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Journal ArticleDOI

Effects of an eco-silica source based activator on functional alkali activated lightweight composites

TL;DR: In this article, an eco-olivine nano-silica is applied to prepare sustainable silicate based activators to replace commercial sodium silicates, and the results show the positive effect of density and Na2O content on strength.
About: This article is published in Construction and Building Materials.The article was published on 2019-08-10 and is currently open access. It has received 8 citations till now. The article focuses on the topics: Portland cement.

Summary (4 min read)

1. Introduction

  • Lightweight concrete has been widely applied as both structural and non-structural components in a wide range of weights and strengths for various applications [1,2], due to its properties such as low density, good thermal insulation and fire resistance [3].
  • In addition, Portland cement is commonly used as binding material for lightweight con- crete, but its production is responsible for around 7% of the global carbon emissions and high energy costs [5,6].
  • In order to reduce the negative environmental impacts, the development of sustainable alternatives such as alkali activated materials has been investigated because of the excellent mechanical properties, durability, thermal resistance together with low energy and carbon costs [7– 9].
  • The commercial process of sodium silicate production includes the melting of sodium carbonate and quartz sand around 1400–1500 C with carbon release of above 400 kg/ton [31].

2.1. Materials

  • The powder raw materials applied in the present work are blast furnace slag and Class F fly ash, and their major chemical compositions are shown in Table 1.
  • Commercial lightweight aggregates produced from natural expanded silicate with three particle sizes are applied: 0.5–1 mm, 1–2 mm and 2–4 mm, with particle densities of 600, 550 and 500 kg/m3, respectively.
  • CEN standard sand is also used as fine aggregate.
  • The SEM pictures of the lightweight aggregates’ surface and internal structure are provided in Fig. 9(A and B).
  • Analytical sodium hydroxide and laboratory prepared olivine nano-silica (19.04% SiO2 and 80.96% H2O by mass) were used to produce alkali activators.

2.2. Sample preparation

  • A fixed activator modulus of 1.4 and a slag/fly ash mass ratio of 8/2 were used based on the previous experiences [35,36].
  • The detailed mix proportions are presented in Table 2, for instance, sample with the label of D15-5.0 means it was designed to have a density level of 1500 kg/m3 and a Na2O content of 5.0%.
  • Specimens were prepared and poured into molds of 40 40 160 mm3, then covered with a plastic film to prevent the moisture loss.

2.3. Testing methods

  • The compressive strength was determined according to EN 196- 1 [37].
  • The early age hydration heat release was investigated by an isothermal calorimeter with TAM Air, Thermometric.
  • The thermal conductivity (k) and the mass heat capacity (c) were measured by Table 2 Mix proportions of alkali-activated slag-fly ash composites (kg/m3).
  • The acoustic absorption coefficient was measured according to EN 10534-2 [39].
  • Microstructure of the reaction products was identified by scanning electron microscopy (SEM) using a JEOL JSM-IT100 instrument, operated with high vacuum mode at an accelerating voltage of 10 kV.

3.1. Compressive strength

  • The relations between the oven dry density and strength are briefly depicted.
  • Fig. 2 depicts the effect of the equivalent Na2O content on 28 d compressive strength of mixtures with two density levels: 1500 and 1200 kg/m3, represented with sample label of D-15 and D-12 in the figure.
  • For mixtures with a Na2O content of 5% in this study, the additionally provided silicate from the activator accounts for around 14.9% of the total silicate within the system, and this activator dosage is commonly used in achieving a high strength [9,20,21,24–26].
  • On the one hand, increasing the alkalinity (Na2O %) will promote the activation of the binder that consequently leads to a higher strength from the aspect of the binder matrix; while on the other hand, the usage of lightweight aggregate limits the strength development by the relatively low crushing strength of the aggregate.

3.2. Reaction kinetics

  • The isothermal calorimeter test was performed on mixtures with the Na2O content of 2.0%, 3.5% and 5.0%, respectively, and lightweight aggregates were added with an aggregate/binder ratio of 0.8 (based on the mixture proportions shown in Table 2), in order to evaluate their effect on the early age reaction.
  • The induction period lasts more than 48 h; the main reaction peak exhibits an obviously broader covered area with a low peak intensity of about 0.34 mW/g, indicating a gradual and slow formation of the reaction products.
  • Thus it can be concluded that the reduction of the reaction process does not present a linear relation with the Na2O content, the shift of Na2O concentration effectively influences the characters of the reaction process such as induction time, reaction intensity, the location and duration of main reaction period.
  • Similar trends are also shown in samples with 3.5% Na2O content, indicating that the effect of lightweight aggregate on the early age reaction is rather limited, and those slight effects are probably attributed to the absorption of small amount of activator during the initial mixing.
  • Similar to the results shown in Fig. 3, as the Na2O content decreases, the effect of lightweight aggregate on heat release becomes more significant.

3.3. Gel structures

  • In order to investigate to the effect of the Na2O content on the gel compositions, the TG/DTG and FTIR analyses were performed and the results are shown in Figs. 5 and 6, respectively.
  • The mass loss between 600 C and 1000 C is partly attributed to the decomposition of reaction products, while the carbonates also play a role.
  • The infrared spectra of the unreacted slag and fly ash, and the reaction products with different Na2O contents are shown in Fig.
  • All mixes show a main absorption band around 950 cm 1, which is the vibration of a non-bridging Si-O bond [59], also commonly recognized as C-A-S-H type gels.
  • The fixed location of the typical bands together with the TG-DTG results indicate that the Na2O blends with different Na2O contents.

3.4. Thermal conductivity

  • In terms of the building materials, a low thermal conductivity contributes to an enhanced indoor thermal comfort, saving the energy cost and preventing the fire caused collapses; while lightweight concrete products based on alkali activated materials are capable of achieving those requirements with a further lowered environmental impact.
  • For mixtures with a density level of round 1500 kg/m3, the thermal conductivity (k-value) is 0.37 W/(m k), this value is lower than the ones from the obtained literatures.
  • This is because that besides the density, the differences in matrix composition, type of binder and aggregates also show an influence on the property of thermal insulation [64,65].
  • As can be seen from the mix proportions shown in Table 2, once the Na2O content is fixed, the difference between different mixes lies in the aggregate type and dosage.
  • Overall, the compressive strengths of around 20–30 MPa together with moderate densities and ideal thermal conductivities indicate a wide and promising application potential of this new lightweight concrete.

3.5. Acoustical absorption

  • Owing to the massive addition of the porous lightweight aggregates, the resulting alkali activated lightweight concrete is expected to exhibit good sound absorption behaviours.
  • Fig. 8 exhibits the sound absorption coefficient as a function of frequency, four mixtures with a Na2O content of 3.5% with different density levels were tested.
  • The mixture with label of D-15 refers to the sample with a density around 1500 kg/m3.
  • The peak absorption coefficient increases to around 0.35 and 0.52 in samples with a density about 1400 and 1300 kg/m3 respectively, while the main absorption frequency range remains similar.
  • It should be mentioned that the medium frequency usually refers to the sound from humans and daily life.

3.6. SEM analysis

  • Scanning electron microscopy images are used to characterize the applied lightweight aggregate and the interfacial transition zones (ITZ) of the reaction products.
  • Due to the fact that the effect of Na2O content on micro scale characteristics is not significant, the reaction products shown in Fig. 9 are having a constant Na2O content of 3.5%.
  • Fig. 9- B depicts a sectional view of the internal structure of the lightweight aggregate, micro pores with different sizes and shapes are clearly presented.
  • Table 3 Calculation on the carbon footprint (kg/m3).
  • This may lead to an increment of the density compared to the designed one, and may slightly reduce the thermal insulation and sound absorption properties.

3.7. Advantages in carbon footprint

  • Applying alkali activated materials shows an advantage in carbon emission towards Portland cement.
  • While within the alkali binder systems, the Na2O content is directly linked to the environmental issues.
  • The assumed recipes are based on the mix proportions shown in Table 2.
  • Moreover, when olivine nano-silica is applied as commercial waterglass replacement, the carbon emission in terms of activator can be further reduced by around 25%.

4. Conclusions

  • This paper evaluates the mechanical properties, thermal property, acoustical absorption and interfacial transition zones of ecofriendly alkali activated slag-fly ash lightweight composites (LWC) with different density classes.
  • The effect of Na2O contents and the utilization of alternative silica source on early age reaction, gel characteristics and carbon footprints on the designed LWC are assessed.
  • Mixtures with 28 d compressive strength up to 32.5 MPa and densities between 1200 and 1500 kg/m3 are resulted, and a direct correlation between strength and density is observed.
  • Compared to the commercial waterglass, the utilization of olivine nano-silica reduces the carbon footprint from activator by around 25%.
  • The lightweight concretes exhibit very low thermal conductivity between 0.16 and 0.37 W/(m k), as well as good sound absorption coefficient up to 0.7 for medium frequencies.

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Citations
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Journal ArticleDOI
TL;DR: In this paper, the shortcomings and application limitations of geopolymer materials were summarized, and their progress was summarized to lay a theoretical foundation for the long-term development of the materials.
Abstract: Geopolymer is a new environment-friendly cementitious material, and the development of geopolymer can reduce the carbon dioxide emission caused by the development of cement industry. Geopolymer materials not only have excellent mechanical properties, but also have a series of excellent properties such as fire resistance and corrosion resistance. Most industrial solid waste and waste incineration bottom ash are piled up at will, which not only occupies land resources, but also has a bad impact on the environment. Recycling them can be used as raw materials for preparing geopolymers. Geopolymer materials can effectively adsorb heavy metals, dyes, and other radioactive pollution, which is very beneficial to society's future development. However, due to the excellent properties of geopolymer materials, its application goes beyond that. Some useful information about geopolymer materials was introduced in this paper. The paper included the geopolymerization, the source of raw materials, the types of activators, the preparation methods, and the different application fields of geopolymer materials. The factors affecting the fresh properties and mechanical properties of geopolymer materials were discussed. In this paper, the shortcomings and application limitations of geopolymer materials were summarized, and their progress was summarized to lay a theoretical foundation for the long-term development of geopolymer materials.

111 citations

Journal ArticleDOI
05 Jun 2021
TL;DR: In this paper, the authors reviewed different types, mechanisms, and result of mechanical and durability properties of alkali-activated materials and geopolymer reported in literature and discussed future projections of waste materials that have cementitious properties and can replace ordinary Portland cement and be used in alkali activated materials.
Abstract: The vast increase in CO2 and waste generation in recent decades has been a major obstacle to sustainable development and sustainability. In construction industry, the production of ordinary Portland cement is a major greenhouse gas emitter with almost 8% of total CO2 production in the world. To address this, Alkali-activated materials and geopolymer have more recently been introduced as a green and sustainable alternative of ordinary Portland cement with significantly lowered environmental footprints. Their use to replace Portland cement products generally leads to vast energy and virgin materials savings resulting in a sustainable concrete production. In doing so, it reuses the solid waste generated in industrial and manufacturing sectors, which is aligned with circular economy. In turn, it reduces the need for ordinary Portland cement consumption and its subsequent CO2 generation. To provide further insight and address the challenges facing the substitution of ordinary Portland cement, this article reviews different types, mechanisms, and result of mechanical and durability properties of alkali-activated materials and geopolymer reported in literature. Finally, it discusses future projections of waste materials that have cementitious properties and can replace ordinary Portland cement and be used in alkali-activated materials and geopolymer.

66 citations

Journal ArticleDOI
TL;DR: In this article, an insulating material synthesized from metakaolin, limestone powder and phosphoric acid was analyzed by using sophisticated tools such as X-ray diffraction, thermogravimetric/its derivative, scanning electron microscopy and energy dispersive analytical Xray.

30 citations

Journal ArticleDOI
TL;DR: In this paper, the authors provide an updated information on recent advances while stressing the sustainability of lightweight geopolymer materials over ordinary Portland cement products that are vastly in use, including perlite, pumice, shale, ceramsite, and slate sand.
Abstract: Alkali-activated materials and geopolymer are major sustainable alternative binding materials to ordinary Portland cement products with higher thermal resistance and often better durability properties. In lightweight form, they have an unmatched lowered thermal conductivity and insulating properties making them a perfect fit for optimized structural components with highest strength to density ratio and major energy savings in green buildings. For them to produce lightweight materials, generally either certain foaming agent or some types of lightweight aggregates in virgin, expanded, or recycled form are utilized that reduce the overall density through higher overall porosity. In accordance, this review provides an updated information on recent advances while stressing the sustainability of lightweight geopolymer materials over ordinary Portland cement products that are vastly in use. In the end, recent mechanical and durability properties developed and documented are reviewed and provided for future applications. Based on the result of this review, the most common lightweight aggregates used in literature are perlite, pumice, shale, ceramsite, and slate sand, in expanded and porous form, along with recycled thermosetting (e.g., rubber), or thermoplastic (e.g., polyethylene) materials. In foam form, chemical and mechanical foaming are the most commonly used foaming techniques to increase porosity of final materials. The pore mechanism of foam-based geopolymer is found to be different from that of lightweight aggregate-based geopolymer. This variation results in different physico-mechanical and durability properties such as better insulation properties (and lower thermal conductivity) for foam-based versus better mechanical properties for lightweight aggregate-based geopolymer.

29 citations

Journal ArticleDOI
TL;DR: In this paper , nano-modified alkali-activated composites (AACs) or geopolymers have attracted attention owing to their excellent performance and modification mechanisms, and the results mostly indicated that while increasing nano-additives proportion to a certain extent improves the mechanical characteristics, including compressive, flexural, tensile, and impact strengths, incorporation beyond that amount deteriorates them.

23 citations

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TL;DR: The activation of fly ash/slag pastes with NaOH solutions has been studied in this paper, where the authors established the equations of the models describing the mechanical behaviour of these pastes as a function of the factors and levels considered.

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Frequently Asked Questions (1)
Q1. What are the contributions in "Effects of an eco-silica source based activator on functional alkali activated lightweight composites" ?

In this paper, alkali activated slag-fly ash lightweight composites with moderate densities between around 1200 and 1500 kg/m are prepared and characterized. The calculation on the carbon footprint shows an evident advantage of using alkali activated materials to replace Portland cement, also the utilization of olivine nano-silica further reduces the carbon emission of the activator by around 25 %.