The hydration of slag,part 1: reaction models for alkali-activated slag

Reaction models are proposed to quantify the hydration products and to determine the composition of C–S–H from alkali-activated slags (AAS). Products of the slag hydration are first summarized from observations in literature. The main hydration products include C–S–H, hydrotalcite, hydrogarnet, AFm phases (C₄AH₁₃ and C₂ASH₈) and ettringite. Then, three stoichiometric reaction models are established correlating the mineral composition of slag (the glass part) with the hydration products. Using the proposed models, quantities of hydration products and composition of C–S–H are determined. The models are validated with a number of experimental investigations reported in literature, yielding good agreement, i.e., these models can successfully predict the hydration reaction of AAS. The models are furthermore applied to calculate the retained water in the hydration products of AAS in different hydration states and a general hydration equation of AAS is derived. As an illustration to one of the model applications, chemical shrinkage of the AAS cement paste in different hydration states are predicted. The chemical shrinkage of AAS is shown to be remarkably higher than OPC. Furthermore, phase distribution in the hardened AAS paste and the porosity are calculated.     

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.     

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.     

Carbonation process of alkali-activated slag mortars

This study analyzes the behaviour of waterglass- or NaOH-activated slag mortars after carbonation. The effect of a superplasticizer based on vinyl copolymer and shrinkage reducing polypropylenglycol derivative admixtures on that process was also examined. The same tests were run on cement mortars for reference purposes. The mortars were carbonated in a chamber ensuring CO₂ saturation for four and eight months, after which ages the samples were tested for mechanical strength; mercury porosimetry and mineralogical (XRD, FTIR) and microstructural characterization (SEM/EDX) were also conducted. The results obtained indicate that alkali-activated slag mortars were more intensely and deeply carbonated than Portland cement mortars. Carbonation took place directly on the gel, causing decalcification. When waterglass was the alkaline activator used, carbonation caused a loss of cohesion in the matrix and an important increase in porosity and decrease in mechanical strength. When a NaOH solution was used as the alkali activator, carbonation enhanced mortar compaction and increased mechanical strength. Finally, in waterglass-activated slag mortars, the inclusion of organic admixtures had no effect either on their behaviour after carbonation or the nature of the reaction products.     

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.     

Improvement of metakaolin on radioactive Sr and Cs immobilization of alkali-activated slag matrix

Effects of metakaolin on simulated radioactive Sr or Cs immobilizing behavior of alkali-activated slag (AAS) matrix were evaluated by cation exchange capacity (CEC), distribution ratio of selective adsorption, leaching test and porosity analyses. The results revealed that the additions of metakaolin into the AAS matrixes largely enhanced their distribution ratios K(d) of selective adsorption on Sr and Cs ions. Meanwhile, this new immobilizing matrix M-AAS showed the lowest leaching rate. Hydration product analyzes by XRD, DTA, FTIR and SEM demonstrated that (Na+Al)-substituted calcium silicate hydrate (CSH) and self-generated zeolite were major hydration products in the M-AAS matrix. These products have better Sr and Cs selective adsorption as compared to C-S-H without any substitution and natural zeolite minerals.     

Immobilization of mercury and zinc in an alkali-activated slag matrix

The behavior of heavy metals mercury and zinc immobilized in an alkali-activated slag (AAS) matrix has been evaluated using physical property tests, pore structure analysis and XRD, TG-DTG, FTIR and TCLP analysis. Low concentrations (0.5%) of mercury and zinc ions had only a slight affect on compressive strength, pore structure and hydration of AAS matrixes. The addition of 2% Hg ions to the AAS matrix resulted in a reduction in early compressive strength but no negative effects were noticed after 28 days of hydration. Meanwhile, 2% Hg ions can be effectively immobilized in the AAS matrix with the leachate meeting the USEPA TCLP mercury limit. For a 2% Zn-doped AAS matrix, the hydration of the AAS paste was greatly retarded and the zinc concentration in the leachate from this matrix was higher than 5mg/l even at 28 days. Based on these results, we conclude that the physical encapsulation and chemical fixation mechanisms were likely to be responsible for the immobilization of Hg ions in the AAS matrix while only chemical fixation mechanisms were responsible for the immobilization of Zn ions in the AAS matrix.     

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.     

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.     

Hydration and properties of sodium sulfate activated slag

Interest in alkali-activated slag as a construction material is increasing, primarily due to its environmentally friendly nature. Although strong alkaline activators, such as sodium hydroxide and sodium silicate solution, are preferred for high strength, none of them exists naturally and their manufacturing process is quite energy intensive. Whilst sodium sulfate (Na₂SO₄) can be obtained from natural resources, the early strength of Na₂SO₄ activated slag is usually low. In this paper, the effects of slag fineness and Na₂SO₄ dosage on strength, pH, hydration and microstructure were investigated and compared with those of a pure Portland cement (PC). Test results indicated that increasing the slag fineness is a more effective approach than increasing Na₂SO₄ dosage for increasing both the early and long-term strength of Na₂SO₄ activated slags. In addition, increasing the slag fineness can also increase the strength without increasing the pH of the hardened matrix, which is beneficial for immobilizing certain types of nuclear waste containing reactive metals and resins.     

碱激发改性矿粉/砒砂岩复合材料

为了实现砒砂岩的资源化利用,首先,通过碱激发法并掺入适量的矿粉将砒砂岩转变成了力学性能较好的改性矿粉/砒砂岩复合材料;然后,系统地研究了矿粉、碱激发剂掺量和龄期对改性矿粉/砒砂岩复合材料抗压强度和软化系数的影响;最后,利用SEM/EDS、FTIR和XRD分析了改性矿粉/砒砂岩复合材料的水化进程和碳化进程。结果表明:改性矿粉/砒砂岩复合材料的90d抗压强度和软化系数分别达到46.0 MPa和0.94,完全满足工程应用的需求;改性矿粉/砒砂岩复合材料主要的水化产物为C-S-H凝胶和地聚物胶体,且C-S-H凝胶存在比较严重的碳化现象;当矿粉和NaOH的掺量分别为20.0wt%和1.5wt%时,C-S-H凝胶的28d碳化率达30.1%,且碳化率随矿粉及NaOH掺量的增加而减小,但碳化率对强度几乎没有影响。改性矿粉/砒砂岩复合材料拥有良好的力学性能及耐水性。      

Nano-TiO2 particles with increased photocatalytic activity prepared by the miniemulsion method

The water-in-oil (W/O) miniemulsion system was used to prepare nano-TiO₂ particles. The nano-TiO₂ particles were characterized by XRD, FTIR, DRS, SEM and BET surface area analysis. The photoactivity of nano-TiO₂ particles was evaluated by photocatalytic degradation of methyl orange. The results show that nano-TiO₂ particles, prepared by miniemulsion, have a higher efficiency in decomposition of methyl orange than P25 and TiO₂ synthesized via the sol-gel method due to the high surface area and optimum phase constitution.     

矿渣对道路混凝土表面透气和吸水性影响

研究不同掺量的矿渣对硅酸盐水泥混凝土表面透气和吸水性能的影响规律,并对其作用机理进行探讨。利用X射线衍射(XRD)、扫描电镜(SEM)和孔隙分析(MIP),对水化生成物、混凝土微观结构和孔结构及其孔分布进行分析。结果表明:添加矿渣可以改善混凝土的表面透气和吸水性能。掺加15%矿渣对改善混凝土表面透气和吸水效果较显著,其透气和吸水系数分别为0.035和0.8,均小于基准混凝土。这是由于矿渣具有微集料效应和潜在水化活性造成的。随着矿渣掺量的增加,混凝土表面的透气和吸水性能随之降低。     

Sintering mechanism of blast furnace slag–kaolin ceramics

A general ceramics processing scheme by cold uniaxial pressing and conventional sintering process have been used to prepare ceramics from mixtures of blast furnace slag (BFS) and kaolin (10%, 30% and 50% kaolin). The properties of the ceramics were studied by measuring linear shrinkage, bulk density, apparent porosity and mechanical properties of samples heated at temperatures from 800 °C to 1100 °C. The formed crystalline phases were characterized using X-ray diffraction (XRD) and scanning electron microscope (SEM). Slag melt formed at relatively low temperatures (800–900 °C) modified the sintering process to liquid phase sintering mechanism. Combination of BFS with 10% kaolin gave the highest mechanical properties, densification and shrinkage at relatively low firing temperatures. The crystalline phases were identified as gehlenite (Ca₂Al₂SiO₇) in both BFS and BFS with 10% kaolin samples. Anorthite (CaAl₂Si₂O₈) phase increased with increasing kaolin contents. In the case of kaolin-rich mixtures (30% and 50% kaolin), increased expansion took place during firing at temperatures in the range 800–1000 °C. This effect could be attributed to the entrapment of released gases.     

Fabrication and characterization of superhydrophobic high opacity paper with titanium dioxide nanoparticles

A new type of paper with superhydrophobic surface was prepared by addition of modified nano-TiO₂ to cellulose pulp. Nano-TiO₂ powder was first dispersed with a high-speed homogenizer, followed by surface modification with the coupling agent, 3-(trimethoxysilyl) propyl methacrylate (MPS). The superhydrophobic and opaque paper was obtained by adding the modified nano-TiO₂ to plant fiber to change its characteristics from hydrophilic to hydrophobic. The effects of the initial content of the coupling agent MPS used, on the weight percentage of MPS attached on the surface of nano-TiO₂ were studied. The obtained paper was characterized by contact angle measurements and SEM. The results showed that the water contact angles for the modified paper ranged from 126.5 to 154.2°, and the sliding angle was <3°. Moreover, many well-dispersed nano-TiO₂ protuberances were observed on the surface of the paper, which further confirmed that the obtained paper was superhydrophobic on account of its nanostructure. Comparative optical studies performed on the paper handsheets revealed a much higher opacity for the sample with the MPS-modified-TiO₂ nanoparticles.     

Alkali-activated slag concrete: Fresh and hardened behaviour

The behaviour of fresh and hardened alkali-activated slag (AAS) and OPC concretes was compared and the effect of mixing time assessed. OPC and AAS concrete slump and rheological results proved to differ, particularly when the slag was activated with waterglass (WG). The nature of the alkaline activator was the key determinant in AAS concrete rheology. Bingham models afforded a good fit to all the OPC and AAS concretes. In OPC and NaOH-activated AAS concretes, longer mixing had an adverse effect on rheology while improving hardened performance only slightly. In WG-AAS concrete, longer mixing times, improved mechanical properties and also rheological behaviour was enhanced, in which those conditions were required to break down the microstructure. Longer mixing raised thixotropy in OPC and NaOH-activated AAS concretes, but lowered the value of this parameter in waterglass-activated slag concrete.     

石膏对石灰石粉水泥基材料水化及硬化性能的影响

针对我国目前非荷载作用下混凝土严重开裂的问题,以"比表面积较低的水泥熟料-比表面积较高的掺合料-足够掺量的石膏"构成的胶凝材料体系为研究对象,通过水化热速率、X射线衍射(XRD)、扫描电子显微镜(SEM)、压汞法(MIP)及热重-差示扫描量热法(TG-DSC)等手段,研究石膏对石灰石粉水泥基材料水化及硬化体微结构的影响。结果表明,石灰石粉能够加速C3A与石膏作用生成钙矾石相,在足量石膏存在的条件下,能够阻碍钙矾石向低硫型硫铝酸钙转变;石灰石粉的掺入与石膏一起延缓了C3A的水化;在石灰石粉和足够石膏同时存在的情况下,C3A水化生成具有膨胀性的水化碳铝酸钙和高硫型硫铝酸钙,补偿了收缩,提高了水泥基材料的抗裂性能;熟料粗磨、掺合料细磨及较高石膏掺量的胶凝材料体系配制的C30和C50等级混凝土,强度能持续增大,从28d到180d,强度分别提高了36.7%和33.3%,混凝土结构紧密、孔隙率低、有害孔含量少。     

Mechanical properties and microstructure of steel/iron slag blended mortar

This study aims to examine the mechanical properties and microstructures of steel/iron slag blended mortar (SISBM), which contains two by-products of the steel and iron industries: steel slag (basic oxygen furnace slag, BOFS) and iron slag (blast furnace slag, BFS). Test results indicate that steel slag will have effective hydration reactions with iron slag and contribute strength. The optimal mixing ratio of steel to iron slags was 3:7 (by weight), and the compressive strength was about 83.59% compared with that of ordinary Portland cement mortar (OPM). The strength development was similar to that of OPM and the strength increased as the curing period increased. The X-ray diffraction analysis results implied that the main products of hydration could be C–S–H, C–A–S–H, CaO–MgO–Al₂O₃–SiO₂, Fe_0.974O, and C₄AF. The scanning electron microscope images indicated that the distribution of Ca(OH)₂ and CaCO₃ increased as the inclusion of steel slag increased, perhaps resulting from insufficient reactions between BFS and Ca(OH)₂ and f-CaO due to excessive BOFS. In addition, the results indicated that the density of OPM was superior to that of SISBM. This may be the reason for lower strength of SISBM compared to OPM.     

Alkali-aggregate behaviour of alkali-activated slag mortars: Effect of aggregate type

The alkali–silica reaction in waterglass-alkali-activated slag (waterglass-AAS) and ordinary Portland cement (OPC) mortars was evaluated using three types of (siliceous and calcareous) aggregates. The tests were conducted to the ASTM C1260-94 standard test method. The mortars were studied by volume stability, mechanical strength and Hg intrusion porosity. The ASR products were studied with XRD, FTIR and SEM/EDX techniques.According to the results obtained, under the test conditions applied in this study, waterglass-AAS mortars are stronger and more resistant to alkali-aggregate reactions than OPC mortars. When the mortars were made with a reactive siliceous aggregate, expansion was four times greater in the OPC than in the AAS material. When a reactive calcareous (dolomite) aggregate was used, no expansion was detected in any of the mortars after 14 days, although the characterization results showed that the dolomite had reacted and calcareous-alkali products (brucite) had in fact formed in both mortars. These reactive processes were more intense in OPC than in AAS mortars, probably due to the absence of portlandite in the latter. When the calcareous aggregate was non-reactive, no expansions were observed in any of the mortars, although a substantial rise was recorded in the mechanical strength of AAS mortars exposed to the most aggressive conditions (1 M NaOH and 80 °C).     

Efficiency of internal curing by superabsorbent polymers (SAP) in PC-GGBS mortars

This paper evaluates the effect of superabsorbent polymers (SAP) on hydration and microstructure of PC-GGBS mortars. Development of autogenous shrinkage, microstructural characteristics (MIP/SEM) and compressive strength were analysed during the first 90 days. Four levels of Portland cement (PC) replacement by GGBS (0%, 25%, 50% and 75%) and two types of SAP with different water absorption capacities were considered. The results proved the efficiency of internal curing by SAPs in PC-GGBS systems due to significant reduction in autogenous shrinkage, especially for higher contents of GGBS. SAP facilitates GGBS hydration activated by portlandite; its products can be deposited into the nano pores leading to a small relative expansion of the hardened bulk volume. This process is initiated during the second week and it lasts until the sixth week. Despite increased total porosity, compressive strength of SAPs modified mortars is comparable to the reference samples for low GGBS contents in advanced ages.