The hydration characteristics of MSWI fly ash slag present in C3S

This paper reports on an investigation of the hydration characteristics of C₃S and the mixing of C₃S with municipal solid waste incinerator (MSWI) fly ash slag. The results can be summarized as follows: TGA observations show lower amounts of CSH and Ca(OH)₂ in samples that incorporated MSWI slag into C₃S, possibly due to the partial replacement of the C₃S by slag with less activity. In general, the incorporation of slag into C₃S decreases the initial hydration reactions, but it increases the pozzolanic reactions at a later stage by consuming Ca(OH)₂. As the hydration precedes, the C₃S peaks decrease, up to 90 days, due to the activation of the slag by liberated Ca(OH)₂. As well, the amount of hydration products (Ca(OH)₂) from the pure C₃S pastes are more than for the C₃S-slag pastes. Moreover, the degree of hydration of the C₃S pastes increases with the curing time, the C₃S-slag paste also shows that lower hydration degree values at all ages.     

The influence of municipal solid waste incinerator fly ash slag blended in cement pastes

This study investigates the effect of mixing Type I, Type II, and Belite cements with municipal solid waste incinerator (MSWI) fly ash slag-blended cement (FASBC). The experimental results showed that a 10-40% slag replacement of by caused an increase in the initial and the final setting time. The toxicity characteristic leaching procedure (TCLP) results show that the heavy metal content met the Environmental Protection Administration (EPA) regulatory limits. From the results, it can be seen that the effect of the replacement of 10-40% of the cement by slag caused an increase in the initial and final setting time. Compressive strength results indicate that the slag-blended cement (SBC) pastes had slower compressive strength development in the early stages, but this strength obviously increased at later ages. Variations in the Portland cements can affect early strength development but have no significant effect on the degree of hydration at later ages. MSWI slag gives a relatively slower increase in early strength but may show a greater degree of reaction at later ages.     

Interaction between BaCO3 and OPC/BFS composite cements at 20 °C and 60 °C

A BaCO₃ slurry, containing radioactive ¹⁴C, is produced during the reprocessing of spent nuclear fuel. This slurry is encapsulated in a Portland-blastfurnace slag composite cement. The effect of BaCO₃ on the hydration of OPC and Portland-blastfurnace slag cements has been studied in this work. Samples containing a simulant BaCO₃ slurry were cured for up to 720 days at 20 and 60 °C and analysed by XRD, SEM(EDX) and ICC. BaCO₃ reacted with OPC to precipitate BaSO₄ from a reaction between soluble sulfate and BaCO₃. Calcium monocarboaluminate subsequently formed from the carbonate released. The monocarboaluminate precipitated as crystals in voids formed during hydration. At 60 °C in OPC, it was not identified by XRD, suggesting the phase is unstable in this system around this temperature. In the Portland-blastfurnace slag cements containing BaCO₃, less monocarboaluminate and BaSO₄ were formed, but the hydration of BFS was promoted and monocarboaluminate was stable up to 60 °C.     

Effect of calcium formate as an accelerator on the physicochemical and mechanical properties of pozzolanic cement pastes

The aim of the present work is to study the effect of calcium formate (CF) as an accelerator on the properties of pozzolanic cement pastes. Three types of cements were used in this investigation. These cements were OPC and pozzolanic cements containing 80 mass% OPC and 20 mass% silica fume (SF) or 20 mass% ground clay bricks (GCB). The dosages of CF were 0.00, 0.25, 0.50, and 0.75 mass% of cement. The compressive strength, total porosity, and hydration kinetics such as free lime and combined water contents were investigated. The results obtained in this study showed that the addition of CF shortens the initial and final setting times and increases the compressive strength and combined water content as well as gel/space ratio at all ages of hydration. On the other hand, it decreases the total porosity. CF activates the liberation of Ca(OH)₂ of OPC pastes. The free lime content of pozzolanic cement in the presence of CF increases up to 7 days, then decreases at the later ages of hydration.     

Influence of polymer on cement hydration in SBR-modified cement pastes

The influence of styrene-butadiene rubber (SBR) latex on cement hydrates Ca(OH)₂, ettringite, C₄AH₁₃ and C-S-H gel and the degree of cement hydration is studied by means of several measure methods. The results of DSC and XRD show that the Ca(OH)₂ content in wet-cured SBR-modified cement pastes increases with polymer-cement ratio (P/C) and reaches a maximum when P/C is 5%, 10% and 10% for the pastes hydrated for 3 d, 7 d and 28 d, respectively. With wet cure, appropriate addition of SBR promotes the hydration of cement, while the effect of SBR on the content of Ca(OH)₂ and the degree of cement hydration is not remarkable in mixed-cured SBR-modified cement pastes. XRD results illustrate that SBR accelerates the reaction of calcium aluminate with gypsum, and thus enhances the formation and stability of the ettringite and inhibits the formation of C₄AH₁₃. The structure of aluminum-oxide and silicon-oxide polyhedron is characterized by ²⁷Al and ²⁹Si solid state NMR spectrum method, which shows that tetrahedron and octahedron are the main forms of aluminum-oxide polyhedrons in SBR-modified cement pastes. There are only [SiO₄]⁴⁻ tetrahedron monomer and dimer in the modified pastes hydrated for 3 d, but there appears three-tetrahedron polymer in the modified pastes hydrated for 28 d. The effect of low SBR dosage on the structure of aluminum-oxide and silicon-oxide polyhedron is slight. However, the combination of Al³⁺ with [SiO₄]⁴⁻ is restrained when P/C is above 15%, and the structure of Al³⁺ is changed obviously. Meantime, the polymerization of the [SiO₄]⁴⁻ tetrahedron in C-S-H gel is controlled.     

Chemistry of cement hydration in polymer-modified pastes containing lead compounds

The role of polymeric additives on the hydration process of cement pastes admixed with a lead compound (Pb₃O₄) was investigated. Three series of pastes were prepared: the reference series, mixing water with Ordinary Portland Cement (OPC), and two series in which whether a styrene–butadiene rubber latex or a superplasticiser based on acrylic-modified polymer was added to the pastes. For each series, 5 and 10 wt% of Pb were mixed with the pastes. Phase analysis and microstructural characterisation were carried out by means of X-ray powder diffraction and SEM–EDX. Thermogravimetric analysis was performed to monitor the hydration degree of the three pastes; indeed, quantitative determination of portlandite and calcite was performed.Dynamic leach tests were performed on solidified monoliths to evaluate the effective immobilisation of Pb₃O₄. After 384 h leaching, excellent results were obtained by pastes mixed with superplasticiser that showed a cumulative release of Pb equal to 0.62 mg/l for samples containing 5 wt% of Pb, and equal to 0.84 mg/l for samples bearing 10 wt% of Pb.     

Estimation of the degree of hydration of blended cement pastes by a scanning electron microscope point-counting procedure

A scanning electron microscope (SEM) point-counting technique was employed to study the hydration of plain portland and blended cement pastes containing fly ash or slag. For plain portland cement pastes, the results for the degree of cement hydration obtained by the SEM point-counting technique were consistent with the results from the traditional loss-on-ignition (LOI) of nonevaporable water-content measurements; agreement was within ±10%. The standard deviation in the determination of the degree of cement hydration via point counting ranged from ±1.5% to ±1.8% (one operator, one sample). For the blended cement pastes, it is the first time that the degree of hydration of cement in blended systems has been studied directly. The standard deviation for the degree of hydration of cement in the blended cement pastes ranged from ±1.4% to ±2.2%. Additionally, the degrees of reaction of the mineral admixtures (MAs) were also measured. The standard deviation for the degree of fly ash reaction was ±4.6% to ±5.0% and ±3.6% to ±4.3% for slag. All of the analyses suggest that the SEM point-counting technique can be a reliable and effective analysis tool for use in studies of the hydration of blended cement pastes.     

Hydration characteristics of waste sludge ash utilized as raw cement material

In this study, the hydration characteristics and the engineering properties of three types of eco-cement pastes, including their compressive strength, speciation, degree of hydration, and microstructure, were studied and compared with those of ASTM type I ordinary Portland cement. The results indicate that it is feasible to use sludge ash and steel-making waste to replace up to 20% of the mineral components of the raw material of cement. Furthermore, all the tested clinkers met the toxicity characteristic leaching procedure requirements. The major components of Portland cement, C₃S (i.e., 3CaO·SiO₂), C₂S (i.e., 2CaO·SiO₂), C₃A (i.e., 3CaOAl₂O₃) and C₄AF (i.e., 4CaO·Al₂O₃·Fe₂O₃), were all found in the waste-derived clinkers. All three types of eco-cements were confirmed to produce calcium hydroxide (Ca(OH)₂) and calcium silicate hydrates (CSH) during the hydration process, increasing densification with the curing age. The thermal analysis results indicate that the hydration proceeded up to 90 days, with the amount of Ca(OH)₂ and CSH increasing. The chemical shift of the silicates, and the resultant degree of hydration, and the increase in the length of the CSH gels with the curing age, were confirmed by ²⁹Si NMR techniques. Compressive strength and microstructural evaluations confirm the usefulness of eco-cement.     

Influence of the availability of calcium on the hydration of tricalcium aluminate (C3A) in seawater-mixed C3A–gypsum system

This work examined the effects of seawater (SW) on the hydration of tricalcium aluminate (C₃A) in C₃A–gypsum and C₃A–gypsum–Ca(OH)₂ systems through the characterization of hydration heat release, the evolution of aqueous phase composition and hydration products with the hydration time. It was found that SW increased the dissolution driving force of C₃A and solubility of gypsum, which accelerated the early hydration of C₃A and the formation of ettringite (AFt), leading to a higher hydration degree of C₃A at an early age compared with the deionized (DI) water–mixed pastes. After gypsum depletion to form AFt, and in the absence of Ca(OH)₂, the formation of chloroaluminate hydrates was slower due to the insufficient Ca resulted in an accumulation of Al in solution. This would delay the subsequent transformation of AFt to monosulfate (SO₄–AFm) and the formation of hydrogarnet (C₃AH₆), which would further reduce the hydration degree of the C₃A at the later ages. However, in the presence of Ca(OH)₂, the hydration degree of C₃A–gypsum–Ca(OH)₂ at later ages was increased, which was similar to that of the corresponding DI pastes. This can be inferred that the amount of Ca available in SW-mixed cement concrete can affect the hydration degree of C₃A in cement.     

The effects of Cr2O3, Cu(OH)2, ZnO and PbO on the compressive strength and the hydrates of the hardened C3A paste

The purpose of this paper is to investigate the effects of Cr₂O₃, Cu(OH)₂, ZnO or PbO on the hydration of C₃A and characterization of its hydrates. C₃A pastes adding the above compound were examined on the basis of the hydration products and their structure, compressive strength and rate of early hydration.     

The impact of Al2O3 amount on the synthesis of CASH samples and their influence on the early stage hydration of calcium aluminate cement

In this work the impact of Al₂O₃ amount on the synthesis (200?°C; 4–8?h) of calcium aluminium silicate hydrates (CSAH) samples and their influence on the early stage hydration of calcium aluminate cement (CAC) was examined. It was found that the amount of Al₂O₃ plays an important role in the formation of calcium aluminate hydrates (CAH) because in the mixtures with 2.7% Al₂O₃ only calcium silicate hydrates (CSH) intercalated with Al³⁺ ions were formed. While in the mixtures with a higher amount of Al₂O₃ (5.3–15.4%), calcium aluminate hydrate – C₃AH₆, is formed under all experimental conditions. It is worth noting that the largest quantity of mentioned compound was obtained after 4?h of hydrothermal treatment, in the mixtures with 15.4% of Al₂O₃. It was proved that synthesized C₃AH₆ remain stable up to 300?°C and at higher temperature (945?°C) recrystallized to mayenite (Ca₁₂Al₁₄O₃₃), which reacted with the rest part of CaO and amorphous structure compound, resulting in the formation of gehlenite (Ca₂Al₂SiO₇). Moreover, the synthesized C₃AH₆ addition induced the early stage of CAC hydration. Besides, in the samples with an addition, the induction period was effectively shortened: in a case of pure CAC (G70) paste, hydration takes about 6–6.5?h, while with addition – only 2–2.5?h. The synthesized and calcinated compounds was characterized by using XRD and STA analysis.     

-3 ℃养护下考虑矿物掺合料影响的水泥水化程度计算模型

针对多年冻土地区工程施工时混凝土养护的问题,采用10%、20%、30%的矿粉和粉煤灰替代量等量替代水泥,测试了-3 ℃恒温养护条件下0.38水胶比水泥浆体在各个龄期的水泥水化热,计算了水泥水化程度;分析了龄期及矿物掺合料对水泥水化程度的影响规律,建立了综合考虑龄期和矿物掺合料替代量的水泥水化程度计算模型.结果表明:-3 ℃恒温养护下,矿物掺合料等量替代水泥,水泥浆体的水化程度会降低,粉煤灰降低水化程度的值要比矿粉高;在相同矿物掺合料替代量下,随着龄期的增长,矿物掺合料对水泥水化程度的影响逐渐减弱;同一龄期时,随着矿物掺合料的增加,矿物掺合料对水泥水化程度的影响逐渐增强;利用建立的模型计算了分别掺入15%矿粉和粉煤灰的水泥水化程度,与实测值相比,计算值偏离值较少,预测精度较高.     

Predicting Ca(OH)2 content and chemical shrinkage of hydrating cement pastes using analytical approach

A semiempirical model is proposed to predict the evolution of chemical shrinkage and Ca(OH)₂ content of cement paste at early age of hydration. The model is based on chemical equations and cement compound hydration rates. Chemical shrinkage and Ca(OH)₂ amount are computed using the stoichiometric results of the hydration reactions considered in the model and the density of hydration products and reactants. The model validation is conducted by comparison between computed and experimental results achieved on ordinary cement pastes with different water-to-cement (w/c) ratios (0.25, 0.30, 0.35 and 0.40) cured at 10, 20, 30, 40 and 50 °C, respectively. Hydration degree and Ca(OH)₂ content are determined using the thermogravimetric analysis (TGA) and chemical shrinkage evolution using a gravimetric method.The comparison reveals a good consistency between modelled and experimental data at early age of hydration.     

Investigation of blended cement hydration by isothermal calorimetry and thermal analysis

Hydration of portland cement pastes containing three types of mineral additive; fly ash, ground-granulated slag, and silica fume was investigated using differential thermal analysis, thermogravimetric analysis (DTA/TGA) and isothermal calorimetry. It was shown that the chemically bound water obtained using DTA/TGA was proportional to heat of hydration and could be used as a measure of hydration. The weight loss due to Ca(OH)₂ decomposition of hydration products by DTA/TGA could be used to quantify the pozzolan reaction. A new method based on the composition of a hydrating cement was proposed and used to determine the degree of hydration of blended cements and the degree of pozzolan reaction. The results obtained suggested that the reactions of blended cements were slower than portland cement, and that silica fume reacted earlier than fly ash and slag.     

Preparation and ladle slag resistance mechanism of MgAlON bonded Al2O3 -MgAlON-Zr2Al3C4-(Al2CO)1-x(AlN)x refractories

MgAlON bonded Al₂O₃-MgAlON-Zr₂Al₃C₄-(Al₂CO)_1-x(AlN)_x refractories were prepared at high temperatures from 1300?℃ to 1600?℃ in N₂-flowing based on the design of Al-AlN core-shell. The refractory prepared at 1500?℃ was chosen to conduct the ladle slag resistance test at 1600?℃ in air. All the refractories are composed of MgAlON, Zr₂Al₃C₄, (Al₂CO)_1-x(AlN)_x and Al₂O₃. The ladle slag resistance test for the chosen refractory presents a good result and there exist two different reaction layers —— the reacted slag layer and the spinel solid solution layer. The reacted slag layer consists of spinel solid solution, Ca-ZrO₂ and gehlenite/gehlenite solution, where the formation of those phases has changed the chemical composition and phase composition of the original ladle slag. The compact spinel solid solution layer forms by the decomposition of MgAlON into rich-Al₂O₃ spinel and the incorporation of Mn²⁺ and Fe^2+/3+ into rich-Al₂O₃ spinel, which plays an important role in slag penetration resistance.     

Hydration of cement — role of triethanolamine

Following addition of 0.1, 0.25, 0.35, 0.5 and 1.0 per cent triethanolamine, studies have been made of the hydration and hardening characteristics of (a) tricalcium aluminate, (b) tricalcium aluminate + gypsum, (c) tricalcium silicate, (d) dicalcium silicate, and (e) portland cement. Triethanolamine (TEA) accelerated the hydration of 3CaO.Al₂O₃ and 3CaO.Al₂O₃-CaSO₄.2H₂O systems and extended the induction period of the hydration of 3CaO.SiO₂. In portland cement paste TEA decreased the strength at all ages and setting characteristics were drastically altered, especially at higher TEA contents. Evidence was obtained also of the formation of a complex of TEA with the hydrating silicate phase.     

Thermodynamic simulation models for predicting Al2O3-MgO castable chemical corrosion

The chemical corrosion of two Al₂O₃-MgO castables (containing distinct binder sources: hydratable alumina or calcium aluminate cement) were evaluated in this work via thermodynamic calculations. Two simulation models were proposed according to the following procedures: (1) firstly the matrix and later the aggregates of the castables were placed, separately, in contact with an industrial basic slag, and (2) the overall chemical composition of the design castables was directly reacted with the molten slag. The theoretical results were further compared with experimental data collected after corrosion cup tests. Although the thermodynamic evaluation of the overall castable compositions was able to identify the phase transformations correctly, a two-step analysis of the matrix components and aggregates particles seems to be the best alternative to evaluate the binder source effect on the corrosion performance of the two Al₂O₃-MgO refractory materials.     

Influence of slag chemistry on the hydration of alkali-activated blast-furnace slag — Part II: Effect of Al2O3

The hydration and microstructural evolution of three alkali activated slags (AAS) with Al₂O₃ contents between 7 and 17% wt.% have been investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na₂SiO₃·5H₂O. The formation of C(A)–S–H and hydrotalcite was observed in all samples by X-ray diffraction, thermal analysis and scanning electron microscopy. Higher Al₂O₃ content of the slag decreased the Mg/Al ratio of hydrotalcite, increased the Al incorporation in the C(A)-S-H and led to the formation of strätlingite. Increasing Al₂O₃ content of the slag slowed down the early hydration and a lower compressive strength during the first days was observed. At 28 days and longer, no significant effects of slag Al₂O₃ content on the degree of hydration, the volume of the hydrates, the coarse porosity or on the compressive strengths were observed.     

Solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pore solutions as a function of hydration time

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement (OPC) and white Portland cement (WPC) pastes were measured during the first 28 days of hydration at room temperature. Saturation indices (SI) with respect to various solid phases known to occur in cement pastes were calculated from a thermodynamic analysis of the elemental concentrations, resulting in good agreement between the two pastes. In agreement with other published work, gypsum was saturated during the first several hours of hydration and then undersaturated thereafter, while portlandite was modestly supersaturated after the first few hours. High levels of supersaturation with respect to ettringite and calcium monosulfoaluminate were calculated, particularly prior to the consumption of gypsum at around 10 h. Results are consistent with published thermodynamic studies that show calcium monosulfoaluminate is metastable with respect to ettringite under normal hydration conditions. Three different ion activity product (IAP) equations for C-S-H were applied to the data. From 10 h onward, each of the IAP values declined gradually over time and the values for the OPC and WPC pastes were in close agreement. The same IAP equations were applied to experimental data from the pure CaO-SiO₂-H₂O system, resulting in good agreement between the cement paste pore solutions and the equilibrium between portlandite and the upper, or metastable, C-S-H solubility curve.     

Corrosion mechanisms of Al2O3/MgAl2O4 by V2O5, NiO, Fe2O3 and vanadium slag

The effects of V₂O₅, NiO, Fe₂O₃ and vanadium slag on the corrosion of Al₂O₃ and MgAl₂O₄ have been investigated. The specimens of Al₂O₃ and MgAl₂O₄ with the respective oxides above mentioned were heated at 10 °C/min from room temperature up to three different temperatures: 1400, 1450 and 1500 °C. The corrosion mechanisms of each system were followed by XRD and SEM analyses. The results obtained showed that Al₂O₃ was less affected by the studied oxides than MgAl₂O₄. Alumina was only attacked by NiO forming NiAl₂O₄ spinel, while the MgAl₂O₄ spinel was attacked by V₂O₅ forming MgV₂O₆. It was also observed that Fe₂O₃ and Mg, Ni, V and Fe present in the vanadium slag diffused into Al₂O₃. On the other hand, the Fe₂O₃ and Ca, S, Si, Na, Mg, V and Fe diffused into the MgAl₂O₄ structure. Finally, the results obtained were compared with those predicted by the FactSage software.