Effect of the clinker composition on the threshold limits for Cu,Sn or Zn

The threshold limit corresponds to the maximum amount of trace element that could be incorporated into clinker whilst reaching the limit of solid solution of its four major phases (C₃S, C₂S, C₃A and C₄AF). For Cu, Zn or Sn, these threshold limits in a standard clinker (65% C₃S, 18% C₂S, 8% C₃A and 8% C₄AF) were equal to 0.35, 0.7 and 1 wt.% respectively (Gineys et al., in press). This paper presents the effect of the clinker composition on these threshold limits. Laboratory made clinkers having different mineralogical compositions was characterised by XRD and SEM. Results showed that the threshold limits for Cu, Zn or Sn were consistent. The threshold limit for Sn was affected by the Bogue content in interstitial phases. On the other hand, the threshold limit for Zn was affected by the Bogue content in C₃S of clinker while that of Cu was unaffected by any modifications of clinker composition.     

Impact of triethanolamine on the sulfate balance of Portland cements with mixed sulfate carriers

The differences between the hydration of Portland cements with single and with mixed sulfate carriers in the presence of triethanolamine (TEA) were investigated, and possible mechanisms were proposed. Without TEA, cements with different types of sulfate carriers (gypsum, hemihydrate, anhydrite, and mixture of these) have a comparable hydration process at the same molar amount of calcium sulfate. At a TEA dosage of 0.5 wt.%, the sample with a mixture of three sulfate carriers shows substantially stronger retardation of the C₃S (This publication uses the cement chemist notation: C₃S = Ca₃SiO₅, C₂S = Ca₂SiO₄, C₃A = Ca₃Al₂O₆, C₄AF = Ca₂(Al, Fe)₂O₅.) hydration than the cements with only one of these sulfate carriers, which is likely caused by the rapid formation of ettringite and the fast depletion of all sulfate carriers. These effects indicate that TEA influences the balance of sulfate carriers with aluminate-containing clinker phases. On the one hand, TEA can disturb the original sulfate balance due to the accelerated dissolution of aluminate-containing clinker phases, especially C₄AF. On the other hand, these effects are closely related to the types and amounts of the sulfate carriers in the cement. A higher amount of sulfate carriers can minimize the TEA-related retardation of the C₃S hydration, and hemihydrate shows the strongest impact at the same calcium sulfate quantity.     

Incorporation of trace elements in Portland cement clinker: Thresholds limits for Cu, Ni, Sn or Zn

This paper aims at defining precisely, the threshold limits for several trace elements (Cu, Ni, Sn or Zn) which correspond to the maximum amount that could be incorporated into a standard clinker whilst reaching the limit of solid solution of its four major phases (C₃S, C₂S, C₃A and C₄AF). These threshold limits were investigated through laboratory synthesised clinkers that were mainly studied by X-ray Diffraction and Scanning Electron Microscopy. The reference clinker was close to a typical Portland clinker (65% C₃S, 18% C₂S, 8% C₃A and 8% C₄AF). The threshold limits for Cu, Ni, Zn and Sn are quite high with respect to the current contents in clinker and were respectively equal to 0.35, 0.5, 0.7 and 1 wt.%. It appeared that beyond the defined threshold limits, trace elements had different behaviours. Ni was associated with Mg as a magnesium nickel oxide (MgNiO₂) and Sn reacted with lime to form a calcium stannate (Ca₂SnO₄). Cu changed the crystallisation process and affected therefore the formation of C₃S. Indeed a high content of Cu in clinker led to the decomposition of C₃S into C₂S and of free lime. Zn, in turn, affected the formation of C₃A. Ca₆Zn₃Al₄O₁₅ was formed whilst a tremendous reduction of C₃A content was identified. The reactivity of cements made with the clinkers at the threshold limits was followed by calorimetry and compressive strength measurements on cement paste. The results revealed that the doped cements were at least as reactive as the reference cement.     

Influence of precuring on high pressure steam hydration of tricalcium silicate in the presence of other constituents of portland cement

The influence of precuring at room temperature on the autoclave hydration of C₃S in the presence of other constituents of clinker and of gypsum was studied. C₃A, C₄AF and β-C₂S hampered the formation of C₃SH_1.5 and, especially for short precuring times, favored the formation of α-C₂SH. Gypsum hampered the formation of both the crystalline hydrated silicates. When the steam treatment took place after a long precuring, C-S-H was the prevailing hydrated silicate formed.     

The influence of an ion-exchange resin on the kinetics of hydration of tricalcium silicate

The addition of a finely-ground ion-exchange resin makes it possible to modify the hydration kinetics of C₃S pastes. Analyses of the liquid phase in pastes and more dilute suspensions show that the resin exchanges calcium ions for sodium ions very rapidly during the early stage of hydration and therefore the concentration of silica in solution increases. The resin impacts the hydration of C₃S by other mechanisms which depends on the resin quantity added. For a high resin quantity, the induction period is very short, but the longer-term hydration is enhanced compared to a reference sample without resin. We hypothesize that the surface of the resin can provide sites for the nucleation and growth of C-S-H hydrates and/or portlandite far away from the surface of the C₃S grains. This consequently increases the quantity of hydrates that can precipitate before a continuous hydrate layer forms over the surfaces of C₃S particles.     

Effect of melt composition on tricalcium silicate: Its formation and composition

This study describes the effect of several clinker melt phases on the kinetics of tricalcium silicate (C₃S) formation. At a given burning temperature, a maximum amount of C₃S was apparently formed for a given melt. The iron-rich melt phases promoted the consumption of lime and the rate of formation of C₃S in the order C₂F > C₄AF > C₆A₂F > C₃A. Results of the microprobe analyses of C₃S and melt phases suggest that iron and alumina entered the C₃S lattice during clinkering. Silica also apparently entered the melt phase, although conclusive evidence could not be obtained.     

Effect of prolonged heating at elevated temperatures on the phase composition and textures of portland cement clinker

Two kinds of portland cement clinker with widely different MgO and SO₃ content were reheated for a long time at elevated temperature. With the clinker rich in MgO and SO₃, alite increased while belite decreased in quantity after reheating. The alite crystals, overgrown with new precipitates, gave zonal structures. Thin platy hexagonal crystals were occasionally nucleated and grown separately in the bulk liquid. High degree of supercooling produced dismembered dendritic crystals of belite. The C/S ratio of the interstitial phase decreased with reheating. The above changes occur in association with the process in which the interstitial liquid, initially variable in basicity, is transformed to the uniform and most acidic one. This process is controlled by the counterdiffusion of CaO and SiO₂, the rate of which is such that, in normal clinker processing, the clinkering reaction terminates before reaching this stage. Rise in firing temperature increases the concentration of SiO₂ in the interstitial liquid and leads to higher solid C₃S/C₂S ratio in clinker. The presence of MgO and SO₃ in abundance lowers the viscosity of the liquid and hence accelerates the changes. No appreciable change could be recognized for the clinker low in MgO and SO₃ content.     

Investigations on the relationship between porosity structure and strength of hydrated portland cement pastes III. Effect of clinker composition and gypsum addition

In as series of cements made out of clinkers with variable C₃A/C₄AF ratios and containing different amounts of gypsum, the strength development and the composition of the hydrated material were studied. For a single clinker composition the obtained strength appears to be just a function of porosity. Variations in the C₃A/C₄AF content affected both the structure and intrinsic properties of the formed hydration products and thus the position of the pertinent strength porosity plots.     

On the role of free calcium oxide in expansive cements

Results of research, tending to elucidate the effect of free CaO content in portland cement clinker upon the expansion of specimens of expansive cements, prepared in semi-commercial scale by intergrinding of portland cement clinker, C₄A₃$\text{S}$-phase containing special clinker and gypsum, by means of examination of liquid phase composition and porosity of mortars, have been presented. Special clinker was obtained by burning a mix of limestone, fly ash with high Al₂O₃ content and gypsum. Obtained results confirm the advantageous effect of free lime upon the hydration process and properties of expansive cements. The concentration of CaO in the liquid phase seems to influence rather the rate of ettringite formation than the size of its crystals.     

Characterization by solid-state NMR and selective dissolution techniques of anhydrous and hydrated CEM V cement pastes

The long term behaviour of cement based materials is strongly dependent on the paste microstructure and also on the internal chemistry. A CEM V blended cement containing pulverised fly ash (PFA) and blastfurnace slag (BFS) has been studied in order to understand hydration processes which influence the paste microstructure. Solid-state NMR spectroscopy with complementary X-ray diffraction analysis and selective dissolution techniques have been used for the characterization of the various phases (C₃S, C₂S, C₃A and C₄AF) of the clinker and additives and then for estimation of the degree of hydration of these same phases. Their quantification after simulation of experimental ²⁹Si and ²⁷Al MAS NMR spectra has allowed us to follow the hydration of recent (28 days) and old (10 years) samples that constitutes a basis of experimental data for the prediction of hydration model.     

Effect of alkalies on portland cement hydration: I. Alkali oxides incorporated into the crystalline lattice of clinker minerals

Na₂O and K₂O in absence of SO₃ are incorporated preferentially into the C₃A phase of portland clinker altering its modification from cubic to orthorhombic. The hydration of Na₂O doped C₃A is retarded and that of K₂O doped C₃A accelerated in the resultant cements. The progress of C₃S hydration and strength development are not altered noticeably. The setting time is extended in Na₂O-doped and shortened in K₂O-doped cements.     

The influence of polymers on the hydration of portland cement phases analyzed by soft X-ray transmission microscopy

The soft X-ray transmission microscope, which allows the in situ observation of wet samples of cement at normal pressures with high spatial resolutions (25 nm), was used to observe and compare the effects of two polymers — a water soluble polymer (HPMC — hydroxypropyl methylcellulose) and a latex [EVA-poly(ethylene-co-vinyl acetate)] on the early hydration of C₃S and C₃A. These polymers are used to modify the properties of fresh and hardened mortars and concretes, especially when adhesive characteristics are required. The images show that the cellulose ether delays the hydration of the cementitious particles and promotes the formation of inner products rather than outer products. On the other hand, EVA particles agglomerate around the hydrating C₃S grains, and act as nucleation agents in the development of the composite microstructure. While HPMC slightly changed the aspect of C₃A hydration, EVA inhibited or even prevented the formation of ettringite crystals during the early stage of hydration, and resulted in a cloud of small, bright particles concentrated around the hydrating C₃A grains.     

城市污水厂剩余活性污泥生产生态水泥

引言 随着我国对城市生活污水处理能力以及污水处理率的不断提高,污水处理厂的污泥产量也不断增长.由于其产量较大,含水率高,同时含有大量有机质、病菌、寄生虫和重金属等[1-2],如果处理处置不当,会给环境带来严重的二次污染.     

The effect of cooling rate on the fluidity of mortar made from kiln clinker

The effect of cooling rate on the fluidity of mortar is investigated. This study proposed a model for the early hydration of C₃A In this model, the amount of C₃A hydrated initially is almost proportional to the size of C₃A and the C₃A content. This model explained all the experimental results.The C₄AF crystallite size calculated by the Rietveld method can be used to measure the cooling rate of clinker. These measurements are consistent with the conventional microscopical method. This method can be a tool to predict the fluidity of mortal.     

Role of delayed release of sulphates from clinker in DEF

Two clinkers rich in sulphate burned in the pilot plant rotary kiln and cements prepared from them were investigated. Clinker richer in sulphate (SO₃=3.6%) contained independent anhydrite grains as well as inclusions of anhydrite in belite. The mortar from it expanded after heat treatment at 90 °C and the addition of Na₂SO₄ or NaOH accelerated and increased this expansion. The expansion occurred irrespective of the fact that the clinker contained only 3% of C₃A, although the C₄AF content was 13%. The second clinker with 2.6% SO₃ contained mainly calcium langbeinite and expanded only when 2% of Na₂SO₄ was added. The SEM examination of the mortars revealed the presence of numerous bands of massive ettringite around sand grains. Agglomerates of cracked ettringite in cement gel were also present. In addition, microcracks were seen inside the darker C-S-H gel. The conclusion is that anhydrite forming inclusions in belite gives an expanding mortar after heat treatment at 90 °C independently of the tricalcium aluminate content. Such clinkers are not typical of industrial conditions. The expansion is caused by the bands of massive ettringite as well as its agglomerates present in the cement gel and nanometric ettringite in the C-S-H phase.     

The effects of seeding C3S pastes with afwillite

The addition of 1–4% s/s (dry solids by mass of C₃S) of afwillite (C₃S₂H₃) seeds to C₃S pastes made with two different commercial polyacrylate-based superplasticizers (SP) allows the pastes to be cast at low water/C₃S mass ratios (w/c) and overcomes the hydration retardation produced by the SPs. SP-free C₃S pastes seeded with afwillite at an initial w/c of 0.50 gave about 30% lower 28-day compressive strengths than the unseeded controls, due to higher porosities. However, at w/c = 0.35, with the addition of 0.4% s/s SP, the afwillite-seeded pastes gave similar or higher strengths than the unseeded controls at all ages tested. Hydration rate data obtained by chemical shrinkage measurements suggest that this is because the degrees of hydration of the C₃S in the low w/c afwillite-seeded pastes made with added SP reach higher values than in the unseeded controls, compensating for the difference in density of the hydrates.     

The Effect of Magnesium and Zinc Ions on the Hydration Kinetics of C3S

Impure tricalcium silicate (C₃S) in portland cement may contain various foreign ions. These ions can stabilize different polymorphs of C₃S at room temperature and may affect its reactivity. In this paper, the effects of magnesium and zinc on the polymorph type, hydration kinetics, and the hydrate morphology of C₃S were investigated. The pure C₃S has the T1 structure while magnesium and zinc stabilize polymorphs M3 and T2/T3, respectively. The two elements have distinct effects on the hydration kinetics. Zinc increases the maximum heat released. Magnesium increases the hydration peak width. The C–S–H morphology is modified, leading to longer needles in the presence of zinc and thicker needles in the presence of magnesium. Zinc is incorporated into C–S–H, while magnesium is only incorporated slightly, if at all, but rather seems to inhibit nucleation. Implementing experimentally measured parameters for C–S–H nucleation and growth in the μic hydration model captured well the observed changes in hydration kinetics. This supports C–S–H nucleation and growth to be rate controlling in the hydration of C₃S.     

Early hydration of cement constituents with organic admixtures

The hydration of C₂S, C₃S, C₃A, C₄AF and type 1 portlant cement in the presence of calcium lignosulfonate and salicylic acid was studied at a high(20/1) water-cement ratio. The effect of these admixtures on the development, microstructure and surface area of the hydration products was investigated.     

The hydration of Portland cement,C3S and C2S in the presence of a calcium complexing admixture (EDTA)

The effect of EDTA, a calcium chelating agent, on the early hydration of Portland cement, C₃Sand β-C₂S has been studied by solution analysis and electron microscopy. EDTA is a retarded of cement hydration. Under normal conditions of hydration, the silica levels in solution are very low (<0.05 M) but in the presence of EDTA an initial flush of silica appears in the bulk aqueous phase. On continued hydration, following the saturation of EDTA with calcium, the appearance of ‘free’ calcium causes precipitation of C-S-H gel from the bulk solution and changes in microstructure of the colloidal gel around clinker particles in C₃S and β-C₂S pastes are observed. The action of EDTA as a retarding admixture is explained in terms of the membrane model of cement hydration.     

Tetracalcium aluminoferrite hydration in the presence of lime and gypsum

The influence of lime and/or gypsum on the C₄AF hydration was examined and the results were compared with those obtained for the C₃A hydration. Gypsum is more effective than CH in retarding the hydration of C₄AF. Ettringite produced in the C₄AF hydration in the presence of C$\text{S}$·H₂ seems to be more stable than that produced in the C₃AC$\text{S}$·H₂H₂ system. The consumption of gypsum, the transformation of ettringite into monosulfate and the hydration of C₄AF in the C₄AFC$\text{S}$·H₂H₂O system are much slower than those in the C₃AC$\text{S}$·H₂H₂O system. The retardation of hydration of C₄AF or C₃A in the presence of C$\text{S}$·H₂ is further increased by the add$\text{i}$tion of CH.