Characterisation of ground hydrated Portland cement-based mortar as an additive to alkali-activated slag cement

The sustainable development of cement manufacturing requires extension of the raw material base, including large-tonnage waste. Hydrated mortar waste is a promising mineral resource for the production of Portland cements and alternative binders, such as alkali-activated slag cement. The influences of ground-hydrated mortar aged for 3 months on the properties of alkali-activated slag fresh and hardened pastes were performed. The results show that the properties are dependent on the concentration (2.5–60%), cement:sand ratio (1:1–3) and fineness (200–600 m²/kg) of the ground hydrated mortar; the alkali activator (sodium carbonate and sodium silicate); and the curing conditions (normal conditions and steam curing). The fresh paste properties that we considered in this study included the water requirement and the setting time; the hardened paste properties we considered were the water absorption, the density, and the compressive strength after 2, 7, 14, 28, 180 and 360 days of ageing. The ground hydrated mortar improved the early strength and the long-term strength of the alkali-activated slag paste and replaced the slag up to 50%. The factors that affecting the strength of the alkali-activated slag cement with ground hydrated mortar as an additive were, in order of influence, alkali activator type > curing conditions > cement:sand ratio > ground-hydrated mortar fineness.     

Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash

The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.     

Shrinkage characteristics of alkali-activated fly ash/slag paste and mortar at early ages

The purpose of this study is to investigate the shrinkage characteristics of alkali-activated fly ash/slag (henceforth simply AFS) and the factors affecting it. A series of tests were conducted to determine the chemical shrinkage, autogenous shrinkage and drying shrinkage. The microstructures and reaction products were also characterized through XRD and SEM/EDS analyses. An increase in the slag content from 10% to 30% resulted in a denser matrix and showed a higher Ca/Si ratio of C–N–A–S–H in the microstructure. Higher sodium silicate and slag contents in a mixture caused more chemical, autogenous, and drying shrinkage, but led to a higher compressive strength. From the test results, it can be concluded that the autogenous shrinkage of AFS mortar occurs mainly due to self-desiccation in hardened state rather than volume contraction by chemical shrinkage in fresh state. The AFS paste showed higher drying shrinkage than ordinary Portland cement (OPC), which may be caused by the higher mesopore volume of the AFS paste compared to that of OPC paste.     

Characterisation of pore structure development of alkali-activated slag cement during early hydration using electrical responses

This paper describes the results of a study investigating early age changes in pore structure of alkali-activated slag cement (AASC)-based paste. Capillary porosity, pore solution electrical conductivity and electrical resistivity of hardened paste samples were examined and the tortuosity determined using Archie's law. X-ray computed micro-tomography (X-ray μCT) and Scanning electron microscope (SEM) analysis were also carried out to explain conclusions based on electrical resistivity measurements. AASC pastes with 0.35 and 0.50 water-binder ratios (w/b) were tested at 3, 7, 14 and 28 days and benchmarked against Portland cement (PC) controls. Results indicated that for a given w/b, the electrical resistivity and capillary porosity of the AASC paste were lower than that of the PC control, whilst an opposite trend was observed for the pore solution conductivity, which is due to AASC paste's significantly higher ionic concentration.Further, capillary pores in AASC paste were found to be less tortuous than that in the PC control according to estimations using Archie's law and from the results of X-ray μCT and SEM analysis. In order to achieve comparable levels of tortuosity, therefore, AASC-based materials are likely to require longer periods of curing. The work confirms that the electrical resistivity measurement offers an effective way to investigate pore structure changes in AASC-based materials, despite threshold values differing significantly from PC controls due to intrinsic differences in pore solution composition and microstructure.     

Understanding the drying shrinkage performance of alkali-activated slag mortars

In this work, drying shrinkage of four alkali-activated slag (AAS) mortars, prepared using various types/dosages of activator, was characterized at four different levels of relative humidity (RH) and two drying regimes (i.e. direct and step-wise drying). The results show that drying shrinkage values of AAS are significantly dependent on the drying rate, as AAS shrinks more when the RH is decreased gradually, instead of directly. At high RH, the drying shrinkage of AAS exhibits a considerable visco-elastic/visco-plastic behavior, in comparison to ordinary portland cement (OPC). It is concluded that the cause of high-magnitude shrinkage in AAS mortar is due to the high visco-elastic/visco-plastic compliance (low creep modulus) of its solid skeleton. Furthermore, the activator affects the shrinkage behaviors of AAS by influencing the pore structure and mechanical properties.     

Influence of limestone content,fineness, and composition on the properties and microstructure of alkali-activated slag cement

The influence of the fineness, concentration, and chemico-mineralogical composition of limestone on the workability, reaction kinetics, compressive strength, microstructure, and binder gel characteristics of sodium carbonate–based waste-activated waste slag cement pastes was investigated in this work. Alkali-activated slag cements incorporated with limestone, containing 33–100% of calcite, at a content of up to 60% with a 28-day compressive strength of 26.2–48.8 MPa were proposed. The main reaction products of hardened alkali-activated cement pastes and those incorporated with limestone are CSH, CaCO₃, Na₂Ca(CO₃)₂·5H₂O, and Na₂CaSiO₄. “Physically active” limestone does not chemically react with the binder gel but it can improve the physical structure. The higher packing density of mixed cement, without an increase in the water demand, the satisfactory binding strength of limestone with the binder gel lead to the improvement in the physical structure and compressive strength of alkali-activated slag paste.     

Rheology of alkali-activated slag pastes. Effect of the nature and concentration of the activating solution

An understanding of the rheological behaviour of OPC-based products has been widely studied, for it is essential to determining and predicting the fresh and hardened characteristics and properties of pastes, mortars and concretes. The rheology of alkali-activated material (AAM) systems has been much less intensely researched, however.The present study aimed to ascertain the effect of factors such as the nature and concentration of the alkaline activator on the rheological behaviour of alkali-activated slag (AAS) pastes, with a comparison between the rheological parameters and fluidity of these pastes to the same parameters in OPC. More specifically, the study explored how paste rheology was affected by the nature of the alkaline activator (NaOH, 50/50 wt% NaOH/Na₂CO₃ or waterglass – Wg), its concentration (3–5% Na₂CO₃ of slag weight) and, in the waterglass solution, the SiO₂/Na₂O ratio.The findings showed that AAS paste rheology is affected by the nature of the activator. The rheological behaviour in AAS pastes activated with NaOH alone or combined with Na₂CO₃ was similar to the rheology observed in OPC pastes, and fit the Bingham model. Conversely, the AAS pastes activated with waterglass fit the Herschel–Bulkley model and their rheology proved to depend on both the SiO₂/Na₂O ratio and the Na₂O concentration. Moreover, regardless of the activator used (NaOH, Na₂CO₃ or waterglass), an increase in Na₂O concentration implies a raise of shear stress.The formation of primary C–S–H gel in Wg–AAS and its effect on paste rheology were confirmed. Gel formation was likewise shown to be related to the SiO₂/Na₂O ratio and activator concentration.     

Establishment of a preconditioning regime for air permeability and sorptivity of alkali-activated slag concrete

This study reports an experimental investigation designed to assess the influence of near-surface moisture contents on permeation properties of alkali-activated slag concrete (AASC). Five different drying periods (5, 10, 15, 20 and 25 days) and three AASC and normal concretes with compressive strength grades ranging from C30 to C60 were considered. Assessment of moisture distribution was achieved using 100 mm diameter cores with drilled cavities. Results indicate that air permeability of AASC is very sensitive to the moisture content and its spatial distribution, especially at relative humidity above 65%. To control the influence of moisture on permeation testing, the recommendation of this paper is that AASC specimens should be dried in controlled conditions at 40 °C for 10 days prior to testing. It was also concluded from this study that AASC tends to perform less well, in terms of air permeability and sorptivity, than normal concrete for a given strength grade. This conclusion reinforces the need to further examine AASC properties prior to its widespread practical use.     

Performance of blended metakaolin/blastfurnace slag alkali-activated mortars

This paper studies the effect of silicate content on the mechanical and durability-related properties of metakaolin (MK) and metakaolin/blastfurnace slag (BFS) alkaline activated mortars. A reference mortar based on the alkaline activated MK was compared to 60/40 MK/BFS mortars containing different SiO₂/Na₂O molar ratios in the activator. The properties assessed were compressive strength, porosity (water saturation), porosity and pore size distribution by Mercury Intrusion Porosimetry (MIP) and water capillary sorption. The microstructure was assessed using SEM and x-ray computerized micro-tomography (μ-CT). Results show that the addition of BFS significantly alters the microstructure of alkali-activated mortars, promoting a reduction of porosity and capillary sorption. In addition, an optimum SiO₂/Na₂O molar ratio in the activator is required to produce better durability mortars, which however do not necessarily present the highest mechanical strength.     

Compressive strength and microstructure of alkali-activated fly ash/slag binders at high temperature

This paper reports the results of the compressive strength and microstructure of various alkali-activated binders at elevated temperatures of 300 and 600 °C. The binders were prepared by alkali-activated low calcium fly ash/ground granulated blast-furnace slag at ratios of 100/0, 50/50, 10/90 and 0/100 wt.%. Specimens free of loading were heated to a pre-fixed temperature by keeping the furnace temperature constant until the specimens reached a steady state. Then the specimen was loaded to failure while hot. XRD, SEM and FTIR techniques were used to investigate the microstructural changes after the thermal exposure. The fly ash-based specimen shows an increase in strength at 600 °C. On the other hand, the slag-based specimen gives the worst high-temperature performance particularly at a temperature of 300 °C as compared to ordinary Portland cement binder. This contrasting behaviour of binders is due to their different binder formulation which gives rise to various phase transformations at elevated temperatures. The effects of these transformations on the compressive strength are discussed on the basis of experimental results.     

Very early-age reaction kinetics and microstructural development in alkali-activated slag

The early age reaction kinetics and microstructural development in alkali-activated slag binder are discussed. In-situ isothermal calorimetry was used to characterize the reaction progression in sodium hydroxide and sodium silicate-activated slag binders cured at ambient temperature. Microstructure and strength development were monitored to correlate the heat evolution with the property development. In-situ isothermal calorimetric data for sodium hydroxide-activated systems exhibited only one major heat evolution peak with no dormant period. Sodium silicate-activated pastes exhibited multiple peaks and extended dormant periods. Microstructural evolution, monitored using BSE–SEM, showed rapid product formation on the surface of slag grains in sodium hydroxide-activated systems, forming thin reaction shells—the thickness of which was related to the activator concentration—and leading to diffusion controlled hydration at a very early stage. Sodium silicate-activated systems exhibited slow and progressive product formation, predominately nucleated from the solution. These results are supported by electron mapping and electron dispersive X-ray spectroscopy.     

Measurement and prediction of thermal properties of alkali-activated fly ash/slag binders at elevated temperatures

This paper presents a comprehensive experimental study of thermal properties of various alkali-activated binders at ambient and elevated temperatures. The binders were prepared using alkali-activated low calcium fly ash/ground granulated blast-furnace slag at ratios of 100/0, 90/10, 50/50 and 0/100 wt%. These binders can be considered as a composite of solid, water and air. Accordingly, a three-phase model is applied to predict thermal conductivity of the binders at ambient temperature. At elevated temperatures, the Hashin–Shtrikman model is used to estimate the bounds of thermal conductivity for alkali-activated binders containing of fly ash. To validate the above models, a transient plane source measurement technique was applied to measure the thermal conductivity and heat capacity at temperatures ranging from 23 to 600 °C. Data generated is then utilised to develop analytical expressions for estimating thermal properties as a function of temperature. The simplified relationships can be used for estimating the fire resistance of structural elements made from alkali-activated cementitious materials.     

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.     

Sr(NO_3)_2对碱矿渣材料结构和性能的影响

为探明碱矿渣胶凝材料(AAS)固化核素Sr2+后,固化体性能的变化。利用矿渣粉、水玻璃、水和Sr(NO3)2制备了碱矿渣固化体,并研究了Sr(NO3)2对AAS固化体结构和性能的影响。结果表明,Sr(NO3)2使得AAS固化体的结晶程度和水化放热均降低,延缓了浆体的凝结硬化;Sr2+破坏了Si—O—Al基团的形成过程,阻碍了二次C-S-H的形成。Sr(NO3)2的掺入降低了AAS固化体的3 d抗压强度;但当Sr2+掺量小于0.4%时,AAS固化体的28 d抗压强度增加。     

Interfacial transition zone of cement composites with steel furnace slag aggregates

As previous studies of mortar and concrete with steel furnace slag (SFS) aggregates have shown increases or decreases in the bulk mechanical properties, this study investigated the microstructural cause of these opposing trends through characterization of the interfacial transition zone (ITZ) with quantitative image analysis of backscatter electron micrographs. Three SFS types – basic oxygen furnace (BOF), electric arc furnace (EAF), EAF/ladle metallurgy furnace (EAF/LMF) – were examined as aggregates in a portland cement mortar. The ITZ size for all SFS mortar mixtures was similar, with the ITZ of BOF and EAF/LMF being slightly more porous than mortar mixtures with EAF or dolomite. Microstructural examinations of the SFS particle revealed that BOF and EAF/LMF aggregates have different outer and interior compositions, with the outer composition consisting of a porous layer, which likely contributes to the reduced strength relative to EAF. The imaging results demonstrated that the type of SFS and its spatial composition greatly influences the bulk properties of mortar and concrete, mainly as a function of porosity content in the ITZ and the outer layer and interior porosity of the SFS aggregate.     

A model of phase stability,microstructure and properties during leaching of portland cement binders

A new model of 3D cement paste microstructure development is described and used to simulate the influence of leaching on hydrated cement pastes. In contrast to recent leaching models that have used empirical rules for phase dissolution, this model uses continual thermodynamic speciation and phase stability calculations to guide the microstructural changes that happen throughout hydration and subsequent exposure to low-pH solutions. This novel aspect of the model enables it to predict not only the well-known phase instability of calcium hydroxide at the onset of leaching, but also the detailed compositional and volumetric changes of C–S–H gel and other calcium, aluminate, and sulfate phases. Besides tracking the compositional and microstructural changes, we use the evolving microstructure as input to calculate changes in the relative diffusivity and effective Young’s modulus of the binder using established finite difference and finite element models. The results are broadly consistent with previous experimental and modeling investigations of leaching. In particular, the leaching process can be roughly divided into initial, intermediate, and final stages, each of which has distinct degradation characteristics and consequences for mechanical and transport properties. The thermodynamic basis of the model makes it readily extensible to simulate a wide range of cementitious materials and degradation phenomena, so we discuss its potential as a virtual microprobe for use with continuum-scale service life models of concrete elements.     

碳酸化钢渣和矿渣作硅酸盐水泥混合材水化性能研究

通过对钢渣碳酸化前后的硅酸盐相提取及水化放热性能和将碳酸化钢渣和矿渣作为混合材的硅酸盐水泥的胶砂强度和水化产物种类的测定,以及对它们微观形貌的观察,研究了碳酸化钢渣对胶凝体系水化性能的影响.结果表明,碳酸化使钢渣中硅酸盐相的含量由47.06％下降至14.38％;碳酸化促进了钢渣的早期水化,抑制其后期水化;在配比相同的条件下,碳酸化钢渣-矿渣-硅酸盐熟料体系试样的3、28d抗压强度较未碳酸化钢渣-矿渣-硅酸盐熟料体系试样的高;碳酸化生成的CaCO3促进了熟料的水化;碳酸化钢渣促进了胶凝体系中AFt的生成,且生成水合碳铝酸钙.     

Comparative performance of alkali activated slag/metakaolin cement pastes exposed to high temperatures

This paper presents a study on the effect of temperature exposure of binders of blast furnace slag (BFS) and metakaolin (MK) in BFS-MK weight ratios of 100-0, 50-50, and 0-100 activated with sodium silicate of modulus Ms = SiO₂/Na₂O = 1 and 5, 10 and 15% Na₂O. A blended ordinary CPC-30R Portland cement reference was used. Pastes were subjected to exposure up to 1200 °C and the performance was evaluated in terms of compressive strength, residual strength, volumetric shrinkage, physical appearance and microstructural changes at different temperatures. All the binders retained more than 30 MPa after exposure to 800 °C for 4 h; specimens of MK and CPC-30R experienced the highest strength losses of 42 and 56% respectively, while those of 100-0 and 50-50 showed minor losses of ∼20%. After heating at 1200 °C the samples showed microstructural damage and more than 65% of strength losses. XRD indicated that the 100-0 and 50/50 binders are prone to form crystalline phases as akermanite, nepheline and nosean at temperatures greater than 1000 °C, while 0-100 geopolymeric binders preserved mostly an amorphous structure even at 1200 °C with some traces of mullite. The dehydration of C-A-S-H and N-A-S-H altogether with the crystallization of the binder gel induced the formation of highly porous microstructures.     

High volume limestone alkali-activated cement developed by design of experiment

Herein, we report on the development of a cement comprising ground granulated blast furnace slag, soda ash (sodium carbonate), and up to 68 wt.% granular limestone. Mixture Design of Experiment (DOE) was utilized, with analysis of compressive strength, modulus of elasticity, hydraulic properties, cost, CO₂ production, and energy consumption. Models were derived to understand the impact of mix design on performance and for optimization. Successful formulations are hydraulic and cure at room temperature, with strengths as high as 41 MPa at 3 d and 65 MPa at 28 d. These formulations, compared to OPC, are competitive in cost and performance and can reduce both CO₂ production and energy consumption by up to 97%.     

Utilization of steel slag for Portland cement clinker production

The aim of the present research work is to investigate the possibility of adding steel slag, a by-product of the conversion of iron to steel process, in the raw meal for the production of Portland cement clinker. Two samples of raw meals were prepared, one with ordinary raw materials, as a reference sample ((PC)(Ref)), and another with 10.5% steel slag ((PC)(S/S)). Both raw meals were sintered at 1450 degrees C. The results of chemical and mineralogical analyses as well as the microscopic examination showed that the use of the steel slag did not affect the mineralogical characteristics of the so produced Portland cement clinker. Furthermore, both clinkers were tested by determining the grindability, setting times, compressive strengths and soundness. The hydration products were examined by XRD analysis at 2, 7, 28 and 90 days. The results of the physico-mechanical tests showed that the addition of the steel slag did not negatively affect the quality of the produced cement.