Showing papers by "Francisca Puertas published in 2014"

Rheology of alkali-activated slag pastes. Effect of the nature and concentration of the activating solution

Abstract: An understanding of the rheological behaviour of OPC-based products has been widely studied, for it is essential to determining and predicting the fresh and hardened characteristics and properties of pastes, mortars and concretes. The rheology of alkali-activated material (AAM) systems has been much less intensely researched, however. The present study aimed to ascertain the effect of factors such as the nature and concentration of the alkaline activator on the rheological behaviour of alkali-activated slag (AAS) pastes, with a comparison between the rheological parameters and fluidity of these pastes to the same parameters in OPC. More specifically, the study explored how paste rheology was affected by the nature of the alkaline activator (NaOH, 50/50 wt% NaOH/Na2CO3 or waterglass – Wg), its concentration (3–5% Na2CO3 of slag weight) and, in the waterglass solution, the SiO2/Na2O ratio. The findings showed that AAS paste rheology is affected by the nature of the activator. The rheological behaviour in AAS pastes activated with NaOH alone or combined with Na2CO3 was similar to the rheology observed in OPC pastes, and fit the Bingham model. Conversely, the AAS pastes activated with waterglass fit the Herschel–Bulkley model and their rheology proved to depend on both the SiO2/Na2O ratio and the Na2O concentration. Moreover, regardless of the activator used (NaOH, Na2CO3 or waterglass), an increase in Na2O concentration implies a raise of shear stress. The formation of primary C–S–H gel in Wg–AAS and its effect on paste rheology were confirmed. Gel formation was likewise shown to be related to the SiO2/Na2O ratio and activator concentration.

Sodium silicate solutions from dissolution of glasswastes. Statistical analysis

Abstract: It has studied the solubility process of four different waste glasses (with different particle sizes, 125 µm) in alkaline solutions (NaOH and NaOH/Na₂CO₃) and water as a reference and under different conditions of solubility (at room temperature, at 80°C and a mechano-chemical process). Have established the optimal conditions of solubility and generation of sodium silicates solutions, and these were: the smaller particle size (<45 µm), with NaOH/Na₂CO₃ solution and with temperature during 6 hours of stirring time. The statistical analyses of the results give importance to the studied variables and the interactions. Through ²⁹Si NMR MAS it has confirmed the formation after dissolution processes of monomeric silicate, suitable for use as an activator in the preparation of alkaline cements and concretes.

Historical Aspects and Overview

Abstract: Cement and concrete are critical to the world economic system; the construction sector as a whole contributed US$3.3 trillion to the global economy in 2008 [1]. The fraction of this figure which is directly attributable to materials costs varies markedly from country to country – particularly between developing and developed countries. Worldwide production of cement in 2008 was around 2.9 billion tonnes [2], making it one of the highest-volume commodities produced worldwide. Concrete is thus the second-most used commodity in the world, behind only water [3]. It is noted that there are certainly applications for cement-like binders beyond concrete production, including tiling grouts, adhesives, sealants, waste immobilisation matrices, ceramics, and other related areas; these will be discussed in more detail in Chaps. 12 and 13, while the main focus of this chapter will be large-scale concrete production.

Other Potential Applications for Alkali-Activated Materials

Abstract: The focus of this chapter is the discussion of a variety of niche applications (other than as a large-scale civil infrastructure material) in which alkali-activated binders and concretes have shown potential for commercial-scale development. The majority of these applications have not yet seen large-scale AAM utilisation, except as noted in the various sections of the chapter. However, there have been at least pilot-scale or demonstration projects in each of the areas listed, and each provides scope for future development and potentially profitable advances in science and technology. In addition to the applications specifically discussed in this chapter, there are also commercial and academic developments in alkali-activation for specific applications including a commercial product which is being marketed as a domestic tiling grout showing some self-cleaning properties [1], as well as alkali-activated metakaolin binders as a vehicle for controlled-release drug delivery [2, 3]. Although undoubtedly promising and of commercial interest, these are rather specialised applications, and so the focus of this chapter is instead on broader categories of research and development rather than in providing detailed analysis of specific products. The areas to be discussed will include lightweight materials, well cements, fire-resistant materials, and fibre-reinforced composites.

Durability and Testing – Degradation via Mass Transport

Abstract: 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.

C-A-S-H gels formed in alkali-activated slag cement pastes. Structure and effect on cement properties and durability

Abstract: The development of alternatives to traditional Portland cement produced with more eco-efficient processes (lower energy consumption and CO 2 gas emissions) is an item on climate change and innovation agendas. Alkaline cements and concretes are an effective alternative to traditional cements.The structure of the C-S-H gel in Portland cements consists mostly of 14-nm tobermorite (with a chain length of five) and jennite (2-link chain). The mechanical properties of C-S-H gels can be explained in terms of the three types of packing found in these gels: low density (LD), high density (HD) and ultra-high density (UHD).The main reaction product in alkali-activated slag (AAS) cements is a C-A-S-H gel, which adopts different structures depending on the nature of the alkaline activator. When the activator is a NaOH solution (4 % Na 2 O by slag weight), the C-A-S-H gel formed has an intermediate structure between 14-nm tobermorite with a chain length of five links and 11-nm tobermorite with 14 links. When the activator is a waterglass solution (4 % Na 2 O by slag weight), traits characteristic of 14-nm tobermorite with 11-link chains and 11-nm tobermorite with a chain length of 14 co-exist in the structure of the C-A-S-H gel formed. This densely packed structure (with three HD states) yields excellent mechanical properties. Like the C-A-S-H gels obtained in NaOH (4% Na2O)-activated AAS paste, the C-A-S-H gels forming in AAS gels activated with waterglass have no UHD states.The structure and composition of these C-A-S-H gels determine strength development in AAS mortars and concretes as well as their resistance to aggressive chemicals.