only when aggregates are very well packed.

w − w550 ⎞ Cw = ⎛ 40 w550 ⎠ ⎝ ⎜

⎟

where WX is the percentage of mass loss at temperature x°C. During the data mining effort to test the concept, we found that the bound waterto-mixing water ratio had been quickly presented by Powers and Brownyard [111], who decided to adopt the gel-space ratio instead. Gel-space ratio, despite being more precise, requires measurement of the degree of hydration and densities of each individual phase, which is not easy specially in composite cements. cwf is much more practical, since requires only two simple measurements: combined water at any age – which can be achieved by TG or in a simple ventilated oven – and the mixing water. These measurements can be performed in almost any reasonably equipped laboratory. Fig. 3 shows the correlation between cwf and compressive strength of 1:3 mortar cubes. Fig. 3 (a) shows 92 data from our lab with variable w/c ratio, half of them with dispersants, testing accordingly to [107] whereas Fig. 3 (b) shows 147 results from literature with ages varying from 1 to 360 days. Data includes a variety of exotic SCMs, including nano clay, graphene admixture, high amount (70%) of clinker substitution by limestone fillers, variations around LC3 formulation. Data in Fig. 3 (a) grouped in function of the cement type show that, despite the good correlation (R2 ≥ 0.94) between compressive strength and cwf, the trendlines seem to be parallel among them and slightly shifted. For the same cwf the maximum variation among the cements is about 10 MPa (Fig. 3-a) and the slope of the curve related to each cement families is slightly different. For instance, acid slag and fly ash cements show similar behavior, whereas for filler up to 48% and OPC it is possible to note that for the same cwf the former reach a compressive strength ~9 MPa higher. Analysing all data (Fig. 3-b) it is possible to

5.2.2. Measuring combined water Combined water is a fundamental property of a cement, directly related to solids fraction increase which is responsible for strength growth. It measures the amount of hydration of the binder fraction in the period between mixing and testing. It is sensitive to relevant and common situations related to cement hydration such as: clinker dilution by filler, partially or slowly reactive SCMs or even the influence of admixtures on hydration kinetics and degree. Thermogravimetry is the most used research method to measure combined water because it gives more information regarding hydrated phases, evidencing pozzolans or other SCMs reaction [49,107]. If the 9

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Fig. 3. – Results of cwf versus compressive strength: (a) 92 results from our own data; (b) 239 samples 1:3 mortar from 10 sources [113–121] including our own data. Data includes an assortment of SCMs and mixtures (LC3 cement being one of them) with ages from 1 to 360 days. 0.5 32.5

42.5

52.5

62.5

equipment. However, pre-drying at 110 °C gives another value, the evaporable water content, which is significantly lower than the actual chemically-combined water [112,123]. Heating up to 1000 °C will also decompose the CO2 and cause some mass gain due to phase oxidation in the uncontrolled atmosphere, particularly if slag is present. Perhaps a variation of TG sample treatment with isopropanol presented before, can overcome the problems associated to drying at 110 °C. It is certainly possible to develop a standardized reproducible and useful test based on this platform. Chemical shrinkage also is a secondary measurement tool [123,124], possibly correlated with combined water. Measuring the combined water require careful handling but is not as labour intensive as mechanical strength and seems it can be easily fully automatized. It is not sensitive of moulding procedures and presence of defects such as small air bubbles, but sampling handling and conditioning is critical. It requires much smaller amount of materials and storage space than compressive strength. Currently available TG machines are not optimized for the productivity required in the industrial environment, but at least one producer already has a sample carrousel that keeps samples in a N2 atmosphere until the test. The integration of sample crushing, milling and treating with isopropanol within such machine will be highly desirable. Because the combined water does not depend significantly on the amount of mixing water - provided enough water and space (w/ c > 0.4) - the method allows determining the mixing water demand for constant rheology being carried after starting the combined water tests. These tests can be accelerated by increasing curing temperature to 40 °C [122], therefore reducing curing time by a factor of 6.

72.5

Combined Water (g/g)

0.4

0.3

0.2

0.1

0

0.3

0.4

0.5 0.6 Water demand (g/g)

0.7

0.8

Fig. 4. –The two routes available to control cement strength class: adjusting combined water capacity or controlling mixing water/cement (binder + filler) demand. The green area is higher than the maximum combined water of Portland clinker. Model built by the average equation from Fig. 3, considering cement classes according to 28 days cement compressive strength in 1:3 mortar. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

test is carried up to 1000 °C, the amount of CO2 due to limestone decomposition is measured. The amount of combined water measurement error is between ± 5 and ± 10% and it is associated with the problematic of properly sampling small amounts, being recommended a minimum 50 mg of hydrated paste to minimize this issue [107]. However, sample preparation, handling and storage can introduce larger errors. Best guide on cement testing is the one published by Scrivener et al. [109] as result of an effort of Nanocem team and chapter 5 is dedicated to TGA [107]. Sample crushing and grinding is best to be carried quickly on CO2 free atmosphere and hydration being stopped by isopropanol followed by washing with diethylene ether and immediately transferred to N2 atmosphere of the TG equipment. It can be accelerated to give just the information needed, by increasing heating rate from the usual 10 °C/min to 20 °C/min and limiting the testing temperature to a maximum of 600 °C. Simple small electric ovens, commonly used for ceramics in labs, may be good replacement of a TG machine, [122] and have the advantage of the possibility of using larger samples in a much cheaper

5.2.3. Measuring mixing water demand Struble [125] already had emphasized water demand: “Because water requirement is such an important property of hydraulic cement, adding a specification on water requirement for standard consistency...”. Mixing water demand of cement can be measured by consistency index from the flow-table test in standard mortars, as it has been specified in the past cement standards as previously mentioned in 5.1. For the development of cwf, our own data set was obtained for a fixed spread of 240 ± 10 mm (considering that initial diameter is 125 mm as defined by NBR 13276). This value was adopted because resulted in workable mortars suitable for moulding of a dozen cements, with and without dispersant, without problems due to extreme segregation or lack of water and cohesion [96,97]. Fig. 5 shows a continuous and 10

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Fig. 5. – Images, after flow-table test, of Brazilian standard 1:3 mortars prepared with commercial cements and varying w/c for fixed spread diameter of 240 ± 10 mm. Mortars without dispersant: (a) OPC, similar to CEM 1, (b) 50FA, composite cement with 50% of fly ash and (c) 49DE, composite cement with 49% of diatomaceous earth. Mortars with dispersing admixture: (d) OPC_dis, (e) 50FA_dis, (f) 49DE_dis. Values of w/c are indicated in the legends. Originally from [96,97].

rheometers dedicated for cement pastes can be developed and employed to determine water demand, either by the adoption of a standard torque (or shear stress) level or by the identification of the fluidity point [132] in the mixing curve. The mixing rheometers allow measuring a more complete characterization of rheological behaviour and even measuring mixing energy requirement, which can be a relevant parameter. In the cement plant of the future with robotics and AI, the determination of water demand will be the first step of the series of tests to be performed in the cement paste, and this paste can then be possibly used for heat of hydration, automatic Vicat, rheometry, TG/DSC or chemical shrinkage performed by a robotic system integrating these different experimental techniques. Nevertheless, using the fixed spread on the flow-table, the paste of normal consistency defined by Vicat's plunger, or an automatized sophisticated mixing rheometer, the most important thing is that tests can be performed with or without the presence of admixtures, which is crucial to effectively employ cwf and extend the potential of cement formulations. Coherent relations between the water demand of pastes and those of mortars and concretes will only be established if employed methods have the option to use dispersants. This approach has already been applied to “provide sufficient workability…and consistent relationship between paste and mortar/concrete rheology” as mentioned by Bentz and Ferraris [134].

cohesive mortar with no signs of substantial phase separation. Thus, this method seems promising and could easily enable the implementation of water demand and cwf concept for cement classification, especially because it employs already established and widely used 1:3 mortars and flow-table apparatus are cheap and available in almost all labs. Another option already standardized and commonly used is the amount of water determined for paste of normal consistency (ABNT NBR 16606, ASTM C187, EN 196-3). The test is cheap, simple and, most importantly, employs the single point “constant rheology” concept, which seems reasonable when comparing a great diversity of cements with different physical features – mainly particle size and surface area – that greatly affect water demand. However, in the opinion of some professionals from the cement industry, the test may be not sensitive enough. Other traditional test is the mini-slump test developed by Kantro [126] and extensively used for dosage of dispersants; it is fast, simple and cheap allowing the estimation of yield stress with good correlation with rheometer results with a coefficient of variation lower than 3% [127]. The test is currently under standardization by ASTM WK63516. Although the consistency at flow-table can be effective and induce a smooth transition from strength class to other concept, probably it is not the best option for a 4.0 industry scenario. The digital production of cement-based materials will require stricter and meaningful control of time-resolved rheological behaviour. Establishing rheological behaviour more precisely requires measuring more than one parameter [125], which may become relevant. Additionally, since automation will be the rule, it is desirable easily automated methods. Flow table does not seem simple to automatize. It also requires reference sands, which increases cost and creates environmental impacts including waste. Other options based directly on measuring the paste are probably better. Paste of normal consistency defined by standard test with the Vicat penetration plunger and Kantro's mini-slump are cheap but not easy to be automated. A best option from technical point of view will be assessing the mixing behaviour of the paste as a function of water addition. Mixing rheometers have been used in other areas for a long time [128] with equipment available in the market capable of measuring the mixing behaviour of wet granular materials [129] and concentrated suspensions as mortars and concretes [130,131] as exemplified in Fig. 6. Hence, it is reasonable to expect that automatic mixing

5.3. Setting time Cement technology is all based on the capacity of the powder to react with water, then to solidify and develop substantial mechanical strength over time. Apart from false setting [125], cement set is a progressive change mainly – but not only – controlled by chemical reactions of the cement, also affected by particle agglomeration. It aims to define the workable period or open-time of the cementitious material before the suspension starts to solidify, which is an important performance requirement. Its importance had been already noted by scientists and engineers in the past. Vicat apparatus completed 200 years in 2018 as it was devised by L.J Vicat in 1818 for dealing with the setting time of hydraulic limes [27]. Original needle diameter (2.54 mm) and weight (1360 g) were different (larger and heavier) than the ones of the contemporary device, but stress at the needle point remains similar and it 11

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Fig. 6. – Example of mixing curves to determine water demand, measured by PHESO rheometer, of mortars (25%vol of fines and 75% ag) containing different types of fine raw materials: hydrated lime (Volumetric Surface Area = 15 m2/cm3, D50 = 10 μm), lime stone fine filler (VSA = 8 m2/cm3, D50 = 3 μm), lime stone medium filler (VSA = 3 m2/cm3, D50 = 23 μm) . After an initial water addition, agglomeration, dispersion and homogenisation take place resulting in the mixing kinetics. Additional water contents are introduced, causing immediate torque reduction, until the pre-established torque level is reached. Adapted from [133].

Fig. 7. – Comparison of setting time results measured by Vicat (ASTM C191) and by peak stress rheometry method. Originally from [134].

“Vicat setting times are arbitrary, in that they do not correspond exactly to any specific change in properties or to any specific levels of hydration reaction” [125]. However, despite the current diverse high-technology measuring techniques available, the 200-year old test still is the rule for technological control of setting by the industry, apart from the less used calorimetry test of ASTM C1679 that provides a “thermal indicator” to estimate initial setting time. For research and development, though, different methods are employed to evaluate the physical effects of setting like ultrasonic wave transmission [135,136] - a non-destructive physically-based test that continuous monitors the microstructural changes of the paste; rheometry - rotational shear flow or oscillatory [134,137–141]; and calorimetry is also used to investigate the relation of the reaction kinetics with setting [134,138–141]. Furthermore, this equipment can be adapted for industrial use and their cost reduces by production scale. Different types of rheological measurements are employed to evaluate setting of cements. The stress growth technique determines an approximation of the yield stress at different elapsed times after mixing (thus requiring new samples for each measurement) and the inflection point of the peak stress vs. time curve is considered as the initial setting

still is the widely used standard test method to determine setting times. These are defined by established values of penetration of the needle in a cement paste with standard consistency – determined with the same apparatus but with a plunger of 10 mm in diameter. The adopted parameter for final setting time is the same in Europe, Brazil and USA (as in the year 1818): 0.5 mm of penetration in the paste surface (EN 196-3, NBR 16607, ASTM C191). But initial setting time in the USA is considered as a needle penetration of 25 mm (stays 15 mm from the bottom plate), while in Europe it is 36 mm of penetration (4 mm from the bottom) and slightly different in Brazil, 34 mm of penetration (6 mm from the bottom) according to the mentioned national standards. These values are defined by convention and have changed over time, in Brazil for example it was 1 mm in 1937 and is currently 6 mm. Alternative test using Gillmore needles, as prescribed by ASTM C266, employs the same principle but with different masses and needle geometry. Vicat needle is unquestionably useful for determination of setting time, but the evolution of cement compositions, the use of admixtures and the development of new construction techniques have increased complexity and importance of cement setting. As stated by L. Struble, 12

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require constant consistency. A very interesting approach is the association of rheometry and calorimetry, enabling to distinguish physical and chemical contributions to setting [139–141] by analysing G'/accumulated heat vs. time curves, which can be useful to evaluate complex cement formulations that contain different types of admixtures (dispersant and set regulators for example) and a variety of SCM's. None of these methods are perfectly suited for the 4.0 industry scenario, which requires automation and the possibility of scaling up the number of tests, except perhaps the ultrasound test. Nevertheless, these methods cannot compete with an automated Vicat when only cost is considered.

5.4. Admixture and cement standards Most of the cement sold in bulk will be mixed with admixtures, specially dispersants. In many practical applications, admixtures increase the efficiency of cement reducing the environmental impact, and the CO2 footprint in particular up to 20% [100]. The new cementitious products, such as concretes and mortars with high filler and low water, UHPC and the trend of 3D printing [142] are not possible without a combination of admixtures. Apart from dispersants as superplasticizers, which are dominant in concrete, there are plenty of others. For mortars, another important market, dispersants are employed only in specific product types (mainly self-levelling compositions), but other admixtures -some also used in concretes - as air-entraining, viscosity-enhancing and redispersible polymers powders are commonly used and affect rheological behaviour and, therefore, water demand in different ways. Set regulators are becoming also important for digital production. Cement characteristics, including but not limited to surface area, may influence the amount of admixture needed to achieve desirable effect. A full dispersion of a cement can be achieved with 0.3 to 1.3% of admixture expressed on the mass of fines below 100 μm. Incompatibility between cement and admixtures is a relevant market problem [98] and, in the case of superplasticizers, can cause an unpredictable retardation of cement hydration with many practical consequences. In certain cases, this problem is known to be related to sulphate and aluminates interaction and presence of soluble alkalis [143,144]. However, the problem is considered by Marchon and Flatt [98], to be one of the least well-understood topic and, considering the ever-growing importance of dispersants, it certainly needs urgent attention. Despite being in use for long time in conjunction with cement and being crucial for many users, there is no requirement or even standardized test to measure cement compatibility with admixtures, particularly with the widely used dispersants. The development of such testing methods is certainly a priority for the future of the industry. Cement composition influences superplasticizers dosage significantly, by a factor of three as shown in Fig. 8 (left), a fact that has important economic consequences. Dispersants are dosed by deflocculation curves usually by a mini-slump test [126] that is being currently standardized by ASTM. Deflocculation curves [145] can be obtained in a paste rheometer, using apparent viscosity and/or yield stress data [96,97,146], a test that requires careful paste preparation for proper repeatability. The admixture stability over time can be measured by flow or oscillatory rheometry as well but also by simple test such as mini-slump. The eventual effect of admixtures on the cement hydration and to setting time is best measured by isothermal calorimetry [143]. One problem for developing a standard compatibility test between dispersant admixtures and cements is the growing variety of admixture products that behaves very differently with a given cement (Fig. 8 right), even under the same generic molecule type. Better understanding of the fundamental phenomena is needed.

Fig. 8. – Deflocculation curves. (left) Effect of cement composition for commercial Brazilian cements in polycarboxylate ether dosage (11MS is a manganese slag-blended cement; 16DE and 49DE are natural pozzolan blended cements with different content of diatomaceous earth [96,97]). (right) Effect of admixture type – M is melamine; L is lignosulphonate and modified polyacrylic resin for the same Italian OPC cement [146]. Deflocculation is reached when yield stress or viscosity approaches the minimum or becomes relatively insensitive to dosage increase.

time; when a substantial change of consolidation kinetics occurs. Compared to ASTM C191 setting time by Vicat needle, the stress growth technique resulted in considerable shorter times (as shown in Fig. 7), indicating a higher sensitivity of the rheometry-based method [134,138]. Small amplitude oscillatory shear (SAOS) rheometry does assess rheological changes continuously (in a single sample) without disrupting the developing paste microstructure and provides information regarding elastic (G') and viscous (G") components of the viscoelastic behaviour of the paste. G' is the most important parameter related to the set of cement (transition from fluid to solid) and yield stress vs. time curve can also be drawn to point out the inflection point representing the initial setting time [139]. The use of such tests at industrial scale demands further research and cost reduction of the equipment. The heat of hydration infers the setting time from chemical activity based on previous correlations with Vicat setting time. However, it may not be sensitive to estimate the setting time when agglomeration plays the major role in that period, particularly for cements with dispersants in which setting can be strongly affected by the admixture, especially when it loses efficiency, agglomeration is favoured and the consequences on rheology of these suspensions with low water content are intensified. By the other side, calorimetry does not depend on the determination of water for normal consistency. Rheology-based methods measure the combined effects of agglomeration and chemical reaction related to changes on the paste microstructure, but as does Vicat, they 13

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5.5. Particle physical characterization

5.6. Other tests

5.5.1. Density Density is a property that governs the volume of cement and, in a world dominated by clinker blended with SCMs of various densities, changing the density has important implications in mix design [125] and on economics, since variations higher than 10% are possible with some pozzolans and fillers. Therefore, it is reasonable to mandatorily inform clients. In most of the current cement standards, density measurement is part of Blaine fineness test. The most common test prescribed is the Le Chatelier flask, a time-consuming test that requires the use of volatile organic substances, generating vapours and waste. This test therefore is not suited to routine measurement [125], neither can be easily automated. Gas pycnometers are almost universal in other industries and allowed as alternative method in some standards. They are more precise than Le Chatelier but pycnometer gives densities 1% higher implying that helium penetrates in pores inaccessible to kerosene [147].

Besides the ones discussed here, there are many other relevant tests needed to better describe the cement and cement paste, both fresh and hardened, generating fundamental data that will allow performancebased mix-design concrete. Struble [125] has conducted a comprehensive discussion of all those, like heat of hydration dimensional stability and including some crucial durability aspects that are relevant in some specific applications and regions. We have to agree with the author state that “many of the tests described … are empirical and need a better fundamental basis. In some cases, the fundamental knowledge is available but just needs to be applied in standards.” And certainly, many of the available tests are not suited for a more automated industry, so innovation is welcome. Chemical and mineralogical characterization has been evolving rapidly with the electronics, including the now ubiquitous XRF to QXRD machines that can describe the material with unprecedent detail in a very short time and at low cost. Nevertheless, most specifications of compositional limits are not compatible with performance standards.

5.5.2. Surface area and particle size distribution Particle size distribution and surface area are relevant information for modern mix-design models based on packing [36,148], and this will become more important in the future. The current standard method for surface area is the air-permeability Blaine test, invented in 1943 [149], which assumes spherical particles and gives only comparative results in a same set of materials. In standards, particle sizes are measured solely by sieve residue tests. Both Blaine and sieving tests are limited for use in the new mix design models based on packing and mobility, besides being time-consuming and not fully automated. Industry adopted laser diffraction as quality control method for particle size distribution and in-line LD equipment for process optimization. Therefore, it seems to be the natural candidate for standardization [149]. However, for accurate results, full dispersion is needed and it is not clear how agglomerated the dry particles are when diverted for the in-line industrial test, but Ferraris and Garboczi [150] consider that only wet test can ensure full dispersion of bagged cement. Our own results show that at least in some cases the use of dispersant is advisable for wet testing in water [96]. Particle size distribution by laser diffraction certainly is helpful to technical costumers if informed by cement producers and, hence, needs to become a cement standard test method. Nevertheless, standardization of laser diffraction of cement would definitely be a challenging project, since many aspects like sample preparation, dispersing and testing methods, significant variation in calculation algorithms of equipments from different manufacturers, and the scattering theory applied to analyse the results [151] must all be considered, studied and specified.. Maximum size, as currently prescribed in standards, may remain as a proxy to control segregation risk. The BET adsorption test is the most fundamental specific surface area (SSA) test and ideally will be adopted by the industry. However, it may take half-day to get results [149] with nowadays available equipment, that also it is not automated. Therefore, without innovation on BET equipment, it is not going to be the specific surface area test for the 4.0 cement industry. However, for a same cement there is a linear strong correlation between BET and surface area estimated from laser diffraction or Blaine, but since the last two consider particles as perfect spheres they give smaller areas and are not sensitive to the diversity of particle shapes and textures associated to original minerals characteristics and their modification during chemical reaction [149]. By assessing both laser diffraction and BET surface areas of large sets of cements, it could be established a correlation for each product, being useful to shorten testing time. Moreover, the shape factor (SSABET/ SSALD) [36,96,105] is another important information provided by the combination of methods to better understand particle morphology and texture, since dynamic image analysis (DIA) technique [152] is not yet fully resolved for cements as recently faced by the authors.

6. Conclusions This paper explored possible course of actions to better adapt the cement standards for the two most important megatrends that may disrupt society and, therefore, shape Portland cement future: environmental sustainability, more specifically resource efficiency and climate change and the revolution of the digitalization on industry – the 4.0 industry. Rather than presenting definitive answers, the aim was to identify opportunities and challenges and encourage discussion, research and innovation. Industry 4.0 artificial intelligence systems require easily automated cement testing methods, capable of delivering larger data sets than the conventional tests currently do. Most of the existing tests are not easy to automatize and are difficult to scale up. Digital construction probably will demand cements with new performance requirements. Departing from the traditional prescriptive composition of cement standards to performance-based standards, limited to mixtures of clinker Portland and any set of SCMs including inert ones, will allow industry to explore local available SCMs without waiting for timeconsuming changes on prescriptive standards. Such standards must cover performance requirements in broad sense, including mixing, environment and durability aspects. This will, thus, help to reduce clinker factor and environmental impact in a world where blast furnace slag and fly ashes are scarce. It will also facilitate performance reliablebased concrete design. But perhaps the most important conclusion is that the fixed water/ cement, almost universally adopted to classify cements according to compressive strength, is hindering the industry to explore one of the most promising and cheap strategies to decreased clinker factor: reducing the mixing water demand whilst maintaining good rheological behaviour, by engineering the physical effect of particles therefore controlling the physical effect of particles. This technology is better explored by compensating dilution of binders by inert filler or weakly reactive SCMs by reducing the water demand through combining dispersant admixtures with improved particle packing (by particle size and shape). Therefore, there is a benefit of allowing adjusting the mixing water to its actual demand when classifying cement by strength and informing clients about mixing water demand. We had shown that varying combined water – by binder dilution or grinding – and mixing water allows to formulate cements with more degrees of freedom than the current strategy based only in chemical reactivity and fineness. The combined water fraction (cwf) – an index that Powers overlooked favouring the more complex gel-space ratio – is strongly and linearly correlated with compressive strength, and possibly also correlated with other porosity-related relevant properties. Therefore, combined water divided by the mixing water demand – a 14

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proxy of the physical effect - can become a fundamental criterion for a new classification of cements. These to parameters that form cwf can be easily measured, using traditional or innovative, easily to automate equipment. Industry can also benefit from rethinking other tests. Specific surface area and particle size distribution are becoming relevant information for modern mix design tools based on packing. Since laser particle size distribution is already available in most cement plants it seems reasonable that the method replaces the sieving testing still present in many standards. Informing cement particle density is also important because the differences of density between cements not only influences packing models, but also economics. Compatibility between admixtures and cement is also a desirable aspect to be considered part of future standards. In summary, society and cement-related industries will have a better future if both, industry and academy, start to prioritize the development of science-based performance test protocols for cement and its products.

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Acknowledgments This paper is dedicated to Dr. Ellis Gartner, a bright, inspiring and generous person, with whom V.M. John first formulated the fundamentals of combined water fraction, cwf, as a classification tool for cements. Authors would like to thank Rafael Cecel and Liz Zanchetta for their contribution on key experiments; Arnaldo Battagin, from Brazilian Portland Cement Association (ABCP) for providing historical data on cement standards and test methods. InterCement funded part of the work. This research is part of the CEMtec project – National Institute on Advanced Eco-Efficient Cement Based Technologies, supported by CNPq - Conselho Nacional de Desenvolvimento Científico e Tecnológico - Brazil (Process 485340/2013-5) and FAPESP São Paulo Research Foundation (Process 14/50948-3). P.C.R.A. Abrão work was partially supported by CNPq scholarship. M. Quattrone and F.A. Cardoso work was supported by a CAPES/BRAZIL EMBRAPII scholarship. The information presented in this study are those of the authors and do not necessary reflect the opinion of CNPq, FAPESP, CAPES or EMBRAPII. References [1] J.V. Tyrrel, Consensus standards formulation, Research and Innovation in the Building Regulatory Process: Proceedings of the Second NBS/NCSBCS Joint Conference, National Bureau of Standards, National Conference of States on Building Codes and Standards, Bozemen, Montana, 1977, pp. 161–164 https:// nvlpubs.nist.gov/nistpubs/Legacy/SP/nbsspecialpublication518.pdf. [2] R.D. Hooton, Bridging the gap between research and standards, Cem. Concr. Res. 38 (2008) 247–258, https://doi.org/10.1016/j.cemconres.2007.09.012. [3] G. Frohnsdorff, J. Clifton, Cement and Concrete Standards of the Future Report from the Workshop, Gaithersbury (1995). [4] K.L. Scrivener, V.M. John, E.M. Gartner, Eco-efficient cements: potential economically viable solutions for a low-CO2 cement-based materials industry, Cem. Concr. Res. (2018), https://doi.org/10.1016/j.cemconres.2018.03.015. [5] K. Breene, Can the circular economy transform the world's number one consumer of raw materials? World Economic Forum Global Agenda, 2016 https://www. weforum.org/agenda/2016/05/can-the-circular-economy-transform-the-world-snumber-one-consumer-of-raw-materials. [6] B. Lothenbach, D.A. Kulik, T. Matschei, M. Balonis, L. Baquerizo, B. Dilnesa, G.D. Miron, R.J. Myers, Cemdata18: a chemical thermodynamic database for hydrated Portland cements and alkali-activated materials, Cem. Concr. Res. 115 (2019) 472–506, https://doi.org/10.1016/j.cemconres.2018.04.018. [7] SJR, Cement and Concrete Research, Scimago Journal & Country Rank, https:// www.scimagojr.com/journalsearch.php?q=23833&tip=sid&clean=0, (2019). [8] M. Hilbert, P. Lopez, The World's technological capacity to store, communicate, and compute information, Science 332 (2011) 60–65, https://doi.org/10.1126/ science.1200970. [9] E. Brynjolfsson, A. McAfee, The Second Machine Age: Work, Progress, and Prosperity in a Time of Brilliant Technologies, W.W. Norton & Company, New York London, 2016. [10] The Economist, A third industrial revolution - special report: manufacturing and innovation, The Economist, http://www.economist.com/node/21552901, (2012) , Accessed date: 11 December 2012. [11] A third industrial revolution - special report: manufacturing and innovation, The Economist, http://www.economist.com/node/21552901, (2012) , Accessed date:

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