Slag Cement Pastes Research Articles

Correction: Comparison of thermogravimetry response of alkali-activated slag and Portland cement pastes after stopping their hydration using solvent exchange method

Correction: Comparison of thermogravimetry response of alkali-activated slag and Portland cement pastes after stopping their hydration using solvent exchange method

Phase evolution and performance of sodium sulfate-activated slag cement pastes

This study evaluates the reaction kinetics, phase assemblage, and microstructure evolution of Na2SO4-activated slag cements produced with three commercial slags. The main reaction products identified are ettringite and calcium aluminosilicate hydrates, alongside a poorly crystalline SO42- intercalated Mg-Al-layered double hydroxide (LDH) phase. Results revealed that the Al2O3 slag content alone does not correlate with the cement performance. While pastes made with a higher Al2O3 content slag exhibit faster reaction kinetics, those made with a slag with a higher Mg/Al ratio developed superior compressive strength and reduced porosity over extended curing periods. Thermodynamic modelling simulations indicate that sulfate consumption occurs via ettringite and LDH phase formation, influencing the slag reaction degree, pH value, and porosity reduction in these cements. This research highlights the critical role of slag composition in controlling microstructure and, consequently, performance of sodium sulfate activated slag cement pastes.

Comparison of thermogravimetry response of alkali-activated slag and Portland cement pastes after stopping their hydration using solvent exchange method

AbstractThe critical step for any subsequent instrumental analysis of cementitious binders is to stop their hydration reactions, i.e., to remove free water. One of the most available techniques is a solvent exchange method. However, the solvents are known to be strongly bound in ordinary Portland cement (OPC) paste and alter the results of thermogravimetric analysis (TGA) and sensitive hydrates, while their effect on TGA response of alkali-activated slag (AAS) has not been comprehensively investigated. Therefore, the objective of this paper is to track the effects of fundamental aspects of the solvent exchange on the TGA response of AAS with different sodium activators (hydroxide, carbonate, waterglass) and to support these results by X-ray diffraction and effluent gas analysis. All solvents used (acetone, diethyl ether, isopropyl alcohol, ethanol, and methanol) affected the TGA response of all tested pastes, and their effect was enhanced by prolonged immersion time. All solvents induced an additional mass loss at around 800 °C and, especially for OPC paste, increased in situ carbonation, even in an inert atmosphere. Methanol and ethanol had a detrimental effect on ettringite and decreased the basal distance of the C–(A)–S–H gel, while they only marginally affected gaylussite. For AAS, hydration stoppage by washing out the alkali-rich pore solution with water was also investigated and can usually be recommended (except for its detrimental effect on gaylussite), as it is more efficient than organic solvents, which lack solubility for activators. Methanol and ethanol are the most suitable alternatives, particularly for NaOH.

Temporal effects on the micro- and nano-structural cement-slag pastes subjected to microwave curing over 7 years

This study examines the changes in the micro- and nano-structures of microwave-cured cement and slag pastes over 7 years. The properties of aged samples were compared with those of young pastes made from the same mixtures. Both young and aged samples were analyzed by XRD, DTA-TG, SEM-EDX, and Solid-state MAS NMR of 29Si and 27Al. It is found that, in addition to the usual components (C-S-H, calcium hydroxide, monosulfoaluminate, trisulfoaluminate, and hydrotalcite-like phases (HT)), the aged samples contained calcium magnesium aluminate hydrate and gibbsite (AH3). Over time, some HT or AH3 integrated into the C-S-H nanostructure as additional layers. These findings suggest that microwaves accelerate the general hydration processes in cement due to thermodynamic effects and selectively activate reactions in the aluminate components. This discovery highlights the potential for using microwaves to manipulate cement reactions.

Effect of nano-silica on mechanical properties and microstructure of high-volume ground granulated blast furnace slag cement paste

Effect of nano-silica on mechanical properties and microstructure of high-volume ground granulated blast furnace slag cement paste

Effect of Diethanol-Isopropanolamine and Typical Supplementary Cementitious Materials on the Hydration Mechanism of BOF Slag Cement Pastes

This study investigated the effects of lithium slag and iron tailings on the hydration mechanism of Basic Oxygen Furnace (BOF) slag cement paste with the addition of 0.06% diethanol-isopropanolamine (DEIPA). This study examined the fluidity, compressive strength, pore solution pH, and hydration products of BOF slag-based composite cementitious materials. The results showed that DEIPA facilitated the conversion from ettringite (AFt) to monosulphate (AFm) and improved the early compressive strength of the BOF slag–cement mortar. Incorporating lithium slag into the DEIPA-containing BOF slag–cement system promoted AFt formation, increased calcium-silicate-hydrate production, and enhanced the microstructure. BOF slag, lithium slag, and iron tailings exhibit synergistic effects in cement pastes. BOF slag and lithium slag provided the reactive components SiO2 and Al2O3. In the early hydration stages, the iron tailings primarily served as fillers, accelerating the system’s reactions.

Correlation between Slag Reactivity and Cement Paste Properties: The Influence of Slag Chemistry

Correlation between Slag Reactivity and Cement Paste Properties: The Influence of Slag Chemistry

The pore structure and water absorption in Portland/slag blended hardened cement paste determined by synchrotron X-ray microtomography and neutron radiography.

The pore structures of hardened Portland/slag cement pastes (>75 wt% slag content), and the initial capillary absorption of moisture through these pores, were monitored using ex situ synchrotron X-ray computerised microtomography and in situ quantitative neutron radiography. The pore structure becomes more constricted as the cement hydrates and its microstructure develops. This mechanism was effective even at a slag content as high as 90 wt% in the cementitious blend, where the lowest total porosity and a significant pore refinement were identified at extended curing ages (360 d). By combining this information with neutron radiographic imaging, and directly quantifying both depth and mass of water uptake, it was observed that 90 wt% slag cement outperformed the 75 wt% slag blend at 90 days in terms of resistance to capillary water uptake, although the higher-slag blend had not yet developed such a refined microstructure at 28 days of curing. The assumptions associated with the "sharp front model" for water ingress do not hold true for highly substituted slag cement pastes. Testing transport properties at 28 days may not give a true indication of the performance of these materials in service in the long term.

Pore characteristics of sulfate-activated coal gasification slag cement paste backfill for mining.

With the mass production of coal-based solid waste, coal mine filling can effectively consume it. The coal gasification slag is modified and prepared as coal mine filling material to meet the relevant technical requirements, which can realize the recycling of coal mine → coal chemical industry → coal mine. In this paper, in order to explore the evolution law of the mechanical properties and pore structure characteristics of the modified coal gasification slag-cement cemented paste backfill (MCGS-CPB) prepared by sodium sulfate excitation coal gasification slag, a combined macro-meso-micro testing method is used. MCGS-CPB with different sodium sulfate contents (1~5%) were prepared and tested for uniaxial compressive strength (UCS), mercury intrusion (MIP) and microscopic tests. The results show that sodium sulfate has a significant effect on the UCS and pore structure characteristics of MCGS-CPB. The mechanical properties and pore structure characteristics of MCGS-CPB were best when sodium sulfate was doped at 3%; the mechanical properties and pore structure characteristics of MCGS-CPB were deteriorated when the addition of sodium sulfate is higher than 3%. On the meso-scale, when sodium sulfate was doped with 3%, the more harmful pores of MCGS-CPB gradually changed into harmless, less harmful, and harmful pores, and the macroscopic mechanical properties were gradually improved; when the addition of sodium sulfate is higher than 3%, the harmless, less harmful, and harmful pores of MCGS-CPB gradually changed into more harmful pores, and the macroscopic mechanical properties were deteriorated. On a microscopic scale, sodium sulfate can cause MCGS-CPB to form hydration products with expansion properties. The presence of a reasonable amount of sodium sulfate in the pores of MCGS-CPB is beneficial to the development of mechanical properties. However, excessive presence will lead to the formation of expansion stress, gradual formation of micro-expansion cracks, and deteriorate the macroscopic mechanical properties. Hence, the volcanic ash activity of coal gasification slag excited by external addition of sodium sulfate should not exceed 3%. This study provides a reference value for application ratio of sodium sulfate-stimulated MCGS-CPB used in coal mine filling design.

On the chemo-mechanical evolution process of high-volume slag cement paste

This study investigated the evolution process of high-volume slag cement (HVSC) paste from a chemo-mechanical standpoint. HVSC specimens with a 70 w.t. % slag replacement rate were studied at various ages. Evolution of phase assemblage, microstructure development, and micromechanical properties were analyzed using TGA/XRD/MIP/SEM-EDS and nano-/micro-indentation techniques. A two-scale micromechanical model was built to predict the effective elastic modulus based on the nanoindentation results. Key findings include: 1) Between 7 and 28 days, the formation of calcium silicate hydrate (C-S-H) gel phase improves the effective elastic modulus by filling capillary pores; 2) From 28 to 90 days, the phase assemblage and microstructure remain stable, with a transition from low-density to high-density C-S-H; 3) Between 90 days and 2 years, slag rims produced by slag grains result in increased elastic modulus; 4) The two-scale micromechanical model, combined with nanoindentation data, accurately predicts the effective modulus of HVSC composites, although the unhydrated slag grains-hydrated cement matrix interface may cause an overestimation at an early age. With longer curing time, this interface disappears owing to the continuous hydration of large slag particles and therefore a good match is found between the modelling and experimental results.

Fresh state and hydration properties of high-volume lithium slag cement composites

In this study, the fresh state and hydration properties of 0–60% lithium slag blended cement pastes were investigated at water-binder ratio of 0.47. The workability of the fresh pastes was evaluated by measuring the air content, marsh cone flow, mini-slump flow, setting times, and through rheology tests. A 40% lithium slag cement could produce 91% strength activity index at 28 days; mini-slump pat diameter of 70.54 mm; marsh cone flow efflux time of 145 s; air content 0.6%; hydration heat of 300 J/g (at 72 h). At replacement levels above 40%, the strength activity index, air content, mini-slump flow, hydration heat, and fluidity were significantly reduced. Experimental investigations confirm that the mini-slump test provides the best correlation coefficients (R2 = 0.96) with the maximum shear viscosity of lithium slag cement pastes than the marsh cone flow efflux time and air content. The classical slump and rheological models were used to characterise the mini-slump, yield stress, and plastic viscosity of low to high volume lithium slag cement pastes. The present study recommends that a 40% lithium slag cement paste is a viable option to produce green concrete for optimum fresh, hydration, rheological, and hardened properties.

Effect of P2O5 incorporated in slag on the hydration characteristics of cement-slag system

This paper presents the influence of P2O5 incorporated in slag on the hydration characteristics of cement-slag system. It was found that the gradual addition of phosphorus oxide in slag did not change overall mineralogy of the hydration products. Except hydration retardation in the dormant stage, chemically bound water and portlandite contents, hydration degree of slag, and pore structure at all investigated ages were similar among cement-slag pastes with different P2O5 percentages. Furthermore, significantly higher amount of monosulfate was observed as the P2O5 content in slag increased. In addition, a higher Al/Si atomic ratio was measured in the C-S(A)-H gel phase formed in the cement matrix. However, similar Ca/Si atomic ratio of C-S(A)-H gel phase and Mg/Al atomic ratio of hydrotalcite-like phase were determined in all slag pastes, irrespective of the addition of P2O5. In contrast to magnesium ion which was retained within the original slag boundary, phosphorus ions could migrate into cement matrix. Therefore, P/Si atomic ratio of the C-S-H gel phase increased with the increasing phosphorus oxide content in slag, reaching up to ∼0.08.

Micro-Mechanical Properties of Slag Rim Formed in Cement–Slag System Evaluated by Nanoindentation Combined with SEM

Slag rim mainly consists of secondary precipitations such as C–S–H gel phase and hydrotalcite-like phase, which originate from the hydration of slag. In this paper, the micro-mechanical properties of slag rim were characterized by nanoindentation in combination with SEM. It was found that, compared to the C–S–H gel phase, slag rim showed about a 15 GPa higher modulus of elasticity. At the early age, slag cement paste mainly consisted of low-density C–S–H gel phase, high-density C–S–H gel phase, and unhydrated slag particles, as well as calcium hydroxide based on the results of nanoindentation; at the later age, the system contained C–S–H gel phase, slag rim (a mixture of C–S–H gel phase and hydrotalcite-like phase), and unhydrated slag particles.

Effect of sulfate attack on the composition and micro-mechanical properties of C-A-S-H gel in cement-slag paste: A combined study of nanoindentation and SEM-EDS

Sulfate attack is a common type of corrosion that results in the deterioration of cementitious materials, but its attacking mechanism on the micro-mechanical properties C-A-S-H gel is still not fully understood. This paper utilizes combined techniques of XRD, nanoindentation and SEM-EDS to investigate the composition and mechanical properties evolution of C-A-S-H gel in cement-slag paste under sulfate attack. It is found that the pozzolanic reaction of slag depletes portlandite and produces AFm, which can act as a buffer for the ettringite formation during sulfate attack. Sulfate ions can adsorb on C-A-S-H gel and cause the its compositional decalcification and dealumination. Slag incorporation can decrease the sulfate adsorption capacity of C-A-S-H gel and moderates its compositional change under sulfate attack. The composition change of C-A-S-H gel induced by sulfate attack is accompanied with its micro-mechanical properties’ degradation, and this degradation is a result of its increasing porosity during decalcification and dealumination but not related with its original composition. The incorporation of slag reduces the decalcification and dealumination of C-A-S-H gel in the paste induced by sulfate attack, thus improving its micro-mechanical properties stability in sulfate attacking environment.

The role of hydrotalcite-like phase and monosulfate in slag cement paste during atmospheric and accelerated carbonation

This paper identified carbonation products in the slag-rich cementitious systems upon three different exposure conditions, namely, long term exposure in the field, indoor natural exposure, and accelerated carbonation testing. Overall, mineralogy of the carbonation products was found to be fundamentally similar under different exposure environments. In the fully carbonated areas, no monosulfate and calcium hydroxide was observed, and calcium carbonate, carbonated hydrotalcite-like phase and Ca–Al AFm phases were identified as the main carbonate phases. In the mildly carbonated areas, monosulfate and calcium hydroxide clusters were detected again. With the continuous supply of CO2, monosulfate appeared to be consumed at first. Despite different exposure environments, carbonated Ca–Al AFm phases bound around 5% of CO2 penetrated into the matrix. Hydrotalcite-like phase was able to absorb more than 15% CO2 initially. However, this value decreased to around 10% in the fully carbonated areas. Therefore, more than 20% CO2 entered into hydrotalcite-like phase as well as Ca–Al AFm phases at first, and the involved reactions were harmless without any detrimental effect on cement matrix. Meanwhile, hydrotalcite-like phase was able to keep intact and its Mg/Al atomic ratio did not vary significantly during carbonation.

Evolution of the early compressive strength of cement-slag pastes with CaO-based expansive agent

Evolution of the early compressive strength of cement-slag pastes with CaO-based expansive agent

Comprehensive properties of passive film formed in simulated pore solution of alkali-activated concrete

The study on the properties of passive films in alkali-activated slag cement can provide a theoretical basis for the development and application of low-carbon and environmental friendly concrete materials. In this paper, the passivation behaviour of anti-corrosion rebar (AR) and normal reinforcing steel (NR) in sulfur-free simulated pore solutions of alkali-activated slag cement pastes was investigated using electrochemical, microscale techniques and molecular dynamics modelling. Results showed that the formation time, composition and thickness of the passive film were affected by the types of simulated pore solutions. Molecular dynamics further elucidated the reason for difference in the protective ability of passive film in three types of simulated pore solutions. The results of protection capability of two kinds of rebar obtained from molecular dynamics were consistent with those of electrochemical impedance spectroscopy (EIS).

Effects of Fluorogypsum and Flue-Gas Desulfurization Gypsum on the Hydration and Hardened Properties of Alkali Slag Cement

In order to explore the influence of the types of waste gypsum on the properties of alkali slag cement, fluorogypsum (FG) and flue-gas desulfurization (FGD) gypsum were comparatively investigated. Moreover, the action mechanisms of FG and FGD gypsum on the properties of alkali slag cement were analyzed. The results show that both the FG and FGD gypsum prolonged the setting time of the alkali slag cement paste. However, the prolongation effect of FG was more pronounced than the FGD gypsum. When the compressive strength was maximum, the contents of FG and FGD gypsum were 5 and 6 wt.%, respectively. At 3 and 28 days, compared to the control sample, the compressive strengths increased by 59.3% and 24.3%, and 66.9% and 33.9%, respectively. Furthermore, the XRD, TG-DTA and SEM-EDS results showed that, with the increase in the contents of FG and FGD gypsum, ettringite was more easily produced and the hydration products were more abundant in the system. The greater the gypsum content of the paste, the less accumulated was the heat of hydration. The change of micro-structure caused by the formation of ettringite was the main reason for the difference in the properties of cement.

Durability of Slag–Cement Paste Containing Polyaluminum Chloride

The effect of polyaluminum chloride (PAC) on the durability, microstructure, and corrosion sensitivity of slag–cement pastes was investigated in this research. Various experimental tests were performed to assess the mechanical properties (compressive strength test), durability (rapid chloride migration test, water permeability test, capillary water absorption test, chloride binding test, accelerated carbonation test), microstructure (scanning electron microscopy, X-ray diffraction, mercury intrusion porosimetry), and corrosion sensitivity (accelerated corrosion sensitivity, initial chloride concentration). Experimental results showed that adding 1.5% PAC increased compressive strength by 77.0%, 53.8%, and 19.4% at 3, 7, and 28 days. Resistance to chloride migration, water permeation, capillary water absorption, and carbonation was improved. These results were consistent with the microstructural analysis, which showed that adding PAC facilitated the formation of Friedel’s salt and development of pore structure, which was the key factor for the improved durability of the slag–cement paste. Adding 1.5% PAC had a limited influence on the corrosion sensitivity despite introducing the additional chloride ions into the system. These results promote the further practical application of PAC.

Effects of alkali dosage and silicate modulus on autogenous shrinkage of alkali-activated slag cement paste

Autogenous shrinkage of alkali-activated slag cement (AASC) paste prepared with different levels of alkali dosage and silicate modulus (Ms) is investigated. The results show that autogenous shrinkage of AASC paste increases at a decreasing rate with alkali dosage, which is attributed to enhanced capillary pore pressure and syneresis of C-A-S-H gels. Autogenous shrinkage of AASC paste with a constant alkali dosage increases as Ms increases from 0.5 to 1.0, followed with a reduction as Ms further increases to 2.0. Increasing Ms initially enhances the capillary pore pressure due to the enhanced reaction degree, and promotes the formation of saturated capillary pores and viscoelasticity, which facilitates the autogenous shrinkage of AASC paste. However, excessive silicate in the activator retards the internal moisture consumption, and subsequently decreases the autogenous shrinkage of AASC paste. Furthermore, increasing Ms can also intensify syneresis of C-A-S-H gels, contributing to the early-age autogenous shrinkage of AASC paste.