Residual strength properties of sodium silicate alkali activated slag paste exposed to elevated temperatures

The residual compressive strength behavior of alkali activated slag paste (AASP) after temperature exposures up to 1,200°C was investigated. Strength loss of approximately 60% occurred between 100 and 200°C and a further strength loss in the order of 30% at 800°C. Total loss of strength occurred at 1,200°C. Thermogravimetric studies (TGA/DTG) verified AASP contained no Ca(OH)₂ which governs the chemical mechanism of strength loss for ordinary Portland cement (OPC) and blended slag cement pastes. However, the TGA results showed that AASP had a higher water loss than the other binders between 100 and 200°C and higher thermal shrinkage as indicated by the dilatometry studies. The high thermal shrinkage led to a differential thermal shrinkage gradient within the AASP and induced micro stresses and cracking which was more prominent for larger samples. Differential thermal shrinkage caused by the higher thermal shrinkage of the AAS material was concluded as the mechanism which gives lower residual strength in AASP compared to OPCP.     

Microstructural changes in alkali activated fly ash/slag geopolymers with sulfate exposure

Sulfate attack is recognized as a significant threat to many concrete structures, and often takes place in soil or marine environments. However, the understanding of the behavior of alkali-activated and geopolymer materials in sulfate-rich environments is limited. Therefore, the aim of this study is to investigate the performance of alkali silicate-activated fly ash/slag geopolymer binders subjected to different forms of sulfate exposure, specifically, immersion in 5 wt% magnesium sulfate or 5 wt% sodium sulfate solutions, for 3 months. Extensive physical deterioration of the pastes is observed during immersion in MgSO₄ solution, but not in Na₂SO₄ solution. Calcium sulfate dihydrate (gypsum) forms in pastes immersed in MgSO₄, and its expansive effects are identified as being particularly damaging to the material, but it is not observed in Na₂SO₄ environments. A lower water/binder (w/b) ratio leads to a greatly enhanced resistance to degradation by sulfate attack. Infrared spectroscopy shows some significant changes in the silicate gel bonding environment of geopolymers immersed in MgSO₄, attributed mostly to decalcification processes, but less changes upon exposure to sodium sulfate. It appears that the process of ‘sulfate attack’ on geopolymer binders is strongly dependent on the cation accompanying the sulfate, and it is suggested that a distinction should be drawn between ‘magnesium sulfate attack’ (where both Mg²⁺ and SO₄ ^2? are capable of inducing damage in the structure), and general processes related to the presence of sulfate accompanied by other, non-damaging cations. The alkali-activated fly ash/slag binders tested here are susceptible to the first of these modes of attack, but not the second.     

Mechanical properties of sodium and potassium activated metakaolin-based geopolymers

In this study, a set of mechanical properties of geopolymers, synthesized by alkali (NaOH or KOH) activation of metakaolin and SiO₂ mixture, were characterized at ambient temperature. Samples with K/Al or Na/Al atomic ratios equal to 1, Si/Al atomic ratios in the 1.25–2.5 range and H₂O/Al₂O₃ molar ratios of 11 or 13 are cured at 80 °C for 24 and 48 h before characterization, to determine effect of Si/Al ratio and curing time on the structure and mechanical properties of geopolymers. The structure of synthesized geopolymers characterized using XRD, NMR, SEM, and density measurements was correlated to their mechanical properties, including compressive strength, Young’s modulus, hardness, and fracture toughness. The results of this study suggest a strong effect of Si/Al ratios (in the 1.5–2 range), density, and microstructure on the maximum strength, Young’s modulus, and hardness of geopolymers. There were also notable differences in strength between samples cured for 24 and 48 h, suggesting that the degree of geopolymerization reaction also plays important role in mechanical properties of this new class of inorganic polymers.     

A study of rheological and mechanical properties of mixed alkali activated slag pastes

Slag, a by-product of the iron/metalliferrous industry, is known for its latent hydraulicity. This research is directed to a study of alkali activation of slag, using a combination of sodium silicate or water glass (Na₂SiO₃) and slaked lime. The mechanical and rheological properties of these activated slag pastes are reported. The mechanism of mixed activation is described, and the factors responsible for enhanced reactivity, high strength, and unusual rheology are discussed. Advanced Cement Based Materials 1994, 1, 178–184     

Hydration of alkali-activated ground granulated blast furnace slag

The hydration of ground granulated blast furnace slag (GGBFS) at 25 °C in controlled pH environments was investigated during 28 days of hydration. GGBFS was activated by NaOH, and it was found that the rate of reaction depends on the pH of the starting solution. The main product was identified as C-S-H, and, in the pastes with high pH, hydrotalcite was observed at later stages of hydration. The pH of the mixing solution should be higher than pH 11.5 to effectively activate the hydration of GGBFS. As deduced from very low electrical conductivity measurements, GGBFS pastes had very tortuous and disconnected pores. The effect of the pH of the aqueous solution on the composition, microstructure and properties of alkali-activated GGBFS pastes are also discussed.     

Alkali activated slag pastes with surface-modified blast furnace slag

A surface modification method for blast furnace slag particles is newly proposed to retard the setting time and to mitigate the flow loss in alkali activated slag pastes. BFS particles were treated by a NaOH solution and then were carbonated to modify the surface of the particles. This leads to suppression of the dissolution of the reactive components at the initial stage of the reaction. The effect of the carbonation period and the modifying solution on the physicochemical characteristics of surface-modified BFS particles was investigated. The reaction and mechanical characteristics of AAS pastes produced from surface-modified BFS were also investigated. The test results show that the developed AAS pastes exhibited the delayed setting behavior and the retarded flow loss, and had a compressive strength comparable to those of AAS pastes produced from unmodified BFS.     

Effects of gypsum and phosphoric acid on the properties of sodium silicate-based alkali-activated slag pastes

In this paper, the effects of gypsum and phosphoric acid on the properties of sodium silicate-based alkali-activated slag paste were examined. Alkali-activated slag is recognized as a good performing binder. However, it suffers from fast setting and drying shrinkage. Phosphoric acid has been used as a retarder and gypsum has been used as a drying-shrinkage inhibitor in previous studies. However, it is not known what will happen if one adds both of them to alkali-activated slag. In this study, the effects of gypsum and/or phosphoric acid on the compressive strength development, setting time and drying shrinkage of alkali-activated slag were examined. Experimental results indicated that adding gypsum shortened the setting time, and at the same age, the compressive strength reached a higher value when the amount of used gypsum is higher. Drying-shrinkage decreased when the amount of used gypsum increased; however, when 0.82 M phosphoric acid was added in the activator with gypsum the results were somewhat different. With added phosphoric acid, adding more gypsum shortened the setting time as well. The compressive strength of specimens with added gypsum was lower than that of control specimens under 28 days. Drying-shrinkage increased with phosphoric acid as the amount of used gypsum increased.     

Effect of sodium dodecyl sulfate on flow and electrokinetic properties of Na-activated bentonite dispersions

The present study reports the effect of anionic surfactant sodium dodecyl sulfate (SDS, C₁₂H₂₅ OSO₃Na) upon the electrokinetic (electrophoretic mobility, zeta potential) and rheological (viscosity, yield value) properties of the Ca-bentonitic clay found in Turkey and its Na- activated form. The SDS dispersant was added in different concentrations in the range of 1 × 10⁻⁵−5 × 10⁻² mol/l. The results show that the viscosity and zeta potential values of bentonite dispersion are affected by the addition of anionic surfactant. The obtained data are analysed by considering the kind of exchangeable cations. Thixotropic property effect was observed in bentonite dispersions.     

Mechanical properties,drying and autogenous shrinkage of blast furnace slag activated with hydrated lime and gypsum

This article reports the characteristics of blast furnace slag (BFS) pastes activated with hydrated lime (5%) and hydrated lime (2%) plus gypsum (6%) in relation to compressive strength, shrinkage (autogenous and drying) and microstructure (porosity, hydrated products). The paste mixtures were characterized using powder X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and thermogravimetric analysis (TG/DTG). BSF activated with lime and gypsum (LG) results in larger amounts of ettringite when compared with BFS activated with lime (L). Although the porosities of the L and LG mixtures were about the same, there was a greater pore refinement for the BFS activated with lime, with an increase in mesopores volume with age. The presence of ettringite and the higher volumes of macropores cause the compressive strength of BSF activated with hydrated lime plus gypsum to be smaller than that of BFS activated with lime. For both chemical activators, compressive strength developed slowly at early ages. Autogenous and drying shrinkage were greater for the BFS activated with lime, believed to result from the more refined porous structure in comparison with the mixture activated with gypsum plus lime.     

基于长江下游超细疏浚砂的碱激发矿渣混凝土力学性能

分析了长江下游航道超细疏浚砂的理化性质,并以超细疏浚砂为原料,设计了5种不同疏浚砂掺量的碱激发矿渣混凝土(AASC)配合比,研究了其流动性、抗压强度、劈拉强度和吸水率的变化.通过SEM、XRD和压汞(MIP)技术,分析了AASC的物相组成和微观结构.研究表明:长江下游航道的超细疏浚砂颗粒细度模数在0.1～0.5之间.随...     

Microcracking and strength development of alkali activated slag concrete

Alkali activated slag concrete (AASC) is made by activating ground granulated blast furnace slag with alkalis without the use of any Portland cement. This study investigates the level of microcracking which occurs in AASC when subjected to various types of curing regimes. The corresponding compressive strength developments of AASC were monitored. The level of microcracking were measured using three different types of tests: (1) frequency and size of surface cracks using crack-detection microscope (2) water sorptivity tests measuring absorption of water by capillary attraction and (3) mercury intrusion porosimetry (MIP) tests which measured the pore size distribution of AASC and AAS pastes (AASPs). The results show that the lack of moist curing of AASC increased the level of microcracking measured using all three different tests mentioned above. The strength development of AASC is also significantly reduced by lack of moist curing.     

Development of alkali activated cement from mechanically activated silico-manganese (SiMn) slag

Silico-manganese (SiMn) slag has been used to develop alkali activated cement binder. The reactivity of SiMn slag was altered by mechanical activation using eccentric vibratory and attrition mill. The reaction kinetics during alkali activation of SiMn slag and structural reorganization were studied using isothermal conduction calorimetry and Fourier transform infrared spectroscopy. The particle size after milling was smaller in attrition milled samples but reaction started earlier in vibratory milled samples due to more reactivity. This observation was further supported by compressive strength which was highest in samples prepared from vibratory milled slag. The main reaction product was C–S–H (C = CaO, S = SiO₂, H = H₂O) of low crystallinity of different types with varying Si/Al and Ca/Si ratio. An attempt has been made to relate the microstructure with mechanical properties. The results obtained in this study establish technical suitability of SiMn slag as raw material for alkali activated cement.     

Time-dependent characterization of Na2CO3 activated slag

This study aims to investigate the time-dependent characteristics of sodium carbonate activated slag at the early stage and its compressive strength development. The reaction kinetics of Na₂CO₃ activated slag is analyzed at the curing ages of 1h, 6h, 1d, 2d, 3d, 4d and 7d, respectively, and the reaction products are characterized employing FTIR, XRD, TG-DTG and SEM. The results show that a weak reflection of gaylussite is observed after 1 d of curing, while intensive accumulation of gaylussite and formation of hydrotalcite are observed at 2 d/3 d. Meanwhile, the gelation of C-(A)SH after 3 d of curing is identified by FTIR. The role of pH, CO₃²⁻ anion concentration and the formation of crystals such as gaylussite and hydrotalcite on the reaction process are discussed. In addition, a relationship between the initial alkali concentration and compressive strength at different curing ages of 7 d, 28 d and 180 d is derived.     

Hazardous ions uptake behavior of thermally activated steel-making slag

This study concerns the utilization of waste steel-making slag, a by-product that contains mainly CaO, Fe(2)O(3) and SiO(2). The as-received slag was ground and thermally activated by temperature treatment from 110 to 1000 degrees C for 24 h. Although the as-received slag was amorphous, it became partially crystallized during grinding. These crystalline phases were larnite and iron oxide but other crystalline phases also appeared in addition to larnite after calcination. The uptake of Ni(2+), PO(4)(3-) and NH(4)(+) by the samples was investigated from solutions with initial concentrations of 10 mmol/l. The sample calcined at 800 degrees C showed the highest Ni(2+) uptake (4.85 mmol/g) whereas the highest simultaneous uptake of PO(4)(3-) (2.75 mmol/g) and NH(4)(+) (0.25 mmol/g) was achieved by calcining the material at 700 degrees C. The principal mechanism of Ni(2+) uptake is thought to involve replacement of Ca(2+) by Ni(2+). The mechanism of PO(4)(3-) uptake is mainly by formation of calcium phosphate while that of NH(4)(+) involves sorption by the porous silica surface of the samples.     

Hydration and properties of nano-TiO2 blended cement composites

Two types of nano-TiO₂ particles were blended into cement pastes and mortars. Their effects on the hydration and properties of the hydrated cement pastes were investigated. The addition of nano-TiO₂ powders significantly accelerated the hydration rate and promoted the hydration degree of the cementitious materials at early ages. It was demonstrated that TiO₂ was inert and stable during the cement hydration process. The total porosity of the cement pastes decreased and the pore size distribution were also altered. The acceleration of hydration rate and the change of microstructure also affected the physical and mechanical properties of the cement-based materials. The initial and final setting time was shortened and more water was required to maintain a standard consistence due to the addition of the nano-TiO₂. The compressive strength of the mortar was enhanced, practically at early ages. It is concluded that the nano-TiO₂ acted as a catalyst in the cement hydration reactions.     

Durability assessment of alkali activated slag (AAS) concrete

The environmental impact from the production of cement has prompted research into the development of concretes using 100% replacement materials activated by alkali solutions. This paper reports research into the durability of AAS concrete. The durability properties of AAS have been studied for a range of sodium oxide dosages and activator modulus. Properties investigated have included measurements of workability, compressive strength, water sorptivity, depth of carbonation and rapid chloride permeability. Microstructure studies have been conducted using scanning electron microscopy and energy dispersive X-ray spectroscopy. It was concluded that an activator modulus of between 1.0 and 1.25 was identified as providing the optimum performance for a sodium oxide dosage of 5% and that AAS concretes can exhibit comparable strength to concrete currently produced using Portland cement (PC) and blended cements. However, with regards to the durability properties such as water sorptivity, chloride and carbonation resistance; the AAS concretes exhibited lower durability properties than PC and blended concretes. This, in part, can be attributed to surface microcracking in the AAS concretes.     

Hydration of quaternary Portland cement blends containing blast-furnace slag,siliceous fly ash and limestone powder

In this study the hydration of quaternary Portland cements containing blast-furnace slag, type V fly ash and limestone and the relationship between the types and contents of supplementary cementitious materials and the hydrate assemblage were investigated at ages of up to 182 days using X-ray diffraction and thermogravimetric analysis. In addition thermodynamic modeling was used to calculate the total volume of hydrates. Two blast-furnace slag contents of 20 and 30 wt.% were studied in blends containing fly ash and/or limestone at a cement replacement of 50 wt.%. In all cases the experiments showed the presence of C–S–H, portlandite and ettringite. In samples without limestone, monosulfate was formed; in the presence of limestone monocarbonate was present instead. The addition of 5 wt.% of limestone resulted in a higher compressive strength after 28 days than observed for cements with lower or higher limestone content. Overall the presence of fly ash exerts little influence on the hydrate assemblage. The strength development reveals that amounts of up to 30 wt.% fly ash can be used in quaternary cements without significant loss in compressive strength.     

Chloride binding and mobility in sodium carbonate-activated slag pastes and mortars

This study evaluates the chloride binding capacity and the migration of chloride in sodium carbonate-activated slag cements and mortars. The effect on chloride mobility and binding of adding a calcined layered double hydroxide (CLDH) to the binder mix was also assessed. Significantly improved durability characteristics can be achieved for sodium carbonate-activated slag mortars by the addition of small fractions of CLDH, as a consequence of a higher degree of reaction, higher chloride binding capacity, and the refined pore structures present in these modified materials, in comparison with alkali-activated cements produced without CLDH. The addition of CLDH enables the production of sodium carbonate-activated slag cements with notably reduced chloride ingress compared to silicate activated slag cements.     

Hydrothermal synthesis of sodium tungstate nanorods and nanobundles in the presence of sodium sulfate

Sodium tungstate nanorods and nanobundles have been successfully prepared, for the first time, through a simple salt-assisted hydrothermal route based on the reaction between Na₂WO₄ and HCl in aqueous solution. The resultant sodium tungstate nanorods and nanobundles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electronic diffraction (SAED) techniques. The ingredients of the sample have been detected by energy-dispersive spectrum (EDS) method. It is found that hydrothermal temperature and time play important roles in the control of the morphology and size of the products.