Critical thinking on efflorescence in alkali activated cement (AAC)

Alkali-activated cement (AAC), also known as “geopolymer”, has been extensively investigated over the past 40 years and has been developed from laboratory mock ups to real structural usage in construction in the last decade. While numerous life cycle analyses and carbon accounting studies show the “green potential” of this material compared to Portland cement, some authors state that the high alkali concentration in AAC is a potentially unstable factor which may lead to, for example, efflorescence. This paper presents a critical thinking on the literature and some new experimental work regarding the possibility of efflorescence in AAC products. Subjects of the discussion include: (1) the role of alkalis in AACs, (2) the effect of alkali concentration on efflorescence, (3) the effect of solid precursor selection on efflorescence, (4) the effect of curing scheme and chemical additives on efflorescence, and (5) the impacts of efflorescence on the microstructural properties of AACs. Particular attention is given to the relationship between pore structure and efflorescence behaviour, and consequently the mechanical properties of AACs suffering from either efflorescence or alkali loss (by leaching). The changes in sodium aluminosilicate hydrate (N-A-S-H) gels due to efflorescence or alkali loss are critical to the durability of AACs. This paper emphasizes that the nature of the solid precursor and the pore structure of the resulting AAC are the two most important factors that control efflorescence rate. However, considering its alkaline nature, it seems difficult or impossible to avoid this issue in AAC products, although kinetically controlled diffusion of alkalis using phase transformation techniques may help to mitigate efflorescence. Efflorescence in AAC is a “skin issue” that needs to be carefully treated. It is recognized to be different from the visually similar, but chemically distinct, efflorescence that occurs in Portland cement based materials

Alkali activated materials based on fluid catalytic cracking catalyst residue (FCC): Influence ofSiO2/Na2O and H2O/FCC ratio on mechanical strength and microstructure

Reuse of industrial and agricultural wastes as supplementary cementitious materials (SCMs) in concrete and mortar productions contribute to sustainable development. In this context, fluid catalytic cracking catalyst residue (spent FCC), a byproduct from the petroleum industry and petrol refineries, have been studied as SCM in blended Portland cement in the last years. Nevertheless, another environmental friendly alternative has been conducted in order to produce alternative binders with low CO2 emissions. The use of aluminosilicate materials in the production of alkali-activated materials (AAMs) is an on going research topic which can present low CO2 emissions associated. Hence, this paper studies some variables that can influence the production of AAM based on spent FCC. Specifically, the influence of SiO2/Na2O molar ratio and the H2O/spent FCC mass ratio on the mechanical strength and microstructure are assessed. Some instrumental techniques, such as SEM, XRD, pH and electrical conductivity measurements, and MIP are performed in order to assess the microstructure of formed alkali-activated binder. Alkali activated mortars with compressive strength up to 80 MPa can be formed after curing for 3 days at 65 C. The research demonstrates the potential of spent FCC to produce alkali-activated cements and the importance of SiO2/Na2O molar ratio and the H2O/spent FCC mass ratio in optimising properties and microstructure.Authors would like to thank to the Ministerio de Ciencia e Innovacion (MICINN) of the Spanish Government (BIA2011-26947) and to FEDER for funding, and also to the PROPG - UNESP "Universidade Estadual Paulista Julio de Mesquita Filho'', Brazil.Mitsuuchi Tashima, M.; Akasaki, JL.; Melges, J.; Soriano Martínez, L.; Monzó Balbuena, JM.; Paya Bernabeu, JJ.; Borrachero Rosado, MV. (2013). Alkali activated materials based on fluid catalytic cracking catalyst residue (FCC): Influence ofSiO2/Na2O and H2O/FCC ratio on mechanical strength and microstructure. Fuel. 108:833-839. https://doi.org/10.1016/j.fuel.2013.02.052S83383910

Mechanical, thermal insulation, thermal resistance and acoustic absorption properties of geopolymer foam concrete

This study reports the synthesis and characterization of geopolymer foam concrete (GFC). A Class F fly ash with partial slag substitution was used for GFC synthesis by mechanical mixing of preformed foam. The GFCs exhibited 28 d compressive strengths ranging from 3 to 48 MPa with demolded densities from 720 to 1600 kg/m3 (105 °C oven-dried densities from 585 to 1370 kg/m3), with the different densities achieved through alteration of the foam content. The thermal conductivity of GFCs was in the range 0.15-0.48 W/m K, showing better thermal insulation properties than normal Portland cement foam concrete at the same density and/or at the same strength. The GFC derived from alkali activation of fly ash as a sole precursor showed excellent strength retention after heating to temperatures from 100 to 800 °C, and the post-cooling compressive strength increased by as much as 100% after exposure at 800 °C due to densification and phase transformations. Partial substitution of slag for fly ash increased the strength of GFC at room temperature, but led to notable shrinkage and strength loss at high temperature. Thin GFC panels (20-25 mm) exhibited acoustic absorption coefficients of 0.7-1.0 at 40-150 Hz, and 0.1-0.3 at 800-1600 Hz

Toward an indexing approach to evaluate fly ashes for geopolymer manufacture

Variations between fly ashes can lead to significant differences in the geopolymers derived from them, in both microstructural and mechanical properties. This study assesses the effect of physical, crystallographic and chemical characteristics of fly ash on geopolymerisation performance and the strength of the resulting binders. Physical and glass chemistry factors are combined to develop a comprehensive index to evaluate the suitability of fly ashes for the production of high-strength geopolymers. An equation for this index is proposed, developed using five typical low-calcium fly ashes and then validated against a further eight literature datasets, showing a good relationship between the ranking order of the calculated index and the compressive strengths of geopolymer pastes produced with comparable activator and paste workability. This index can be used to screen the source materials, which is of significant value in moving alkali activated cements towards acceptance in practice

Signal processing and transduction in plant cells: the end of the beginning?

Plants have a very different lifestyle to animals, and one might expect that unique molecules and processes would underpin plant-cell signal transduction. But, with a few notable exceptions, the list is remarkably familiar and could have been constructed from animal studies. Wherein, then, does lifestyle specificity emerge

Efflorescence: a critical challenge for geopolymer applications?

Efflorescence is the formation of white salt deposits on or near the surface of concrete. For ordinary Portland cement (OPC) concrete, efflorescence is generally harmless except for the discolouration, and is best described as being 'a skin trouble and not a deep-seated disease'. However, for geopolymers, as they contain much higher soluble alkali content than conventional cement, efflorescence can be a significant issue when the products are exposed to humid air or in contact with water. In this study, the efflorescence phenomenon of geopolymers that synthesised using different activators, solid materials and curing conditions is observed. The efflorescence product is mainly sodium carbonate heptahydrate (Na2CO3·7H2O). The efflorescence potential has been compared via measurements of cation concentrations by atomic absorption spectroscopy (AAS), and determination of pH and electrical conductivity of geopolymer leaching solutions. At the same alkali content (in terms of Na2O), geopolymers synthesised at high temperature (80°C×28 d) exhibit less efflorescence rate than those synthesised at low temperature (20°C×28 d). NaOH activated geopolymers possess slower efflorescence than the sodium silicate solution activated specimens. Adding 20% slag can effectively reduce the initial efflorescence of a fly ash geopolymer. From a long term view, however, the efflorescence potential of such samples could be equivalent to the activated 100% fly ash when considering the alkali leaching results. Further investigations to prevent efflorescence, or at least to reduce its rate, are urgently required for wider applications of fly ash-based geopolymers

Fly ash-based geopolymers: the relationship between composition, pore structure and efflorescence

This study reports the observation of efflorescence in fly ash-based geopolymers. The efflorescence rate strongly depends on the activation conditions; at the same alkali content under ambient temperature curing, NaOH-activated geopolymers show less and slower efflorescence than sodium silicate-activated specimens. Geopolymers synthesised at high temperature exhibit much lower efflorescence than those synthesised at low temperature, except for the sodium silicate-activated samples. The substitution of 20% fly ash by slag reduces the efflorescence rate. A relationship between alkali leaching from monosized fractured particles and 'efflorescence potential' is proposed. Soluble silicate and slag addition are beneficial in reducing efflorescence rate, but have very limited influence on the overall efflorescence potential, as they appear to have a delaying rather than mitigating effect. The partial crystallisation of geopolymers, by curing at high temperature, appears to be the most effective method of reducing the efflorescence potential

Geopolymer foam concrete: an emerging material for sustainable construction

The development of sustainable construction and building materials with reduced environmental footprint in both manufacturing and operational phases of the material lifecycle is attracting increased interest in the housing and construction industry worldwide. Recent innovations have led to the development of geopolymer foam concrete, which combines the performance benefits and operational energy savings achievable through the use of lightweight foam concrete, with the cradle-to-gate emissions reductions obtained through the use of a geopolymer binder derived from fly ash. To bring a better understanding of the properties and potential large-scale benefits associated with the use of geopolymer foam concretes, this paper addresses some of the sustainability questions currently facing the cement and concrete industry, in the context of the utilisation of foam concretes based either on ordinary Portland cement (OPC) or on geopolymer binders. The potential of geopolymer binders to provide enhanced fire resistance is also significant, and the aluminosilicate basis of the geopolymer binding phases is important in bringing high temperature stability. The standardisation (quality control) of feedstocks and the control of efflorescence are two challenges facing the development of commercially mature geopolymer foam concrete technology, requiring more detailed exploration of the chemistry of raw materials and the microstructural development of geopolymers