Adsorption of superplasticizer admixtures on alkali-activated slag pastes

Alkali-activated slag (AAS) binders are obtained by a manufacturing process less energy-intensive than ordinary Portland cement (OPC) and involves lower greenhouse gasses emission. These alkaline cements allow the production of high mechanical strength and durable concretes. In the present work, the adsorption of different superplasticizer admixtures (naphthalene-based, melamine-based and a vinyl copolymer) on the slag particles in AAS pastes using alkaline solutions with different pH values have been studied in detail. The effect of the superplasticizers on the yield stress and plastic viscosity of the AAS and OPC pastes have been also evaluated.The results obtained allowed us to conclude that the adsorption of the superplasticizers on AAS pastes is independent of the pH of the alkaline solutions used and lower than on OPC pastes. However, the effect of the admixtures on the rheological parameters depends directly on the type and dosage of the superplasticizer as well as of the binder used and, in the case of the AAS, on the pH of the alkaline activator solution. In 11.7-pH NaOH-AAS pastes the dosages of the superplasticizers required to attain similar reduction in the yield stress are ten-fold lower than for Portland cement. In this case the superplasticizers studied show a fluidizing effect considerably higher in 11.7-pH NaOH-AAS pastes than in OPC pastes. In 13.6-pH NaOH-AAS pastes, the only admixture observed to affect the rheological parameters is the naphthalene-based admixture due to its higher chemical stability in such extremely alkaline media.     

Effect of superplasticizer and shrinkage-reducing admixtures on alkali-activated slag pastes and mortars

This paper shows how several superplasticizers (polycarboxylates, vinyl copolymers, melamine and naphthalene-based) and shrinkage-reducing (polypropylenglycol derivatives) admixtures affect the mechanical and rheological properties and setting times of alkali-activated slag pastes and mortars. Two activator solutions, waterglass and NaOH, were used, along with two concentrations—4% and 5% of Na₂O by mass of slag. All admixtures, with the exception of the naphthalene-based product, lost their fluidifying properties in mortars activated with NaOH as a result of the changes in their chemical structures in high alkaline media. The difference in the behaviour of these admixtures when ordinary Portland cement is used as a binder is also discussed in this paper.     

Effect of shrinkage-reducing admixtures on the properties of alkali-activated slag mortars and pastes

The effect of a shrinkage-reducing admixture (SRA) based on polypropylenglycol on the dimensional stability of waterglass-activated slag mortars was studied. The analysis also showed the effect of the admixture on pore structure of the mortars as well as on the mineralogical composition and microstructure of the alkali-activated slag pastes.The SRA reduced the shrinkage by up to 85 and 50% when the alkali-activated slag mortar specimens were cured at relative humidities of 99 and 50%, respectively. The mechanism primarily involved in shrinkage reduction is the decrease in the surface tension of pore water prompted by the admixture. The SRA also modified the pore structure - under both curing conditions - increasing the percentage of pores with diameters ranging from 1.0 to 0.1 μm. Capillary stress is much lower in these pores than in the smaller capillaries prevailing in mortars prepared without admixtures.Microstructurally, the SRA occasioned a slight increase in the proportion of Si units Q² in the CSH gel and a decrease in the percentage of Al replacing the Si in the gel structure. The admixture did not, however, modify the mineralogical composition of the pastes.Finally, the SRA admixture retarded the alkaline activation of the slag, more intensely at higher admixture dosages. While the admixture did not significantly alter the degree of reaction in pastes cured for 7 days at RH = 99%, the value of this parameter dropped by 7% in the presence of the admixture in pastes cured at 50% relative humidity.     

Impact of chemical variability of ground granulated blast-furnace slag on the phase formation in alkali-activated slag pastes

The influence of ground granulated blast-furnace slag (GGBS) chemical variability on phase formation in sodium hydroxide-activated GGBS pastes has been investigated using X-ray total scattering and subsequent pair distribution function (PDF) analysis. Crystalline phase identification based on reciprocal space analysis reveals that despite large chemical variations in the neat GGBSs the secondary reaction products are quite similar, with the majority of pastes containing a hydrotalcite-like phase. However, PDF analysis reveals considerable differences in short range atomic ordering of the main calcium-sodium aluminosilicate hydrate (C-(N)-A-S-H) gel phase in the pastes. Quantitative analysis of these local structural differences in conjunction with published PDF data identifies the important role calcium plays in dictating the atomic structure of disordered silicate-rich phases in cementitious materials. This study serves as a crucial step forward in linking GGBS chemistry with phase formation in alkali-activated GGBS pastes, revealing key information on the local structure of highly-disordered cementitious materials.     

CO2 binding capacity of alkali-activated fly ash and slag pastes

Quantification of the CO₂ binding capacity of reinforced concrete is of high importance for predicting the carbonation potential and service life of concrete structures. Such information is still not available for alkali activated materials that have received extensive attention as a sustainable substitute for ordinary Portland cement (OPC)-based concrete. To address this gap, this paper evaluates the CO₂ binding capacity of ground powders of alkali activated fly ash (FA) and ground granulated blast furnace slag (GBFS) pastes under accelerated carbonation conditions (1% v/v CO₂, 60% RH, 20?°C) for up to 180 days. The CO₂ binding capacity, the gel phase changes, and the carbonate phases are investigated with complementary TG-DTG-MS, FT-IR and QXRD techniques.Five mixtures with different FA/GBFS ratio are considered. CEM I and CEM III/B pastes are also studied to provide a baseline for comparisons. The results showed that the alkali-activated pastes have a lower CO₂ binding capacity in comparison to cement-based pastes. Furthermore, alkali-activated pastes have similar CO₂ binding capacity regardless of the FA/GBFS ratio. It was observed that the silicate functional groups corresponding to the reaction products in the pastes were progressively changing during the first 7 days, after which only carbonate groups changed. It was also found that the CO₂ bound in the alkali-activated pastes occurs to a substantial extent in amorphous form.

Microanalysis of alkali-activated fly ash-CH pastes

Samples of a Class F fly ash and calcium hydroxide (CH) hydrated in pH 13.2 sodium hydroxide solution were analyzed using backscattered electron, scanning Auger, and X-ray microanalysis. The Class F fly ash, composed mainly of aluminosilicate glass and silica, was reacted for 8, 14, and 78 days at various temperatures. These samples represent both long-term and early-age stages of hydration. Results show that a hydrate product with calcium to silica ratio near 1.4 and katoite are formed. X-ray and scanning Auger microanalysis show evidence of the formation of hydrate product on the surface of both fly ash and CH particles at early ages. This finding suggests a new mechanism to explain prior data that shows that the hydration rates increase with increasing CH-ash content in the starting mixture.     

Pore solution in alkali-activated slag cement pastes. Relation to the composition and structure of calcium silicate hydrate

In this work, the relationship between the composition of pore solution in alkali-activated slag cement (AAS) pastes activated with different alkaline activator, and the composition and structure of the main reaction products, has been studied. Pore solution was extracted from hardened AAS pastes. The analysis of the liquids was performed through different techniques: Na, Mg and Al by atomic absorption (AA), Ca ions by ionic chromatography (IC) and Si by colorimetry; pH was also determined. The solid phases were analysed by XRD, FTIR, solid-state ²⁹Si and ²⁷Al NMR and BSE/EDX.The most significant changes in the ionic composition of the pore solution of the AAS pastes activated with waterglass take place between 3 and 24 h of reaction. These changes are due to the decrease of the Na content and mainly to the Si content. Results of ²⁹Si MAS NMR and FTIR confirm that the activation process takes place with more intensity after 3 h (although at this age, Q² units already exist). The pore solution of the AAS pastes activated with NaOH shows a different evolution to this of pastes activated with waterglass. The decrease of Na and Si contents progresses with time.The nature of the alkaline activator influences the structure and composition of the calcium silicate hydrate formed as a consequence of the alkaline activation of the slag. The characteristic of calcium silicate hydrate in AAS pastes activated with waterglass is characterised by a low structural order with a low Ca/Si ratio. Besides, in this paste, Q³ units are detected. The calcium silicate hydrate formed in the pastes activated with NaOH has a higher structural order (higher crystallinity) and contains more Al in its structure and a higher Ca/Si ratio than those obtained with waterglass.     

Si and Al NMR study of alkali-activated slag

This paper presents the results of the investigation of the hydration of alkali-activated slag (AAS) by nuclear magnetic resonance spectroscopy (NMR). The cross-polarization (CP) technique was used in combination with magic-angle spinning (MAS). This research was part of a systematic study of alkaline activation of slag by several different techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with X-ray microanalysis of energy dispersive spectra (EDS), differential thermal analysis (DTA) and calorimetry. This NMR study provides information on the polymerization of silicates, the role of aluminates in cement hydration and the nanostructure of C-S-H gel.     

The effect of matrix composition and calcium content on the sulfate durability of metakaolin and metakaolin/slag alkali-activated mortars

The work in hand presents the results of an experimental research on the effect of different precursors (binders) used in alkali-activated materials (AMM) and its composition (i.e. SiO₂/Al₂O₃ molar ratio) on their sulfate durability. A reference matrix is formed from the activation of metakaolin (MK); this matrix was modified by the partial replacement of MK with either 20 wt% silica fume (SF) or 20 and 40 wt% blast furnace slag (BFS), so that the SiO₂/Al₂O₃ molar ratio of calcium-free and calcium-rich AAM changed from 3.0 to 3.9. The properties assessed prior to the durability testing were: density (pycnometry), compressive strength, capillary sorption and oxygen permeability. The sulfate durability was investigated by exposing the matrices to a magnesium sulfate solution for 30, 90 and 180 days of attack, after which the residual compressive strength was determined. The reductions in strength after each period of testing were correlated with variations in the pH of the sulfate solutions and with geometry changes (expansion) measured in cylinders exposed to the durability tests. X-Ray diffraction was used to determine the minerals formed onto the surface of the samples after magnesium sulfate attack. The results show that the MK-based AAM present a higher resistance to magnesium sulfate attack. Furthermore, the partial replacement of MK with BFS is responsible for reductions in the mechanical properties after attack to sulfate. This is associated with the formation of ettringite and gypsum in the presence of calcium from BFS, besides the loss of alkalinity from the migration of alkali (Na⁺) to the solution.     

Hydration and durability of sulphate-resisting and slag cement blends in Caron's Lake water

The problem of aggressive attack of sulphate and chloride ions has been of considerable scientific and technological interest because this attack is one of the factors responsible for damage to concrete. The corrosive action of chlorides is due to the formation of chloroaluminate hydrates, which causes softening of concrete. Sulphate ions can enter into chemical reactions with certain constituents of concrete, producing sulphoaluminate hydrates and gypsum, which cause the expansion of concrete. The aim of the present work is to study the hydration and the durability of mixed cement (sulphate-resisting and slag cement blends) pastes and mortars in Caron's Lake water. Different mixes of sulphate-resisting cement (SRC) with various proportions of slag cement were prepared and immersed in tap water for 3, 7, 28 and 90 days. The durability of the cement mortars was followed by curing the samples in tap water for 28 days (zero time) then immersed in Caron's Lake water for 1, 3, 6, 9 and 12 months. The hydration behavior was measured by the determination of the compressive strength, free lime, evaporable and nonevaporable water, total chloride and total sulphate contents at each curing time. The increase of substitution of SRC with blast-furnace slag cement (BFSC) up to 30% increases slightly the total pore volume. The free lime contents decrease sharply in the first months of immersion then slightly up to 1 year. The blended cement pastes made of SRC with BFSC up to 30 mass% have lower values of total chloride and total sulphate, while the mortars containing only SRC have lower values of compressive strength than those of all blended cement mortars at all curing ages of immersion under Caron's Lake water. Useful conclusions and recommendations concerning the use of 70 mass% of SRC with 30 mass% slag cement produces a highly durable mixed cement.     

Autogenous and drying shrinkage of alkali-activated slag mortars

Shrinkage of alkali-activated slag (AAS) cement is a critical issue for its industrial application. This study investigated the mechanisms and effectiveness of shrinkage-reducing agent (SRA) and magnesia expansive agent on reducing autogenous and drying shrinkage of AAS mortars that were activated by liquid sodium silicate (LSS) solution with modulus (SiO₂/Na₂O molar ratio) of 0-1.5. The results showed that the autogenous shrinkage of AAS mortars increased with the increase of LSS modulus from 0 to 0.5, then decreased as modulus increased up to 1.5. The drying shrinkage consistently increased with the increase in the modulus of LSS. The oxyalkylene alcohol-based SRA could significantly reduce the autogenous and drying shrinkage of AAS mortars while the magnesia expensive agent was comparatively less effective. The autogenous shrinkage of AAS mortars was inversely proportional to the internal relative humidity, while the drying shrinkage was more related to the mass loss of samples. Mathematical models were established to describe the autogenous and drying shrinkage behavior of AAS mortars.     

Resistance of alkali-activated slag concrete to acid attack

This paper presents an investigation into the durability of alkali-activated slag (AAS) concrete exposed to acid attack. To study resistance of AAS concrete in acid environments, AAS concrete was immersed in an acetic acid solution of pH=4. The main parameters studied were the evolution of compressive strength, products of degradation, and microstructural changes. It was found that AAS concrete of Grade 40 had a high resistance in acid environment, superior to the durability of OPC concrete of similar grade.     

Strength, pore structure and permeability of alkali-activated slag mortars

This paper deals with the strength development, pore structure development, rapid chloride permeability and water permeability of alkali-activated slag mortars activated by 6% (by mass of Na₂O) NaOH, Na₂CO₃ and Na₂SiO₃. The Na₂SiO₃-activated slag mortars exhibited the highest strength at both early and later ages, even much higher than a typical commercial Type III portland cement. NaOH-activated slag mortars exhibited the lowest strength. The pore structure measurements were consistent with strength results. Four common strength-porosity equations: Balshin's, Ryshkevitch's, Schiller's, and Hasselmann's equations, fit the experimental results from alkali-activated slag mortars with sufficient efficiency; of which Hasselmann's equation fit best. The charge passed through the mortar specimens in the rapid chloride ion permeability test appeared to be dependent more on the chemistry of pore solution than on the pore structure of the mortars. Limited results from water permeability testing appeared to be consistent with strength and pore structure measurements.     

A comparison between alkali-activated slag/fly ash binders prepared with natural seawater and deionized water

In this research, the effects of natural seawater (SW) on the properties of alkali-activated slag/fly ash (AASF) are studied. AASF prepared with deionized water is set as the reference mixture. The results showed that the use of natural SW resulted in a prolonged setting time and lower heat release, but no obvious impact on the flowability of AASF specimens. The long-term compressive strength became higher when SW was used, whereas the corresponding flexural strength and fractural toughness turned lower. The use of SW induced the formation of new products that were not identified in the reference mixture, such as Cl–hydrocalumite and gypsum. In addition, it is evidenced that the dissolution of fly ash (FA) particles was significantly delayed with the incorporation of SW. All these results were related to the various ions introduced by the natural SW and their interactions with the alkaline activator as well as the precipitation of salts on slag and FA surfaces or in the matrix.     

The alkali-silica reaction in alkali-activated granulated slag mortars with reactive aggregate

The expansion of alkali-activated granulated blast furnace slag (AAS) cement mortars with reactive aggregate due to alkali-silica reaction (ASR) was investigated. The alkaline activator used was NaOH solution with 4% Na₂O (by mass of slag). These results were compared to those of ordinary portland cement (OPC) mortars. The ASTM C1260-94 Standard Test Method based on the NBRI Accelerated Test Method was followed. The nature of the ASR products was also studied by SEM/EDX. The results obtained show that the AAS cement mortars experienced expansion due to the ASR, but expansion occurs at slower rate than with OPC mortars under similar conditions. The cause of the expansion in AAS cement mortars is the formation of sodium and calcium silicate hydrate reaction products with rosette-type morphology. Finally, in order to determine potential expansion due to ASR, the Accelerated Test Method is not suitable for AAS mortars because the reaction rate is initially slow and a longer period of testing is required.     

A model for the C-A-S-H gel formed in alkali-activated slag cements

For first time, an experimental and computational study has been conducted to define a structural model for the C-A-S-H gel forming in alkali-activated slag (AAS) pastes that would account for the mechanical properties of these materials. The study involved a comparison with the C-S-H gel forming in a Portland cement paste.The structure of the C-A-S-H gels in AAS pastes depends on the nature of the alkali activator. When the activator is a NaOH, the structure of the C-S-H gel falls in between tobermorite 1.4 nm with a mean chain length of five, and tobermorite 1.1 nm with a mean length of 14. When waterglass is the activator the structure of the C-A-S-H gel is indicative of the co-existence of tobermorite 1.4 nm with a chain length of 11 and tobermorite 1.1 nm with a chain length of 14. This very densely packed structure gives rise to excellent mechanical properties.     

Immobilization of Cr, Cd, Zn and Pb 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 Cr⁶⁺ were lower (ca. 99.0%). The strength development as well as microstructure observations are presented in the paper.     

矿渣微粉对混凝土耐久性的影响

掺加活性掺合料尤其是矿渣微粉可有效解决水泥混凝土的耐久性问题,本试验即是从硬化混凝土的收缩、混凝土抗碳化陛、混凝土抗渗性能三方面研究矿渣微粉对混凝土性能的影响。     

Effect of binder content on the performance of alkali-activated slag concretes

This paper assesses the mechanical and durability performance of concretes produced using alkali silicate-activated ground granulated blast furnace slag as sole binder. Alkali-activated concretes are formulated with 300, 400 and 500 kg slag per m³ of fresh concrete, and their performance is compared with reference concretes produced using Portland cement (OPCC). Regardless of the binder content, the alkali-activated slag concretes (AASC) develop higher compressive strength than the comparable reference concretes. A higher binder content leads to increased strength in both AASC and OPCC at 28 days. However, at 90 days, the performance penalty for low binder content is more significant in the OPCC than AASC samples. Permeability, water sorption and carbonation resistance properties are also improved at higher binder contents. By controlling mix design parameters, it is possible to produce AASC with mechanical strength and durability comparable to conventional Portland cement concretes.     

石膏掺量对碱激发矿渣水泥砂浆性能的影响

研究了石膏掺量对碱矿渣水泥砂浆流动性、力学性能、干缩及水化性能的影响。结果表明,掺入1%～5%的石膏,碱激发矿渣水泥砂浆的流动度下降;石膏掺量在2%以内时,可提高砂浆的强度,但当掺量超过2%后,强度开始下降;石膏掺量在1%～5%范围内递增时,砂浆的干缩率随之降低。交流阻抗谱分析表明,在碱矿渣砂浆中掺入1%～5%的石膏时,Nyquist图形从30 min～1 d的非Randles图形逐渐过渡到3～28 d的准Randles曲线,表明砂浆内部的电化学反应与其水化反应相匹配,交流阻抗参数R_1、R_2在3 d后随着石膏掺量增大而增大,表明石膏在一定程度上促进了砂浆的水化。