Marta Palacios

Alkali-activated cementitious materials: Alternative matrices for the immobilisation of hazardous wastes: Part II. Stabilisation of chromium and lead

Abstract This research focuses on the study of the stabilisation/solidification capacity of a cementing matrix, which has been made using alkali activation of fly ash, in the presence of toxic elements chromium and lead. Such capacity has been compared with that of Portland cement. Leaching tests carried out proved that this new matrix is able to stabilise and solidify lead in a very efficient way (analysed lead concentrations from leaching are in parts per billion). However, it does not present itself as efficient concerning chromium fixation since this element strongly disturbs the alkali-activation mechanism of the ash.

Rheology and Setting of Alkali-Activated Slag Pastes and Mortars: Effect of Organic Admixture

The rheology of waterglass-(Na 2 O·nSiO 2 ·mH 2 O) and NaOH-activated slag pastes and mortars depends on the nature of the alkaline activator used: in waterglass-activated slag pastes and mortars, the extensive structural breakdown under shear makes the Herschel-Bulkley model a better fit to the down ramp of the flow curve, whereas NaOH-activated pastes and mortars, such as portland-cement pastes and mortars, behaved like Bingham fluids. Admixtures were unable to reduce the yield stress of waterglass-activated slag pastes, but the inclusion of a naphthalene derivative admixture in NaOH-activated slag pastes reduced the yield stress by 80%. The problem of undesirably short setting times for waterglass-activated slag mortars and concretes could be overcome by an extended mixing time, giving an initial set of nearly 3 hours.

Effectiveness of Mixing Time on Hardened Properties of Waterglass-Activated Slag Pastes and Mortars

A previous study concluded that the setting of waterglass-activated slag pastes could be controlled and lengthened by increasing the mixing time, solving one of their main problems—poor rheological properties. The aim of this study was to explore the effect of increasing mixing time on the microstructure, mechanical properties, and drying shrinkage of waterglass-activated slag pastes and mortars, as well as on the nature of the respective reaction products. The findings showed that increasing mixing time to up to 30 minutes improves matrix cohesion and compactness, thereby enhancing mortar mechanical strength by approximately 11%. Moreover, drying shrinkage in these mortars was 16% lower than in mortars mixed for shorter times, essentially due to the decline in the percentage of pores under 0.05 μm (2.0 μin.). Lastly, longer mixing times were not observed to modify the chemical or mineralogical composition of the reaction products.

Understanding silicate hydration from quantitative analyses of hydrating tricalcium silicates

Silicate hydration is prevalent in natural and technological processes, such as, mineral weathering, glass alteration, zeolite syntheses and cement hydration. Tricalcium silicate (Ca3SiO5), the main constituent of Portland cement, is amongst the most reactive silicates in water. Despite its widespread industrial use, the reaction of Ca3SiO5 with water to form calcium-silicate-hydrates (C-S-H) still hosts many open questions. Here, we show that solid-state nuclear magnetic resonance measurements of 29Si-enriched triclinic Ca3SiO5 enable the quantitative monitoring of the hydration process in terms of transient local molecular composition, extent of silicate hydration and polymerization. This provides insights on the relative influence of surface hydroxylation and hydrate precipitation on the hydration rate. When the rate drops, the amount of hydroxylated Ca3SiO5 decreases, thus demonstrating the partial passivation of the surface during the deceleration stage. Moreover, the relative quantities of monomers, dimers, pentamers and octamers in the C-S-H structure are measured.

Durability and Testing – Degradation via Mass Transport

In most applications of reinforced concrete, the predominant modes of structural failure of the material are actually related more to degradation of the embedded steel reinforcing rather than of the binder itself. Thus, a key role played by any structural concrete is the provision of sufficient cover depth, and alkalinity, to hold the steel in a passive state for an extended period of time. The loss of passivation usually takes place due to the ingress of aggressive species such as chloride, and/or the loss of alkalinity by processes such as carbonation. This means that the mass transport properties of the hardened binder material are essential in determining the durability of concrete, and thus the analysis and testing of the transport-related properties of alkali-activated materials will be the focus of this chapter. Sections dedicated to steel corrosion chemistry within alkali-activated binders, and to efflorescence (which is a phenomenon observed in the case of excessive alkali mobility), are also incorporated into the discussion due to their close connections to transport properties.

Compatibility of PC Superplasticizers with Slag-Blended Cements

The aim of this paper was to determine the adsorption isotherms of polycarboxylate (PC) superplasticizers with different structures on slag-blended cement pastes (with a slag content between 0-75%). Also, their effect on the rheological properties and hydration process has been evaluated. The results indicate the adsorption of PCs decreases slightly as the slag content in the cement increases; however, their fluidizing properties are significantly higher in the slag-blended cement. This effect is mainly attributed to the content of C3A (mineralogical phase with the highest affinity for the PCs) which decreases in slag blended cement. Consequently, the amount of PCs consumed and adsorbed by this mineralogical phase also decreases. In this way, most of the PC admixtures are absorbed onto the silicate phases of the clinker and onto the slag particles, inducing an electrosteric repulsion and the concomitant reduction in yield stress. The rheological results show that the highest increase of the fluidity is caused by the admixture with highest molecular weight concluded to be due to the higher steric repulsion expected for thicker adsorbed layers. As a consequence of the adsorption of the PCs, a delay of the hydration process of the cement pastes has been observed.

Micro-reactors to Study the Reaction of Slag in Alkaline Media

A micro-reactor approach to study the hydration of cementitious materials has been developed in our laboratory. The method involves milling gaps that are a few microns in dimension in grains using a Focused Ion Beam. These gaps are then filled with solution, leading to dissolution, nucleation, and growth in the gaps. Hydration is stopped at selected time intervals, and a Scanning Electron Microscope is used to image the gaps. Information is obtained about dissolution-growth kinetics and hydrate morphology. Using this technique, we were able to obtain significant insights into the factors influencing early age hydration of tricalcium silicate and alite, and the effect of selected chemical admixtures on the same. In this study, we present the use of micro-reactors to study the alkaline activation of slag. The effects of solution pH and of the nature of the alkaline solution on the alkaline activation are discussed. Additionally, the dissolution of slag is studied, and we show that it is strongly affected by the presence of calcium and aluminum in solution. Results are compared and contrasted with those obtained with tricalcium silicate and alite. Originality The micro-reactor approach is probably the only method that offers the possibility of obtaining information about both dissolution and growth processes. We present ways in which factors affecting dissolution and growth may be understood, something that is very difficult or impossible using other techniques. Applying the micro-reactor technique to alkaline activation offers a way to better understand the reaction and the factors influencing it. Additionally, although we only present results with slag here, this technique may be used to study geopolymerization of several precursors with various solution compositions.

Rheology of Limestone Calcined Clays Cement Pastes. A Comparative Approach with Pure Portland Cement Pastes

In this work, we measure and compare the rheological behavior of suspensions of limestone calcined clays cement paste and pure Portland cement pastes at the same solid volume fraction. The limestone calcined clays cement pastes are prepared with two different calcined clays. We first focus on the influence of clay particles on the suspensions yield stress and viscosity in steady flows and we discuss the influence of these particles on both interaction forces and packing properties. We then study the effect of adsorbing polymer addition on these three systems and analyze our results in terms of polymer adsorption or depletion. We finally measure the evolutions of the rheological properties at rest and conclude on the effect of clay particles on early structuration/nucleation features of these suspensions.