Özlem Cizer

Formation of amorphous calcium carbonate and its transformation into mesostructured calcite

Amorphous calcium carbonate (ACC) is a key precursor of crystalline CaCO3 biominerals and biomimetic materials. Despite recent extensive research, its formation and amorphous-to-crystalline transformation are not, however, fully understood. Here we show that hydrated ACC nanoparticles form after spinodal liquid–liquid phase separation and transform via dissolution/(re)precipitation into poorly hydrated and anhydrous ACC nanoparticles that aggregate, forming a range of 1D, 2D and 3D structures. The formation of these structures appears to be achieved by oriented attachment (OA), facilitated by the calcite medium-range order of ACC nanoparticles. Both electron irradiation processes in the TEM and under humid air exposure at room temperature of the latter ACC structures result in pseudomorphs of single crystalline mesostructured calcite. While the high-vacuum/e-beam heating leads to solid-state transformation, the transformation in air occurs via an interface-coupled dissolution/precipitation mechanism. Our results differ significantly from the currently accepted model, which considers that the low T ACC-to-calcite transformation in air and during biomineralization is a solid-state process. These results may help to better understand how calcite biominerals form after ACC and offer the possibility of biomimetically preparing single crystalline calcite structures after ACC by tuning pH2O at room temperature.

Competition between Hydration and Carbonation in Hydraulic Lime and Lime-Pozzolana Mortars

A combined reaction of hydration and carbonation takes place in hydraulic lime and lime-pozzolana mortars. Hydration reactions are the first reaction and carbonation of lime is the complementary reaction in the strength gain. Competition between these two reactions can occur in lime-pozzolana mortars if the pozzolanic material has low reactivity with lime, leading to the consumption of lime by carbonation reaction. The degree and the order of these reactions are strongly influenced by the moisture content. Hydration reactions are enhanced under moist conditions while carbonation is delayed. Curing under dry conditions does not sufficiently increase their strength because the hydration reactions are slowed down or even terminated by the full carbonation of lime in lime-pozzolana mortars. The consequence of this on the mechanical properties of the mortars is remarkable while the same impact is not observed in their porosity. Such mortars require moist conditions to ensure sufficient strength development.

Cementitious binders from activated stainless steel refining slag and the effect of alkali solutions.

With an aim of producing high value cementitious binder, stainless steel refining slag containing a high amount of CaO in γ-dicalcium silicate form was activated with NaOH and Na-silicate as well as KOH and K-silicate solutions, followed by steam curing at 80 °C. Higher levels of alkali-silicate in the activating solution resulted in higher cumulative heat suggesting accelerated reaction kinetics. With respect to compressive strength, higher levels of alkali silicate resulted in higher strength and the mortars with Na activator were found to have higher early strength than the ones with K activator. The long term strength was found to be similar, regardless of the alkali metal. Thermogravimetric, QXRD and FTIR analyses showed an increase in the amount of reaction products (C-S-H type) over time, further confirming the reactivity of the crystalline slag. Batch leaching results showed lower leaching of heavy metals and metalloids with K activator compared to the Na activator. These results demonstrate that the alkali type and the ratio of hydroxide to silicates have a significant impact on the hydration and mechanical strength development of the stainless steel slag. The above findings can aid in the recycling and valorization of these type of slags which otherwise end up landfilled.

Effect of High Cooling Rates on the Mineralogy and Hydraulic Properties of Stainless Steel Slags

This article investigates the effect of chemical composition and cooling rate during solidification on the mineralogy and hydraulic properties of synthetic stainless steel slags. Three synthetic slags, covering the range of typical chemical composition in industrial practice, were subjected to high cooling rates, by melt spinning granulation or quenching in water, and to low cooling rates, by cooling inside the furnace. Both methods of rapid cooling led to volumetrically stable slags unlike the slow cooling which resulted in a powder-like material. Stabilized slags consisted predominantly of lamellar β-dicalcium silicate (β-C2S) and Mg, Ca-silicates (merwinite and bredigite); the latter form the matrix at low basicity and are segregated along the C2S grain boundaries at high basicities. Slowly cooled slags consist of the γ-C2S polymorph instead of the β-C2S and of less Mg, Ca-silicates. Isothermal conduction calorimetry and thermogravimetric analysis indicate the occurrence of hydration reactions in the stabilized slags after mixing with water, while calcium silicate hydrates (C-S-H) of typical acicular morphology are identified by SEM. The present results demonstrate that the application of high cooling rates can result in a stable, environmental-friendly, hydraulic binder from stainless steel slags, rich in β-C2S, without the necessity of introducing any additions to arrest the β polymorph.

Comparison of solvent exchange and vacuum drying techniques to remove free water from early age cement-based materials

In order to study the progress of hydration at early ages in cement-based materials by thermal analysis, FT-IR and XRD, the ongoing hydration reactions should be stopped after different periods of time by complete removal of the free water from the material, i.e. the water which has not yet participated in the hydration reactions. The replacement of the cement paste pore water by a low surface-tension organic fluid is a common technique to stop the hydration prior to thermal analysis. However, it seems that an interaction is taking place between the organic liquid and the cement hydrates which results in the formation of carbonate-like compounds and an underestimation of the real calcium hydroxide content by thermal analysis. The solvent exchange drying technique is studied for different times of storage in the organic solvent and for cement pastes of different ages. TGA and DSC graphs of these pastes are compared with pastes vacuum dried at 2.5×10 mbar.

Assessment of the Compatibility of Repair Mortars in Restoration Projects

In restoration works compatibility requirements of repair mortars are defined based on the original mortar characteristics, but the efficiency and the performance of the repair mortars after application on masonry are not generally evaluated. From this perspective, original mortars and repair mortars from two historic masonry structures were analyzed for their characterization. Compatibility of the repair mortars with the historic mortars is investigated in terms of chemical, mineralogical, and physical point of view. The efficiency of the analytical techniques used for the mortar characterization is discussed. A methodology relying on a basic approach for a mortar analysis is adopted taking into account the added values and the basic requirements from both practical and scientific point of view. This study will contribute to the existing knowledge on mortar analysis and will provide new insights on the assessment of the compatibility of the repair mortars.

Lessons from a lost technology: The secrets of Roman concrete

Abstract “Roman concrete” was used as building material during Roman Imperial times for a great number of famous constructions with different functions. Well-known examples are aqueducts, for example the Pont du Gard in France and the Aqua Alexandrina in Rome, and numerous monuments such as the famous Coliseum and Pantheon in Rome. While visually less apparent today, ancient harbors also were an important part of constructions supporting the power of the Roman imperial system and were invaluable to control the Mediterranean Sea trade. The article “Unlocking the secrets of Altobermorite in Roman seawater concrete” by Jackson et al. (2013) describes the investigation of several extremely interesting samples drilled from 2000-yearold Roman maritime concretes in 11 harbors along the Mediterranean coast [the ROMACONS project 2002-2009 (Oleson et al. 2004)].

Kinetic effect of carbonic anhydrase enzyme on the carbonation reaction of lime mortar

ABSTRACT The effect of carbonic anhydrase enzyme on the precipitation kinetics and phase transformations of calcium carbonate, and on the strength development of lime mortars has been investigated with saturated lime solutions, lime pastes and lime mortars under atmospheric conditions. The results clearly show that carbonic anhydrase catalyzes the reaction between carbon dioxide and aqueous lime, and increases the rate of calcium carbonate crystallization, the yield of the carbonation reaction and mortar strength at early ages. This is most likely a kinetic effect associated with the increased rate of carbonate ions supply to the solution by the enzyme. In addition, this enzyme favors the formation of stable calcite and significantly modifies its morphology by developing new crystal faces. These results suggest a novel approach for accelerating the hardening of lime mortars using carbonic anhydrase enzyme, which may offer a potentially novel approach with significant benefits on the applications of lime mortars in architectural heritage conservation as well as in construction.

Combining Mineral and Polymer Binder Material Science for Sustainability in Construction and Restoration

Abstract The study of microstructure formation in polymer-cement concrete provides opportunities to exploit synergetic actions between cement and polymer, leading to performance improvement and to a wide range of new and innovative properties and applications. Polymers can reduce the impact of construction industry on environment, by decreasing the carbon footprint of cement and concrete production. Renovation and restoration, largely figuring in the concept of sustainable construction development, thank their growing share in construction activity to the input of polymers in repair and binder materials and in rehabilitation procedures. The study of ancient binders and mortars reveals aspects of the origins of the observed long lasting durability of those ancient mortars. It also reveals the interaction mechanisms between carbonation of air hardening components and hydration of hydraulic components, which in turn helps to develop chemical activation methods (i.e. alkaline activation) to improve the hydraulic properties of pozzolans and industrial residues to develop inorganic polymers (i.e. geopolymers) for eventually full replacement of cement in binders.

Concrete with Flash-Calcined Dredging Sediments as a Novel Supplementary Cementitious Material

Maintenance dredging in the port of Antwerp annually generates about 450.000 tons of dry matter sediment, for which suitable disposal solutions or applications are required. Mechanical dewatering of the sediments results in filter cakes, comprising clays (2:1 clay minerals and kaolinite), quartz, calcite and an amorphous phase as major mineral phases. Flash calcination of these filter cakes reduces the total organic carbon fraction and results in a dehydroxylation of the clay minerals. Isothermal conduction calorimetry tests demonstrated the pozzolanic reactivity of the calcined material, being superior to that of a siliceous fly ash. As a result of the pozzolanic reactions, replacing 20, 30 or 40 wt% of cement by calcined dredging sediments leads to a strength development equivalent to a reference mix with Portland cement up to 28 days, despite low strength at early age. This paper presents material characteristics and pozzolanic reactivity of the flash-calcined dredging sediments, as well as their effect on setting time, fresh concrete properties and mechanical characteristics. The initial results clearly show that the flash-calcined clay-rich dredging sediments have great potential to be used as a novel pozzolanic supplementary cementitious material, for the production of sustainable, low-CO2 blended cements and concrete.