Susan A. Bernal

Generalized Structural Description of Calcium–Sodium Aluminosilicate Hydrate Gels: The Cross-Linked Substituted Tobermorite Model

Structural models for the primary strength and durability-giving reaction product in modern cements, a calcium (alumino)silicate hydrate gel, have previously been based solely on non-cross-linked tobermorite structures. However, recent experimental studies of laboratory-synthesized and alkali-activated slag (AAS) binders have indicated that the calcium-sodium aluminosilicate hydrate [C-(N)-A-S-H] gel formed in these systems can be significantly cross-linked. Here, we propose a model that describes the C-(N)-A-S-H gel as a mixture of cross-linked and non-cross-linked tobermorite-based structures (the cross-linked substituted tobermorite model, CSTM), which can more appropriately describe the spectroscopic and density information available for this material. Analysis of the phase assemblage and Al coordination environments of AAS binders shows that it is not possible to fully account for the chemistry of AAS by use of the assumption that all of the tetrahedral Al is present in a tobermorite-type C-(N)-A-S-H gel, due to the structural constraints of the gel. Application of the CSTM can for the first time reconcile this information, indicating the presence of an additional activation product that contains highly connected four-coordinated silicate and aluminate species. The CSTM therefore provides a more advanced description of the chemistry and structure of calcium-sodium aluminosilicate gel structures than that previously established in the literature.

Performance of alkali-activated slag mortars exposed to acids

The acid resistance of 60-day cured alkali-activated slag (AAS) mortars, in comparison with Portland cement, is assessed. Specimens are exposed to hydrochloric, nitric and sulphuric acids at pH 3.0, and acetic acid (CH3COOH) at pH 4.5, for 150 days, with specimens also immersed in water as a control. Negligible changes in compressive strength are identified in Portland cement and AAS binders during exposure to mineral acids, while compressive strength increases during immersion in water. However, exposure to CH3COOH reduces strength, and increases pore volume, in both Portland cement and AAS mortars. AAS performs better than Portland cement in CH3COOH, associated with the lower initial permeability of the specimens, along with the low CaO/SiO2 ratio typical of AAS. Decalcification of the AAS binder through formation of calcium acetate leaves an aluminosilicate type gel that can hinder the further ingress of acids, contributing to the high acid resistance of this binder.

Microstructure and durability of alkali-activated materials as key parameters for standardization

Alkali-activated concrete (AAC) has been commercialized as a low-CO2 construction material, but its adoption still faces several challenges, including standardization, lack of a dedicated supply chain, limited service track record, and the question of whether laboratory durability testing can predict service life. This paper outlines how using different precursors leads to the formation of different AAC phase assemblages, and how AAC can be recognized in standards using a performance-based approach independent of binder chemistry. Microstructural assessment of pastes, strength development, water permeability, and chloride migration of two AACs (100% slag and 1:1 slag:fly ash) are presented, and compared to Portland cement concrete. Manipulation of binder chemistry leads to differences in the properties of the AACs; however, both AACs assessed exhibited technical benefits in a performance-based comparison. AACs can meet the requirements of the equivalent performance concept, independent of the binder chemistry, supporting their scale-up, regulatory acceptance, and wider adoption.

Structure of Portland cement pastes blended with sonicated silica fume

Application of power ultrasound to enhance dispersion of commercial densified silica fume leads to increased compressive strengths and refinement of the pore structure in mortars, compared with those that are untreated. This was attributed to the enhanced pozzolanic reactivity achieved by particle dispersion through sonication, leading to higher consumption of portlandite during curing, and formation of a calcium silicate hydrate gel with a higher degree of cross-linking than is identified in specimens with densified silica fume. This suggests that with the use of sonicated silica fume, it is possible to reduce the required quantity of admixture in blended cements to achieve specified performance, with the additional advantage of the formation of a highly densified structure and refined pore network, contributing to potential improvements in durability.

The resistance of alkali-activated cement-based binders to carbonation

Abstract Carbonation is a degradation process that negatively affects alkali-activated materials, and represents one of the main perceived limitations for their adoption in structural applications. In this chapter the protocols followed to assess the carbonation susceptibility via accelerated methods are presented, along with a critical overview of the role of testing conditions and chemistry of the binding phases in defining the results acquired from those tests. It is highlighted that there is a need to develop testing protocols that better replicate the natural carbonation mechanism of alkali-activated materials, to enable accurate prediction of their long-term performance.

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.

Other Potential Applications for Alkali-Activated Materials

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.

Milestones in the analysis of alkali-activated binders

The use of alkali activation to achieve environmental savings in the production of construction materials is currently an extremely active area of research and development. There is now a diverse range of chemistries and applications that have been developed within the broader theme of ‘alkali-activated materials’, including the subclass of lower-calcium binders which are also known as ‘geopolymers’. Academic research and commercial development have combined to bring these materials to a level of technological maturity where larger scale deployment is now taking place. This paper reviews some of the key aspects of alkali-activation technology which have brought the field to this point, with a particular view towards re-assessing key points and comments which have been raised in several historical reviews and discussions of this class of materials. Conclusions are therefore drawn regarding which among these key questions have been answered, and which remain outstanding in an engineering or scientific sense.

Ba(OH)2 – blast furnace slag composite binders for encapsulation of sulphate bearing nuclear waste

Abstract The present study investigated the feasibility of the immobilisation of sulphate bearing radioactive wastes in blast furnace slag (BFS) based binders. BaSO4–BFS composites were produced via two methods using Na2SO4 as a waste simulant, along with Ba(OH)2 to promote precipitation of BaSO4 in an insoluble sulphate form and the consequent activation of the BFS. BaSO4 was effectively formed by both methods, and solid wasteforms were successfully produced. Although both methods produced BaSO4 embedded in the cement-like composites, different reaction products including ettringite and witherite were produced, depending on the order Ba(OH)2 was mixed with the system. These results show that the immobilisation of soluble sulphate-bearing aqueous wastes is achievable in Na2SO4–Ba(OH)2–BFS composites.

Evolution of phase assemblage of blended magnesium potassium phosphate cement binders at 200° and 1000°C

The fire performance of magnesium potassium phosphate cement (MKPC) binders blended with fly ash (FA) and ground granulated blast furnace slag (GBFS) was investigated up to 1000°C using X-ray diffraction, thermogravimetric analysis and SEM techniques. The FA/MKPC and GBFS/MKPC binders dehydrate above 200°C to form amorphous KMgPO4, concurrent with volumetric and mass changes. Above 1000°C, additional crystalline phases were formed and microstructural changes occurred, although no cracking or spalling of the samples was observed. These results indicate that FA/MKPC and GBFS/MKPC binders are expected to have satisfactory fire performance under the fire scenario conditions relevant to the operation of a UK or other geological disposal facility