Effects of an eco-silica source based activator on functional alkali activated lightweight composites
Abstract: In this paper, alkali activated slag-fly ash lightweight composites with moderate densities between around 1200 and 1500 kg/m3 are prepared and characterized. An eco-olivine nano-silica is applied to prepare sustainable silicate based activators to replace commercial sodium silicates. Na2O contents of 2.0, 3.5 and 5.0 wt% are investigated in order to reach a suitable balance between performance, costs and application. The results show the positive effect of density and Na2O content on strength, while strength increment between Na2O dosage of 3.5% and 5.0% is limited. The reduction of Na2O content shows a dramatic delay of the reaction process up to around 3 d, but shows negligible effect on the typical Si-O bonds. An increased Na2O content benefits the formation of reaction products, including the contents of hydrotalcite and carbonates. Besides, the thermal conductivity and acoustical absorption properties of the lightweight products are characterized; the phase transition zones between lightweight aggregate and binder matrix are evaluated by SEM. 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%.
Summary (4 min read)
- 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 .
- 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 .
- 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 .
- 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 .
- 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 , 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%.
- 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.
Did you find this useful? Give us your feedback
Related Papers (5)
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 %.