Properties Of Alkali-activated Slag Pastes Research Articles

Effect of Rice Husk Ash on the Properties of Alkali-Activated Slag Pastes: Shrinkage, Hydration and Mechanical Property.

In this paper, rice husk ash (RHA) with different average pore diameters and specific surface areas was used to replace 10% slag in the preparation of alkali-activated slag (AAS) pastes. The effect of RHA addition on the shrinkage, hydration, and strength of AAS pastes was studied. The results show that RHA with a porous structure will pre-absorb part of the mixing water during paste preparation, resulting in a decrease in the fluidity of AAS pastes by 5-20 mm. RHA has a significant inhibitory effect on the shrinkage of AAS pastes. The autogenous shrinkage of AAS pastes decreases by 18-55% at 7 days, and the drying shrinkage decreases by 7-18% at 28 days. This shrinkage reduction effect weakens with the decrease in RHA particle size. RHA has no obvious effect on the type of hydration products of AAS pastes, whereas RHA after proper grinding treatment can significantly improve the hydration degree. Therefore, more hydration products are generated and fills the internal pores of the pastes, which significantly improves the mechanical properties of the AAS pastes. The 28 day compressive strength of sample R10M30 (the content of RHA is 10%, RHA milling time is 30 min) is 13 MPa higher than that of blank sample.

The Effect of Hydrogels with Different Chemical Compositions on the Behavior of Alkali-Activated Slag Pastes.

The effect of in-house synthesized hydrogels with different chemical compositions on the properties of alkali-activated slag pastes was examined. It was found that the teabag test and modified teabag test as a direct method and the flow test as an indirect method showed a similar trend in hydrogel absorption; however, the absorption values differ noticeably between the direct and indirect methods. The alkali-activated slag pastes with hydrogels demonstrated a significant reduction in autogenous shrinkage compared to the pastes without hydrogels. The creation of macrovoids by the hydrogels and change in pore structure resulted in a decrease in compressive strength and electrical resistivity of the pastes with hydrogels. The absorption and desorption of hydrogels in the pastes were tracked using X-ray microcomputed tomography (micro-CT), and it was shown that the onset of hydrogel desorption approximately coincided with the final setting time of the pastes.

Effect of ceramic polishing waste on the properties of alkali-activated slag pastes: Shrinkage, hydration and mechanical property

In order to reduce carbon emissions and realize the resource utilization of solid wastes, the ceramic polishing waste (CPW) was applied to replace part of slag to prepare alkali-activated slag/CPW (AASC) composite cementitious material. The influence of CPW contents on the fluidity, setting time, autogenous shrinkage, chemical shrinkage and compressive strength of AASC pastes was investigated. Moreover, the hydration products and microstructure of AASC pastes were also studied by means of XRD, TGA, and BET-BJH method. Experimental results showed that the fluidity and setting time of AASC pastes were continuously increased when CPW contents increased from 10% to 50%, but the autogenous shrinkage decreased from 6.2% to 51.8%. This is due to the dilution effect of CPW, which decreased the early-age chemical shrinkage. In addition, the compressive strength of AASC pastes containing 30% CPW (P30) at 7 d was basically equal to that of the reference sample, being attributed to more amount of C-A-S-H gels. Although the late-age strength of P30 was decreased due to the increase of pore volume, it could still reach over 90% of reference sample. This indicated that maximum proportion of CPW used to replace slag was 30%, which can not only reduce the risk of shrinkage cracking of AASC pastes, but also have less effect on the strength.

Influence of the Fly Ash Content on the Fresh and Hardened Properties of Alkali-Activated Slag Pastes with Admixtures.

A study on the influence of the inclusion of slag or fly ash and five types of superplasticizers on the fresh and hardened properties of alkali-activated cements is presented. Three alkali-activated slag formulations with different fly ash content (0, 15, and 30%) in the presence of five admixtures (vinyl copolymer, melamine, and three polycarboxylates with different chain lengths) were assessed for fluidity control and setting adjustment without loss of mechanical properties. Solid sodium metasilicate was used as an alkaline activator. Their fresh and hardened properties were studied through slump, setting time, isothermal calorimetry, mechanical strengths, and porosity tests. The results showed that the increase of fly ash content delayed the reaction and improved workability but reduced compressive strengths. Concerning the admixtures, these maintained fluidity especially for the one based on polycarboxylate with very long chains. The melamine and polycarboxylate with very long chain admixtures did not have a drastic impact on mechanical properties at early ages; even a gain of flexural and compressive strength was noted.

The Effect of Biochar on the Properties of Alkali-Activated Slag Pastes

This paper examines the influence of biochar on the properties of alkali-activated slag pastes using two activator solutions, namely NaOH and Na2CO3. The biochar demonstrated different absorption kinetics in the mixture of slag and the two activator solutions. The pastes with biochar showed a delay in the heat flow peak, compared to the pastes without biochar, but the cumulative heat release in these pastes at later hours was increased, compared to the pastes without biochar. It was found that the use of biochar reduced autogenous shrinkage in the pastes and the reduction in autogenous shrinkage was more pronounced in the alkali-activated slag with NaOH, compared to Na2CO3. The void structure of the pastes was investigated using x-ray micro-computed tomography. It was found that refined pore structure due to reduced effective solution/slag in the pastes with biochar was able to compensate for the decreasing effect of biochar voids on compressive strength. The electrical resistivity was shown to be lower in the pastes with biochar.

An initial study about the effect of activated carbon nano-sheets from residual biomass of olive trees pellets on the properties of alkali-activated slag pastes

This study is the first attempt to investigate the possibility of incorporating nano-activated carbon (nAC), prepared from residual biomass of olive trees, on the properties of alkali-activated slag (AAS) pastes. After suitable preparation of nAC, it was added into AAS pastes at various levels starting from 0.5 to 3.5, by weight. The results of workability, compressive strength, total porosity and drying shrinkage were measured. Different techniques such as X-ray diffraction (XRD), thermogravimetric analysis (TGA) and its derivative (DTG), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM) were used to identify the hydration products and the microstructural development of the AAS pastes with/without nAC. The results showed a negative effect of nAC on the workability of the mixtures. The incorporation of 1.5% nAC showed the highest compressive strength, the lowest total porosity and drying shrinkage. The incorporation of 3.5% nAC led to a reduction in the compressive strength and an increase in the total porosity and drying shrinkage. It can be concluded that the incorporation of suitable ratios of nAC can be beneficially used to resolve one of the main problems of AAS (i.e. drying shrinkage). Thus, the addition of nAC could enhance the probability of utilizing AAS as a good alternative to Portland cement.

Understanding the rheological properties of alkali-activated slag pastes from the cohesion and friction interactions

The workability and pumping performance of alkali-activated slag (AAS) concrete are affected by its rheological properties, which are related to the interactions between slag particles in the pastes. The rheological properties of AAS paste with different equivalent sodium oxide (Na2Oeq) dosage activated by sodium hydroxide (NaOH) and sodium silicate (waterglass) with different SiO2/Na2O molar ratios were investigated in this study. The rheological mechanism was studied from the cohesion and friction interactions by calculations from parameters such as the Zeta potential of particles in the pastes, surface energy, hydrogen bond, and friction coefficient of the alkaline activators. The results indicated that the yield stress of AAS pastes was lower than that of the paste made with slag and water, whereas the plastic viscosity was higher. The yield stress of AAS pastes was mainly determined by the friction interactions between particles. The plastic viscosity was mainly affected by attractive interactions, including van de Waals interactions and Lewis acid-base interactions. The electrostatic repulsion interactions were felt at a very short distance because the AAS pastes were systems with the high ionic strength, which means that the electric double layer were severely compressed, therefore, the attractive interactions were predominant. Taken together, these results suggest that the improvement of plastic viscosity of AAS pastes should be achieved by changing the attractive interactions.

A novel titania/graphene composite applied in reinforcing microstructural and mechanical properties of alkali-activated slag

Alkali-activated slag (AAS) binder with low carbon footprint is a promising alternative to high energy-embodied ordinary Portland cement binders. However, wider applications are hindered by their ceramic-type fractural behaviours. This study involves improving the microstructure and mechanical properties of alkali-activated slag pastes by using an in-situ fabricated titania/graphene composite assisted with a novel titania intercalating method. The result indicates that titania located in between the layers of graphene could make graphene homogeneously dispersed in an aqueous solution, which is conducive to obtaining well-dispersed graphene nanoparticles in AAS pastes. Besides, the highly dispersed titania/graphene composites lead to a less disordered microstructure of AAS paste compared to the normal counterpart. Moreover, both the compressive strength, flexural strength, Young's modulus were improved as the content of titania/graphene composites (0%-0.03 wt%) increased. In addition, the fractural behaviours were also improved remarkably evidenced by a 89.7% increase of flexural toughness for AAS pastes with 0.03% titania/graphene composite. Scanning electron microscopy observations confirmed that the microstructure of the AAS pastes with titania/graphene composites was denser compared to normal AAS which is probably due to the effects of crack bridging and branching. Overall, the developed new intercalating method could be used to improve the dispersibility of graphene which is then employed effectively for reinforcing microstructure and mechanical properties of AAS binders.

Effect of high temperature on physical, mechanical and microstructure properties of alkali-activated slag pastes blended with ceramic waste material

This paper presents the influence of ceramic waste powder (CWP) inclusion on the physico-mechanical properties and microstructure analysis of alkali-activated pastes prior and after exposure to elevated temperatures. Such geopolymer specimens were designed upon replacing ground granulated blast-furnace slag (GGBFS) with CWP at ratios of 0%, 5%, 10%, 15% and 20%, by mass. The water absorption and compressive strength (wet and dry) of non-exposed samples were determined at curing ages of 1, 7, 14 and 28 days. After that, the specimens were exposed to high temperatures of 200, 400, 600 and 1000 °C with a heating rate of 5 °C/min. The characteristics of tested samples such as visual inspection, residual compressive strength and microstructure were evaluated. Techniques as Fourier Transform Infrared spectroscopy (FTIR), Thermogravimetric Analysis (TGA/DTG) and Scanning Electron Microscopy (SEM) were used for studying molecular and microstructure of geopolymer composite. Results indicated that the inclusion of CWP with slag in composite structure based geopolymer enhanced the physico-mechanical properties and microstructure of mixtures before and after thermal exposure. The specimens made of 15% CWP displayed superior thermal stability in terms of remaining compressive strength and microstructure than the reference ones.

Effect of reactive MgO on hydration and properties of alkali-activated slag pastes with different activators

Activator type significantly affects the properties of alkali-activated slag (AAS) cement. Reactive MgO may have different effects on AAS cement activated with different activators. Four activators, sodium hydroxide (NaOH), water glass (WG), sodium carbonate (Na2CO3) and sodium sulfate (Na2SO4) were used in the present paper. The effect of reactive MgO on the fluidity, setting time of AAS paste and the compressive strength of AAS mortar was studied. The hydration process was assessed with pH and hydration heat. Microstructure and hydrates were also characterized by XRD, SEM and EDS. The results show that the addition of reactive MgO decreases the fluidity and setting time of AAS paste regardless of activator type. In NaOH activated slag, the compressive strength decreases with the increase of reactive MgO. While in the other three activators, there exists an optimal reactive MgO content of about 4%–6%. Now the compressive strength is higher. Reactive MgO increases the pH, heat evolution rate and cumulative hydration heat of AAS cement. Hydrotalcite and C-(A)-S-H gel are the main hydration products. More hydration products are promoted by reactive MgO, which results in the densification of AAS matrix. The above changes due to reactive MgO are especially remarkable for Na2CO3 and Na2SO4 activated AAS cement. AFt is more likely to be formed than hydrotalcite in Na2SO4 activated slag and reactive MgO enhances this trend. Reactive MgO is especially suitable for Na2CO3 and Na2SO4 activated AAS cement. It can accelerate the hydration, adjust the fluidity and setting time, and improve the mechanical properties to a reasonable degree.

Resistance of alkali-activated slag to sulphate attack and elevated temperatures

Thermal stability as well as sulphate resistance of alkali activated slag (AAS) pastes prepared from Ground Granulated Blast Furnace Slag (GGBFS) and 5% fly ash (FA)are examined. AAS pastes are accelerated using alkaline solution of sodium hydroxide with concentration 1, 1.5 and 2 M and sodium silicate with fixed concentration (0.28 mol/Kg).AAS pastes exposed to 5% MgSO4 solution and to high temperature (200, 400, 600 and 800 oC). The compressive strength, bulk density, total porosity, chemically combined water content and phase composition are determined in this study. Concentration of 2M NaOH with 5% FA promotes the best properties of AAS pastes that exhibit the highest resistance to firing up to 400oC by achieving the greatest compressive strength. However, AAS pastes activated by 2M NaOHgive a high sulphate resistance.

Thermal and mechanical properties of alkali-activated slag paste, mortar and concrete utilising seawater and sea sand

This paper presents an experimental study on the thermal properties of alkali-activated slag paste, mortar and concrete utilising seawater and sea sand exposed to elevated temperature. The thermal properties of paste and concrete utilising cement, fresh water and river sand were also investigated for comparison purpose. The samples were heated to different target temperatures up to 1000 °C at a heating rate of 5 °C/min, and tested both under hot and cooled conditions. The thermal properties, including temperature gradient, visual observation, mass loss, thermal strain and mechanical properties (i.e. strength, Young’s modulus and stress-strain curve) were investigated. X-ray CT scanning and scanning electron microscopy (SEM) were conducted to understand the macro/microscopic changes of the paste and concrete in response to heating. Degradation in mechanical properties of slag paste is attributed mainly to the cracks induced by temperature gradient, pore pressure and phase change. The degradation of concrete is caused by thermal mismatch between paste matrix and aggregates regardless of the use of cement or slag, freshwater or seawater, and river sand or sea sand.

The engineering properties of alkali-activated slag pastes exposed to high temperatures

Alkali cementations material is a recent and new type of environmentally friendly and economical material that is produced from industrial waste (e.g., fly ash and slag). This material possesses many excellent engineering properties, including a high compressive strength, light weight and low thermal conductivity.This study used different liquid–solid ratios, alkaline solutions and slag to produce pastes. The findings showed that the slump and slump flow of the alkali-activated slag pastes increased with the alkaline solutions and liquid–solid ratio. Without any slow setting additive, the setting rate could be reduced by reducing the alkaline solutions. After different curing ages at high temperatures, residual compressive strength, ultrasonic pulse velocity and thermal imaging tests. The findings showed that the ultrasonic pulse velocity and compressive strength of the alkali-activated slag pastes increased with the increase in the alkali-activated solution. The engineering properties declined as the liquid–solid ratio increased, but the workability improved. The increase in the alkaline solutions could enhance the compressive strength at high temperatures; the compressive strength of 0.5% alkali-activated at 500°C was 30–50% of that of 1% alkali-activated. The ultrasonic pulse velocity at the high temperature of 500°C was 2171–3322m/s; the ultrasonic pulse velocities of 0.75% and 1% alkali-activated at 800°C were 2605m/s and 2812m/s, respectively, but the ultrasonic pulse velocity of 0.5% alkali-activated 0.5% was only 2211m/s. The alkaline solutions of 0.75% alkali-activated and 1% alkali-activated were sufficient to finish the polyreaction within a short period of time to afford a material that possessed low porosity and high strength material. The results show that research on alkali-activated slag significantly contributes to energy saving and carbon reduction.

Immobilization of Cr 6+, Cd 2+, Zn 2+ and Pb 2+ in alkali-activated slag binders

Granulated blast furnace slag is the main component of alkali-activated slag cementitious materials (AASCs). Calcium silicate hydrates with a low Ca/Si ratio, hydrotalcite-type phase, some amounts of hydrogarnets and sodium zeolites form as main AASC hydration products. The microstructure of alkali-activated slag pastes shows a higher amount of gel pore content compared to OPC pastes and, simultaneously, significantly lower amount of capillary pores. The microstructure and phase composition of hydrated slag indicate that they can play an essential role in the immobilization of heavy metals. The properties of alkali-activated slag pastes in the presence of Zn, Cd, Cr and Pb ions were studied. The leaching TANK test was used to evaluate the level of immobilization of particular elements in mortars made containing these elements. It was found that the degree of Cd, Zn and Pb ion immobilization was very high (exceeding 99.9%). The values for Cr 6+ were lower (ca. 99.0%). The strength development as well as microstructure observations are presented in the paper.