Role of Polycarboxylate-ether superplasticizers on cement hydration kinetics and microstructural development

Polycarboxylate-ether (PCE) superplasticizers are a fundamental constituent of modern cementbased materials due to their impact on the rheology of the fresh mix and mechanical performance of the hardened material. The effect of PCEs on cement hydration kinetics has been known since their introduction in the early 1980s. However, detailed knowledge of the role played by PCE macromolecules on the basic mechanisms of cement hydration (dissolution, diffusion, precipitation) is still lacking. A better understanding of how such mechanisms are influenced by the addition of PCE is no doubt beneficial to the design of novel superplasticizing admixtures. Here, I report on some recent findings about the role of PCE superplasticizers on cement hydration kinetics and microstructural development. The interaction between PCE and C3S pastes was investigated by an ad-hoc kinetic model based on a combination of generalized forms of the Avrami and BNG (Boundary Nucleation and Growth) models. The model is used to fit the rate of C-S-H precipitation measured by in-situ X-ray powder diffraction combined with mass balance calculations. The results show that a switch from heterogeneous to homogeneous C-S-H nucleation occurs in the presence of PCEs and that the C-S-H growth rate decreases proportionally to the amount of PCE used. The predicted switch to homogeneous nucleation is in agreement with experimental results obtained by XRD-enhanced micro-tomography imaging, showing that, in the presence of PCE, C-S-H preferentially forms in the pore space rather than at the surface of clinker particles

Virtual materials for the prediction of concrete mechanical properties

Physical properties such as compressive strength and elastic moduli are of the utmost importance for the structural stability and design of cement-based materials. These properties are strictly related to the microstructure of the binder paste, which in turn varies in time, as a function of the hydration kinetics. Therefore, the development of the elastic properties and mechanical strength can in principle be controlled by affecting the microstructure and hydration kinetics. This can be achieved through an appropriate mix-design, which encompasses a careful selection of phase proportions, grain-size distribution, amount of water and aggregates, and use of additives. Changing such variables by a trial-and-error process can be extremely time consuming and has a significant impact in terms of resources employed. Moreover, a fully quantitative approach to the study of the cement microstructure and hydration kinetics requires significant efforts in terms of experimental testing, often encompassing analytical techniques such as X-ray diffraction, scanning electron microscopy and isothermal calorimetry, among others. In this contribution, an alternative quantitative characterization of the cement paste in time is illustrated, based on the numerical modeling of cement-based systems. Virtual cement pastes and mortars are generated using the software VCCTL (http://www.nist.gov/el/building_materials/inorganic/vcctl.cfm), using as input parameters the clinker phase composition, the water/cement ratio, and the size and shape distribution of the particles. The elastic moduli and compressive strength of such virtual samples is then computed from the developed microstructure by a finite element method. Extensive calibration and testing has been performed against experimental data, and the good agreement between the calculated and measured elastic and mechanical properties shows that VCCTL can be used as a truly predictive tool. Although experimental testing remains a fundamental aspect of concrete science, the coupling of experiments with computational methods provides a viable tool towards a knowledge-based mix design, with a potential reduction of costs and environmental impact

Virtual materials for the prediction of concrete mechanical properties

Physical properties such as compressive strength and elastic moduli are of the utmost importance for the structural stability and design of cement-based materials. These properties are strictly related to the microstructure of the binder paste, which in turn varies in time, as a function of the hydration kinetics. Therefore, the development of the elastic properties and mechanical strength can in principle be controlled by affecting the microstructure and hydration kinetics. This can be achieved through an appropriate mix-design, which encompasses a careful selection of phase proportions, grain-size distribution, amount of water and aggregates, and use of additives. Changing such variables by a trial-and-error process can be extremely time consuming and has a significant impact in terms of resources employed. Moreover, a fully quantitative approach to the study of the cement microstructure and hydration kinetics requires significant efforts in terms of experimental testing, often encompassing analytical techniques such as X-ray diffraction, scanning electron microscopy and isothermal calorimetry, among others. In this contribution, an alternative quantitative characterization of the cement paste in time is illustrated, based on the numerical modeling of cement-based systems. Virtual cement pastes and mortars are generated using the software VCCTL (http://www.nist.gov/el/building_materials/inorganic/vcctl.cfm), using as input parameters the clinker phase composition, the water/cement ratio, and the size and shape distribution of the particles. The elastic moduli and compressive strength of such virtual samples is then computed from the developed microstructure by a finite element method. Extensive calibration and testing has been performed against experimental data, and the good agreement between the calculated and measured elastic and mechanical properties shows that VCCTL can be used as a truly predictive tool. Although experimental testing remains a fundamental aspect of concrete science, the coupling of experiments with computational methods provides a viable tool towards a knowledge-based mix design, with a potential reduction of costs and environmental impact

Role of Polycarboxylate-ether superplasticizers on cement hydration kinetics and microstructural development

Polycarboxylate-ether (PCE) superplasticizers are a fundamental constituent of modern cementbased materials due to their impact on the rheology of the fresh mix and mechanical performance of the hardened material. The effect of PCEs on cement hydration kinetics has been known since their introduction in the early 1980s. However, detailed knowledge of the role played by PCE macromolecules on the basic mechanisms of cement hydration (dissolution, diffusion, precipitation) is still lacking. A better understanding of how such mechanisms are influenced by the addition of PCE is no doubt beneficial to the design of novel superplasticizing admixtures. Here, I report on some recent findings about the role of PCE superplasticizers on cement hydration kinetics and microstructural development. The interaction between PCE and C3S pastes was investigated by an ad-hoc kinetic model based on a combination of generalized forms of the Avrami and BNG (Boundary Nucleation and Growth) models. The model is used to fit the rate of C-S-H precipitation measured by in-situ X-ray powder diffraction combined with mass balance calculations. The results show that a switch from heterogeneous to homogeneous C-S-H nucleation occurs in the presence of PCEs and that the C-S-H growth rate decreases proportionally to the amount of PCE used. The predicted switch to homogeneous nucleation is in agreement with experimental results obtained by XRD-enhanced micro-tomography imaging, showing that, in the presence of PCE, C-S-H preferentially forms in the pore space rather than at the surface of clinker particles

Cation migration and structural modification of Co-exchanged ferrierite upon heating: a time-resolved X-ray powder diffraction study

The in situ time-resolved analysis of Co-exchanged synthetic ferrierite was performed in the range 53-810 degreesC by Rietveld refinements of powder diffraction data. The small contraction of the unit cell volume (-2.35%) confirmed that the ferrierite framework behaves as a noncollapsible framework. Moreover, continuous monitoring of the structural modifications induced by heating showed that cobalt ions migrate to new positions following the dehydration process step by step. Above 500 degreesC, five cation sites were localized; four of these are readily accessible to absorbed molecules and act as Lewis acid sites

L'influenza del gelo-disgelo e della cristallizzazione salina sul comportamento elettrico di campioni di marmi

The causes of deterìoration of the physical and mechaniical proprìeties of stane mateiials in the Italian climate are chiefly due to two factors: -changes in the status of the water inside the pores following varìations in air temperature; - capillary absorption of water with high load of dissolved salts. These processes promote the formation of saline solutions inside the materials due to condensation. The state of aggregation of stane materìah is represented by porosity, a parameter that affects their physical and mechaniral proprìeties. Porosity impìies thè presence of empty spaces (pores and cracks) through which water and saline solutions can penetrate. Important modifications dìie to wecithmng phenomoia mai oca.tr. Of hese salts are distinctively importuni thè diasulphates (abundant in earth), thè chlorìdes (sea water) and thè nitrates (waste waters). Their presence under certain conditions provoke thè formation of crystals, wich detennines an increase in thè pore water pressure (pressure of crystallisation) that if contini/ed far long perioda of (ime would resuìt in devasta-iing effects (sudi as their qualità deterìomtion). Addi-tionally, many of these salts may exist in different states of hydration (Le. different volumes), so that "hydration pressure" is alno exerted on thè wnlls of thè pore. The precipitation of thè salts ma.y take piace on thè exter-nal surface ofthe storie mate-rìal or within thè porous. If thè veloci ty ofdiffiision ofthe water rapour through thè surface is loiver tìian thè velociti of migmtion of thè solution thmugh thè inside pores, thè salts will cr\slallise on thè surface under thè forni of effloresccnces. Alter-natively. crystfillisntion ma} occur beneath thè external surface giving ?isc lo thè formation of suì>florescences. Saline efflorescences altack and destro} thè surface parta, ti'hile thè formation o

Dehydration-rehydration processes in zeolites A: crystal-chemical characterization and possible application to improve the selectivity of gas sensors

The efficiency of hydrophilic zeolite A in its Na-, Na,K- and Na,Ca-form (4A, 3A and 5A, respectively) as an absorbing water molecules filter was tested after repeated thermal rigeneration cycles. The zeolite samples were characterized by means of thermogravimetric (TG) and X-ray diffraction (XRD) analyses. Water absorption capacity for all samples remains approximately the same after repeated dehydration-rehydration cycles and even if the process is accelerated. Rietveld structure refinements revealed no significant structural changes as a function of the number of regeneration cycles; slight variations of framework T-O distances and T-O-T angle were only recorded after the thermal activation

Towards three-dimensional quantitative reconstruction of cement microstructure by X-ray diffraction microtomography

Quantitative characterization of the microstructure of cement-based materials is of fundamental importance for assessing the performance and durability of the final products. However, accessing the three-dimensional microstructural information of hydrating cement pastes without introducing any perturbation is not trivial. Recently, a novel non-invasive method based on X-ray diffraction computed microtomography (XRD-CT) has been applied to cement-based materials, with the aim of describing the three-dimensional spatial distribution of selected phases during the hydration of the cement paste. This paper illustrates a method based on XRD-CT, combined with Rietveld-based quantitative phase analysis and image processing, which provides quantitative information relative to the distribution of the various phases present in the studied samples. In particular, it is shown how this method allows the estimation of the local volume fraction of the phase ettringite within a hydrating cement paste, and construction of a three-dimensional distribution map. Application of this method to the various constituents of a cementitious material, or, more generally, of a composite polycrystalline material, may provide a non-invasive tool for three-dimensional microstructural quantitative characterization