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Concrete construction harms our environment, making it urgent to develop new methods for building with less materials. Structurally efficient shapes are, however, often expensive to produce, because they require non-standard formworks, thus, standard structures, which use more material than is often needed, remain cheaper. Digital fabrication has the potential to change this paradigm. One method is Digital Casting Systems (DCS), where the hydration of self-compacting concrete is controlled on the fly during production, shortening the required setting time and reducing hydrostatic pressure on the formwork to a minimum. This enables a productivity increase for standard concrete production. More importantly, though, it enables a rethinking of formworks, as the process requires only cheap thin formworks, thus, unlocking the possibility to produce optimised structural members with less bulk material and lower environmental cost. While DCS has already proven effective in building structural members, this process faces the challenge of moving into industry. This paper covers the next steps in doing so. First, we present the benchmark and expectations set by the industry. Second, we consider how we comply with these requirements and convert our fast-setting self-compacting mortar mix into a coarser one. Third, we present the next generation of our digital processing system, which moves closer to the industrial requirements in terms of size and the control system. Finally, two prototypes demonstrate how DSC: (a) increases standard bulk production by 50% and (b) can be cast into ultra-thin formworks. We discuss the results and the short-term industrial concerns for efficiency and robustness, which must be addressed for such a system to be fully implemented in industry.

Oxygen consumption measurement was used to study potential oxidative degradation reactions occurring in wooden artefacts from the Viking age Oseberg collection in Norway. Model samples of fresh birch were impregnated with iron, calcium and alum salts to mimic concentrations of such compounds found in Oseberg artefacts and to assess their effect on oxygen consumption rates. The results showed that heated impregnation with alum salt (KAl(SO4)2·12H2O) significantly increased the rate of oxygen consumption, confirming a previously observed link between alum-treatment and wood oxidation. The presence of iron salts in alum-treated wood specimens, even at low concentrations, also substantially increased the oxidation rate. However, the mechanism by which this occurred appeared to be influenced by the alum-treatment. Samples treated with both iron and calcium salts were also studied, in order to investigate a proposed inhibition of iron-induced oxidation by calcium ions. However, these did not appear to consume oxygen at significantly different rates. In Oseberg samples, a large variation in oxygen consumption rates from 0.48 to an apparent 8.2 μg O2 (g wood)−1 day−1 was observed, and these values were consistently higher than those for reference fresh wood. The results demonstrated that oxygen consumption measurement is a viable method of evaluating chemical stability in this case, but is best applied to model samples with limited compositional variability., Heritage Science, 8 (1), ISSN:2050-7445

Digital fabrication with concrete is considered to potentially revolutionize the construction sector and is often presented as a means to reduce its environmental footprint. However, at least in the case of concrete, it encounters significant challenges in terms of material design, since high paste volumes and Portland cement contents are normally used due to process requirements. In this article, the application to layered extrusion of a recently developed low clinker cement containing 50% Portland cement and 50% supplementary cementitious materials, such as limestone, burnt oil shale, and fly ash, is presented. It is found that an accelerator paste composed by Calcium Aluminate Cement (CAC) and anhydrite provides the required hydration and structural build-up for 3D printing, while not compromising the early and long-term compressive strength. Such a low clinker mortar can be successfully retarded, processed, pumped, and extruded just after mixing it in line with the accelerator paste. This accelerated mortar formulation contains only 303 kg/m(3) of Portland cement, which is roughly half the amount used in current accelerated formulations used for digital fabrication with concrete. ISSN:2329-7662 ISSN:2329-7670

Achieving and controlling the desired strength build-up is important for 3D concrete printing. In this paper, we investigate the possibility of using calcium aluminate cement (CAC) as an accelerator for Ordinary Portland Cement (OPC) for 3D extrusion printing. Both ternary and binary cement systems were studied whereby for the former, the third component was a calcium sulfate source (C\$). Experiments are conducted on mixes both in presence and absence of retarders. They focus on hydration progress, evolution of rheological properties and microstructural development. It is concluded that CAC-C\$ can be effectively used as an accelerator to achieve the desired rates of strength build-up. Additionally, it is shown that strength build-up has is directly related to CAC dosage, irrespective of the presence of retarders. Based on microstructure characterization results, AFt and AH3 are found to be formed in the first hour after acceleration and responsible for the observed strength build-up., Cement and Concrete Research, 162, ISSN:0008-8846

RILEM Bookseries, 28, ISSN:2211-0844, Second RILEM International Conference on Concrete and Digital Fabrication. Digital Concrete 2020, ISBN:978-3-030-49915-0, ISBN:978-3-030-49916-7

The article “Relating early hydration, specific surface and flow loss of cement pastes”, written by “Sara Mantellato, Marta Palacios, Robert J. Flatt”, was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 4 January 2019 without open access.

Pore solution compositions dictate the kinetics of cement hydration. Their characterization can therefore provide essential insight into the rate at which cement phases hydrated. One of the most widely used techniques to characterize pore solution composition is ICP-OES. However, cement literature generally overlooks the fact that high accuracy from this technique can only be obtained if proper accounting for matrix effects is carried out. We first present a new method for such a technique, demonstrating that limits of quantification down to 34 times lower than the ones generally reported can be obtained. This method is applied to the study of the early hydration of admixed cement pastes. In particular, we highlight the formation of nanoparticles that may have different origins. The formation can be expected to have important consequences on the working mechanisms of some chemical admixtures.

An unidentified white substance was observed on the exterior parts of the southern side of Hopperstad Stave Church, located in Vik in Sogn and Fjordane (Norway). One of the 28 remaining stave churches in the country, Hopperstad Stave Church was built between 1130 and 1150 and it constitutes an important part of Norwegian cultural heritage. Such a substance was observed for the first time in 2009 and serious concerns about its harmfulness and origin have been raised. A comprehensive study involving non-invasive (portable XRF) and micro-invasive analyses (SEM, ICP-OES, TOC) was undertaken to investigate the physico-chemical nature of this white substance. Tarring, salt migration from the nearby cemetery, atmospheric agents, leakages from the roof, biological infestation, previous treatments with pesticides are among the possible causes of the phenomenon. This work—employing a unique approach in conservation/conservation science—indicates that the white substance is mainly inorganic and crystalline. It was, therefore, possible to exclude some of the possible causes, hence, clarifying some of the conservation issues of the church., Heritage Science, 7 (1), ISSN:2050-7445

Flow loss in superplasticized systems has been mainly explained in qualitative and comparative ways over the past years. This is due to the intrinsic complexity of the underlying mechanism involving a change in the agglomeration degree as a result of cement hydration. The lack of robust and reliable experimental methodologies must have additionally discouraged researchers from attempting to understand the phenomena of flow loss in quantitative terms. Thanks to new analytical methods, it was possible to prove that after the so-called onset point, yield stress increases exponentially with the increase of both heat rate measured by isothermal calorimetry and specific surface. This paper also identifies the existence of a direct proportionality between the increase of heat rate and the increase of specific surface area during the acceleration period, most likely reflecting the nucleation and growth nature at this stage of the cement hydration., Materials and Structures, 52 (1), ISSN:1359-5997, ISSN:0025-5432, ISSN:1871-6873

The effect of sample preparation, including degassing conditions and method to stop cement hydration, on the specific surface area (SSABET) of synthetic ettringite (AFt) was studied. Dehydration of AFt occurs even at moderate temperatures under N2 flow (0% RH). However, partial dehydration associated to a loss of up to 12 water molecules does not imply a change in SSABET. A mild degassing 16 h at 40 °C under N2 flow is suggested mainly to preserve the chemical composition and morphology of calcium sulfate carrier. At higher temperatures, ettringite dehydration involves a significant modification of its crystalline structure and increase of the SSABET. Typical protocols to stop cement hydration, solvent exchange and freeze-drying, lead to larger water removal from the AFt and crystallinity reduction, being significantly higher when the latter is applied. However, only freeze-drying leads to a moderate increase of the synthetic AFt SSABET.

Over the past decade innovative techniques for shaping concrete have emerged, all aiming to use less material and reduce the need for traditional formwork. One very promising method is to shape concrete dynamically: referred to as Smart Dynamic Casting (SDC), this process was pioneered in 2012 as the first robotically-driven system for slipforming bespoke concrete structures. The process has successfully been adapted to produce structures using ultra-thin formworks that are cast using our digital set-on-demand procedure. More generally we frame this approach as Digital Casting Systems (DCS), which allows the user precisely to determine the hydration rate of the material, thus eliminating formwork pressure. This paper highlights the major findings from SDC that led us to continue developing DCS. It lays out the material concepts fundamental the family of DCS, which, by eliminating the need for bulky formworks, has a large potential impact on future construction methods.

The particle size distribution (PSD) and specific surface area (SSA) are complementary parameters that give information about the fineness of a powder. They can be combined to get information about the state of agglomeration and morphology of a powder. The state of agglomeration of a powder has an influence on its dispersibility, its packing and, hence, the rheology of a suspension; this is of critical importance in the fluidity of cementitious materials.

Concrete digital fabrication is an innovative construction approach where infrastructural elements can be built additively without using formwork. This represents a significant advantage, but also introduces materials engineering challenges, as the requirements normally fulfilled by the formwork are now imposed on the concrete. In this paper, it is discussed how admixtures can be employed to achieve the rheological and hydration properties necessary for printable concrete. An overview of various admixtures currently implemented in standard practice is presented. Then, the main required concrete states for extrusion and deposition processes are analyzed with respect to required performances and potential admixtures. Finally, possible side effects and incompatibilities are discussed, as well as how they could be unconventionally used for printable concrete purposes. The main objective is to demonstrate how admixtures will be critical in the development of concrete systems to realize digital fabrication, and to ultimately motivate investigation in the key areas discussed.

The specific surface area (SSA) of anhydrous cement has a direct impact on its reactivity and on the properties of fresh and hardened concrete. However, measuring this physical parameter is non-trivial. The choice of the degassing conditions (temperature, pressure and time) is essential for reliable SSA measurements by nitrogen adsorption because of the dehydration of the gypsum it contains. Such dehydration involves a significant increase of the SSABET, whereas Blaine test is poorly sensitive to the hydration state and structural modification of the calcium sulfate carrier. Because of this, SSABET values may be more variable than Blaine fineness. More consistent SSABET results could be obtained by degassing anhydrous cement samples at 40 °C under N2 flow for 16 h, as at these conditions the cement composition is preserved.

Superplasticizers are essential components of modern concrete, allowing the production of highly durable structures with reduced environmental impact. Their working mechanisms rely on their molecular structures and physical–chemical interactions taking place between them and the surface of cement particles. In some situations, it is difficult or impossible to find a robust combination giving the desired workability and workability retention without excessive retardation. Such situations are described as cement–superplasticizer incompatibilities. An attempt to better define the origin of these incompatibilities is presented in this chapter, highlighting the role of a soluble alkali, C3A, and a specific surface. The mechanistic insight provided along with recommended testing approaches should help to better control the quality of admixed concrete.

Concrete rheology is affected by many factors. The effect of admixtures takes place at the paste level, and propagates through length scales to be felt at the macroscopic level of concrete. Thus studies on paste can provide correct relative properties at the concrete level for different admixed concrete, but cannot directly provide the absolute rheological properties of concrete. Concrete behaves as a material with a yield stress fluid, and the yield stress plays an essential role in many applications. Its viscosity and possible shear thickening must also be taken into account in a significant number of cases, particularly in formulations with low water–cement (w/c). Concrete can exhibit a thixotropic behaviour and, depending on the rate of structural build-up, this must be taken into account and possibly also exploited. Over recent years much progress has been made in understanding concrete rheology from a practical and a fundamental point of view.

Chemical admixtures are nowadays very important for concrete design. This chapter presents an overview of the chemical structures of different organic chemical admixtures, ranging from small organic compounds to large polymers having a certain polydispersity, and of both natural and synthetic origin. The choice is guided by the fact that this is where the real added value of molecular structure comes into play in terms of design of new or modified chemical admixtures. Such admixtures offer the greatest possibility to chemists to modify properties and target improved performance by specific exploitation of structure/property relationships. The overview gives a basis for better understanding of the working mechanisms of these admixtures.

The properties of most of admixtures come from their ability to adsorb on the surface of particles (as observed for water reducers, superplasticizers, and retarders) or on the liquid–vapor interface (e.g., for air-entraining or shrinkage-reducing admixtures). Their adsorption behavior depends not only on their chemical composition, molecular structure, and dosage but also on the characteristics of the adsorbent surface and the composition of the liquid phase. This is particularly important during cement hydration, when the formation of new phases and an evolving ionic activity may alter strongly adsorption efficiency of admixtures. In practice, the determination of an adsorption isotherm allows for comparing the adsorption behavior of admixtures. The most common way to quantify the amount of adsorbed molecules is the solution depletion method. This method and its limitations, as well as the modeling of the adsorption isotherm, are deeply discussed to show that adsorption measurements and their interpretation are not trivial.

Most concrete admixtures on the market are not single molecules but formulated products. Therefore, they contain more than one compound, up to many, for various reasons. A commercial product must balance many requirements. However, this reality is not clearly visible to most end users and is very often misperceived as a “dark art” in the academic community. This chapter presents a brief overview on some aspects of formulation and some typical additions that may be found in commercial admixtures. Some of the aspects discussed include slump retention, defoaming, setting and hardening control, and biocides.