Properties of glass fibres in cement environment

Fibres produced from a soda-silica-zirconia glass were reacted with Portland cement extracts at 20 and 65° C for various lengths of time and their strength and stiffness determined. The results indicate that these glass fibres resist the attack of cement extracts reasonably well at ambient temperatures. Fibre strengths of the order of 1200 to 1300 N mm^–2 are obtainable after 2 years at 20° C, sufficient to reinforce cement, and there is no change in the Young's modulus of the fibre during this period. At higher temperatures both strength and stiffness are reduced but these temperatures are unlikely to be encountered in practice over extended periods of time. When fibres removed from cement composites containing commercially made alkali-resistant glass fibres are examined, it is found that fibre strengths depend very strongly on the environment in which the composites were kept. For air storage, fibre properties remain relatively unaffected but for composites kept under water continuously, an initial loss in fibre strength is observed. This difference in fibre strength is reflected in the relative strength of the cement composites.     

Bentonite-based ammonium polyphosphate cementitious lost-circulation control materials

Bentonite-based cementitious materials have been formulated and evaluated in terms of their properties for use as cementitious lost-circulation control materials in geothermal wells at temperatures up to 300° C. The formulation which appears to have the highest potential as a new cementitious lost-circulation control material is composed of bentonite, ammonium polyphosphate, borax, magnesium oxide, and water. The appropriate combination of these ingredients results in the formation of slurries with viscosities and thickening times adequate to allow placement. After curing at elevated hydrothermal temperatures, the cement produced was characterized by a compressive strength >500 psi (>3.45 MPa) at 2 h age, a permeability to water <2.0×10^–4 Darcy, and a linear expansion >15%. The reaction compound responsible for the strength development at 300° C was found to be an assemblage of interlocking crystals composed of a grown thin-plate crystal. It was inferred that this microcrystalline cluster is associated with montmorillonite and ammonium polyphosphate-based complex formations.     

Properties of high-alumina cement reinforced with alkali resistant glass fibres

The changes in the mechanical properties of cement composites made from high-alumina cement and Cem-FIL AR-glass fibres kept in three different environments up to 10 years are described. While the flexural and impact properties of the composite remained largely unaffected with time in a relatively dry atmosphere, in wet conditions a reduction in strength takes place. In natural weather the 10 year modulus of rupture and impact strength values are 22.8 MIN m^–2 and 6.7 KJ m^–2, respectively, corresponding to the 28 day values of 41.2 MN m^–2 and 22.8 KJ m^–2. These values are significantly better than the corresponding results obtained with Portland cement composites made from Cem-FIL fibres. High-alumina cement composites reinforced by E-glass fibre lose a very large proportion of their flexural and impact strength under wet conditions. The strength reduction with time observed for glass fibre reinforced high-alumina cement composites can be related to two sources: (a) the reduction in the strength of the glass fibre due to chemical corrosion and (b) conversion of the matrix. The latter has greater influence on those composite properties that are matrix controlled such as the Young's modulus whereas any significant reduction in fibre tensile strength is reflected in a corresponding loss in composite tensile and bending strength. Matrix conversion may also influence the fibre-matrix bond.     

Damage in cement pastes exposed to MgCl<Subscript>2</Subscript> solutions

Magnesium chloride (MgCl₂) reacts with cement pastes resulting in calcium leaching and the formation of calcium oxychloride, which can cause damage. This paper examines the damage in different cement pastes exposed to MgCl₂ solutions. Volume change measurement and low temperature differential scanning calorimetry are used to characterize the formation of calcium oxychloride. Thermogravimetric analysis and X-ray fluorescence are used to quantify calcium leaching from Ca(OH)₂ and C-S-H. The ball-on-three-balls test is used to quantify the flexural strength reduction. Calcium oxychloride can form in cement pastes exposed to MgCl₂ solutions with a (Ca(OH)₂/MgCl₂) molar ratio larger than 1. As the MgCl₂ concentration increases, two-stages of flexural strength reduction are observed in the plain cement pastes, with the initial reduction primarily due to calcium leaching from Ca(OH)₂ and the additional reduction due to the calcium leaching from C-S-H (at MgCl₂ concentrations above 17.5 wt%). For the cement pastes containing fly ash, there is a smaller reduction in flexural strength as less Ca(OH)₂ is leached, while no additional reduction is observed at high MgCl₂ concentrations due to the greater stability of C-S-H with a lower Ca/Si ratio. The addition of fly ash can mitigate damage in the presence of MgCl₂ solutions.     

Dehydration and rehydration processes of cement paste exposed to high temperature environments

Microstructural changes of an OPC cement paste after being exposed at various elevated temperatures and further rehydration have been evaluated using ²⁹Si MAS-NMR. Thermogravimetry and XRD are also employed to complement the information. NMR studies of cement paste exposed to high temperatures demonstrate a progressive transformation of C-S-H gel that leads at 450°C, to a modified C-S-H gel. For temperatures above 200°C to a progressive formation of a new nesosilicate. At 750°C, the transformation of C-S-H is complete into the nesosilicate form with a C₂S stoichiometry close to larnite, but less crystalline. Also is observed an increase of portlandite that takes place up to temperatures of 200°C. A progressive increase of calcite formation up to 450°C is noticed. The ettringite disappearance below 100°C is confirmed and the portlandite and calcite are converted to lime at 750°C. The initial anhydrous phases as larnite and brownmillerite remain unaltered during heating. Rehydration of the heated samples (450 and 750°C) shows recrystallization of calcite, portlandite and ettringite, and the C-S-H reformation from the new nesosilicate. The larnite and brownmillerite remain unaltered during rehydration. The developing of damaged due to the formation of microcracking is detected and improved because of rehydration phenomena.     

Interface between zinc phosphate-deposited steel fibres and cement paste

In an attempt to protect steel fibre reinforcements from corrosion and improve their adherence to cement pastes, we deposited a zinc phosphate (ZnPh) conversion coating on the surface of the fibres. At the interfacial contact zones between the cement paste and ZnPh, alkali-induced dissolution caused the dissociation of abundant PO ₄ ^3– ions from the ZnPh. The interaction of PO ₄ ^3– ions with Ca²⁺ ions from the pastes led to the formation of hydroxyapatite and brushite in the vicinity of the dissolved ZnPh surface. Such intermediate calcium phosphate compounds played important roles in (1) improving the cement-fibre interfacial bonds, and (2) repairing the damage of the ZnPh surfaces dissolved by alkali. These processes protected the steel fibre from corrosion.     

Properties of cement composites reinforced with Kevlar fibres

The organic polyamide fibre, Kevlar, is promising as an efficient reinforcement for cementitious matrices. For cement boards, in which chopped fibres are distributed randomly in two dimensions, typical mechanical properties obtained with 1.9 vol% fibre addition are as follows: ultimate tensile strength (UTS) 16 MN m^–2; MOR 44 MIN m^–2; impact strength 17 kJ m^–2. The composite material can be produced by autoclaving if desired and at ambient temperatures they are expected to be durable in most environments. The relatively low decomposition point of Kevlar (as opposed to glass fibres or steel) is a disadvantage for its use in building components which may come into contact with high temperatures, as in a fire. It should be noted that a solvent which is used in the manufacture of the fibre and remains in the fibre in minute quantities has been found to produce cancer in rats. There is no evidence of it causing cancer in humans but the significance of this in terms of a possible health risk, if any, will need to be assessed by the appropriate medical authorities in relation to any applications.     

A new model for the formation of calcium silicate hydrate (C-S-H)

2 S) and tricalcium silicate (C₃S) pastes are generally based on the notion that the dimeric calcium silicate hydrate (C-S-H) that begins to form at the end of the induction period is not a stable phase. It is implied that its formation simply marks the beginning of a rather lengthy equilibration process. This statement is based on the fact that the arrangement of Ca²⁺, OH^– and (Si₂O₇)^6– ions in the C-S-H at this time has no long range order (C-S-H is X-ray amorphous) and the fact that the dimers present in the C-S-H will seemingly polymerize and form dreierketten of the 3n-1 type with time. It has been suggested that the localized layers of C-S-H that contain these dreierketten are structurally related to tobermorite that has been modified in various ways, including omission of bridging tetrahedra and partial replacement of silicate ions by OH^– groups. As an alternative, it is suggested that the dimeric C-S-H that forms soon after setting and hardening may actually be a metastable phase in its own right, a rigid gel precursor phase whose stability is related to its calcium content. Should calcium concentrations fall below the level needed to stabilize the phase (a phenomenon that can be initiated by rising temperatures and/or portlandite precipitation), the dimeric C-S-H will undergo a phase change forming nano-sized fragments of tobermorite/jennite-like C-S-H. The proposed model differs from current models in that it proposes a relatively rapid equilibration followed by a much slower diffusion-controlled phase change process. Although both models predict the same outcome, i.e. a mature paste sample will normally contain a mixture of dimer and dreierketten, the new model gives a raison d’être for the presence of consistently large amounts of dimer throughout the entire hydration process. Received: 17 June, 1999 / Reviewed and accepted: 13 July, 1999     

Brazilian waste fibres as reinforcement for cement-based composites

Fibre reinforced cement-based composites were prepared using kraft pulps from sisal and banana waste and from Eucalyptus grandis pulp mill residues. The study adapted conventional chemical pulping conditions for the non-wood strands and a slurry vacuum de-watering method for composite preparation followed by air-curing. Plain cement paste and Pinus radiata kraft reinforced cement composites were used as reference materials. Mechanical testing showed that optimum performance of the various waste fibre reinforced composites was obtained at a fibre content of around 12% by mass, with flexural strength values of about 20 MPa and fracture toughness values in the range of 1.0–1.5 kJ m⁻². Experimental results showed that, of the waste fibres studied, E. grandis is the preferred reinforcement for low-cost fibre-cement.     

Interfacial debonding and fibre pull-out stresses

Following the development of an improved theoretical analysis of fibre pull-out on the basis of the concept of fracture mechanics in Part II of this paper, the theory has been successfully used to characterize the debonding and frictional pull-out behaviour in cement mortar matrix composites reinforced with steel and glass fibres. It is shown from the plots of partial debond stress, _d ^p , versus debond length, , that these composites are typical of mechanical bonds at the interface. For the steel fibre-cement matrix composites, the theory overestimates the post-debond frictional pull-out stress, _fr, particularly for long embedded fibre length, L, otherwise the prediction agrees well with the experiments for the maximum debond stress, _d ^* . This seems to be a direct result of decay of frictional bonds at the interface region after debonding due mainly to compaction of the porous cement mortar surrounding the fibre, effectively reducing the residual clamping stress, q ₀, arising from shrinkage of the cement matrix. Therefore, a correct theoretical prediction is made for _fr using a lower value of q ₀ while other parameters are kept constant, which gives good agreement with experimental results. For glass fibre-cement matrix composites, an accelerated cure condition promotes rapid hydration of cement and densification of the matrix. This effectively improves the chemical as well as mechanical bonds at the fibre-matrix interface through the formation of CH crystals and large fibre-solid matrix contact area of the interface, and consequently ameliorating the interfacial properties, interfacial fracture toughness, G _ic and q _o in particular. Predictions of _d ^* and _fr taking into account these changes due to cure condition, results in good agreement with experimental results.     

Alkali carbonation of calcium aluminate cements: influence of set-retarding admixtures under hydrothermal conditions

The preferential uptake of aluminium ions by lactone and carboxylic acid groups in glucuronic-6,3-lactone and gluconic acid suggested that these organic admixtures have a high potential as set-retarding admixtures of high-temperature calcium aluminate cement slurry. However, the liberation of abundant free calcium ions caused by the adsorption of aluminium ions by the admixtures, increased the carbonation rate of hydrated cement pastes after exposure to Na₂CO₃-laden water at 300 C. Using inorganic acid admixtures, such as boric acid and sodium tetraborate decahydrate, the retarding ability of colloidal Ca(BO₂)₂ ·nH₂O and aluminium hydroxide yielded by the reaction between admixture and cement was less than that of the reaction products derived from organic acid admixtures. Although Ca(BO₂)₂ ·nH₂O in hot Na₂CO₃ solution was converted into CaCO₃, the rate of alkali carbonation was almost the same as that of admixture-free calcium aluminate cement pastes.     

辅助胶凝材料对硬化水泥浆体结合氯离子的影响综述

辅助胶凝材料富含SiO2和Al2O3,参与二次水化反应生成更多的C-S-H凝胶,降低硬化水泥浆体的碱度,对硬化水泥浆体结合氯离子具有显著的影响。本文重点综述了矿渣、粉煤灰、硅灰、偏高岭土和煤矸石对硬化水泥浆体结合氯离子的影响,并对物理吸附和化学结合氯离子机理进行了深入分析。掺辅助胶凝材料氯离子结合等温线均适合Freundlich非线性吸附关系,最后对辅助胶凝材料结合氯离子研究的不足进行探讨,为后续的研究工作提供理论依据。     

Structure and electrical conductivity relaxation studies of Nb2O5-doped B2O3–Bi2O3–LiF glasses

Glasses with composition (70 − x) B₂O₃·15Bi₂O₃·15LiF·xNb₂O₅ with x = 0–1.0 mol% were prepared by conventional glass-melting technique. The molar volume V _m values decrease and the glass transition temperatures T _g increase with increase of Nb₂O₅ content up to 0.2 mol%, which indicates that Nb⁵⁺ ions act as a glass former. Beyond 0.2 mol% Nb₂O₅ the V _m increases and the T _g decreases, which suggests that Nb⁵⁺ ions act as a glass modifier. The FTIR spectra suggest that Nb⁵⁺ ions are incorporated into the glass network as NbO₆ octahedra, substituting BO₄ groups. The temperature dependence of the dc conductivity follows the Greaves variable range hopping model below 454 K, and follows the small polaron hopping model at temperatures >454 K. σ _dc, σ _ac conductivity and dielectric constant (ε) decrease and activation energy for dc conduction ΔE _dc which increases with increasing Nb₂O₅ content up to 0.2 mol%, whereas σ _dc, σ _ac and (ε) increase and ΔE _dc decreases with increasing Nb₂O₅ content beyond 0.2 mol%. The impedance spectroscopy shows a single semicircle or arcs which indicate only the ionic conduction mechanism. The electric modulus formalism indicates that the conductivity relaxation is occurring at different frequencies exhibit temperature-independent dynamical process. The (FWHM) of the normalized modulus increases with increase in Nb₂O₅ content suggesting that the distribution of relaxation times is associated with the charge carriers Li⁺ or F⁻ ions in the glass network.     

Molecular dynamics simulation to assess the effect of temperature on diffusion coefficients of different ions and water molecules in C-S-H

Diffusion is a particle transportation process beginning from one point of a system to another through random molecular motion. This process depends on various parameters like temperature, concentration gradient, and particle size. The objective of this article is to assess the variation of diffusion coefficients of water molecules, chloride and sodium ions against different temperatures in calcium silicate hydrates (C-S-H) through molecular dynamics simulation. A uniform sodium chloride solution is modeled between cement hydrate layers with no concentration gradient. In such a solution, temperature could affect diffusion process in a significant manner. The two most important crystalline mineral analogues of C-S-H, tobermorite and jennite, are applied in this simulation. Diffusion coefficients of different ions and water molecules are found in different temperatures. It is revealed that diffusion coefficient is higher at high temperatures. Activation energies of chloride and sodium ions transport in cement hydrates are calculated through Arrhenius law. Output values of diffusion coefficients and activation energies are compared to previous experimental and simulation results in the related literature. A multi-scale analysis is run to estimate the penetration depth of \(\mbox{Cl}^{-}\) ions in cement paste through Fick’s second law.     

A study of the effect of molecular weight on the tensile strength of ultra-high modulus polyethylenes

In this paper the influence of molecular weight and molecular weight distribution on the tensile strength (tenacity) of melt spun and drawn linear polyethylene are investigated with the aim of outlining the requirements for a high strength fibre. The tenacity was investigated over the molecular weight average ¯M _w range 60×10³ to 330×10³ with polydispersities ¯M _w/¯M _n ranging from 1.1 to 13.3. It was found that both molecular weight and its distribution affected tensile strength. The drawing conditions were also found to be important, a high draw temperature and a high draw ratio being needed for a high strength, high modulus fibre. By using a polymer of high ¯M _w and low polydispersity, and drawing at the optimum conditions, strengths of 1.65 GPa and moduli of 85 GPa have been achieved for test temperatures of –55° C.     

The effect of glass synthesis route on mechanical and physical properties of resultant glass ionomer cements

Glass ionomer cements (GICs) have potential orthopaedic applications. Solgel processing is reported as having advantages over the traditional melt-quench route for synthesizing the glass phase of GICs, including far lower processing temperatures and higher levels of glass purity and homogeneity. This work investigates a novel glass formulation, BT 101 (0.48 SiO₂–0.36 ZnO–0.12 CaO–0.04 SrO) produced by both the melt-quench and the solgel route. The glass phase was characterised by X-ray diffraction (XRD) to determine whether the material was amorphous and differential thermal analysis (DTA) to measure the glass transition temperature (T _g). Particle size analysis (PSA) was used to determine the mean particle size and X-ray photoelectron spectroscopy (XPS) was used to investigate the structure and composition of the glass. Both glasses, the melt-quench BT 101 and the solgel BT 101, were mixed with 50 wt% polyacrylic acid (M _w, 80,800) and water to form a GIC and the working time (T _w) and the setting time (T _s) of the resultant cements were then determined. The cement based on the solgel glass had a longer T _w (78 s) as compared to the cement based on the melt derived glass (19 s). T _s was also much longer for the cement based on the solgel (1,644 s) glass than for the cement based on the melt-derived glass (25 s). The cements based on the melt derived glass produced higher strengths in both compression (σ_c) and biaxial flexure (σ_f), where the highest strength was found to be 63 MPa in compression, at both 1 and 7 days. The differences in setting and mechanical properties can be associated to structural differences within the glass as determined by XPS which revealed the absence of Ca in the solgel system and a much greater concentration of bridging oxygens (BO) as compared to the melt-derived system.     

The fracture properties of glass polyalkenoate cements as a function of cement age

The fracture properties of two glass polyalkenoate cements based on a short chain-length and on a long chain-length poly (acrylic acid) have been studied as a function of the cement age. The stress intensity factor, K _I, increases with cement age for both cements. The un-notched fracture strength _f increases with cement age, largely as a result of an increase in the Young's modulus accompanying crosslinking of the polyacrylate chains by metallic ions. The toughness G _I remains approximately constant for the short chain-length cement, but reduces with cement age for the long chain-length cement. Analysis of the toughness data using a chain pull-out model leads to the conclusion that chains distant from the fracture plane are involved in fracture, and that the number of chains that take part in chain pull-out decreases as the crack opening displacement reduces with cement age.     

Continuous-fibre CMC fabrication using pre-impregnated sheets

A new fabrication technology is presented for long fibre-reinforced ceramic matrix composites (CMCs) using pre-impregnated (prepreg) sheets. The monolayer prepreg sheets were fabricated by a modified doctor blade method using a Si–Ti–C–O long fibre/Al₂O₃ laminated composite as an example. These were flexible enough to allow handling during subsequent processing steps. At the final step of fabrication, the multilayer-preforms of fibre containing prepreg sheets were pressureless-sintered in the furnace. The key point of this fabrication method is that the SiO₂–B₂O₃-based glass powder is previously mixed with Al₂O₃ powder to be dispersed in the slurry, in order to lower the sintering temperature and thus to avoid the degradation of fibres during sintering of the fibre contained green body. Continuous fibre CMC fabrication using such a technique has the advantage of not requiring expensive fabrication facilities (and is thus cost-effective) in addition to its potential for significantly increasing/tailoring mechanical properties such as static strength, fracture toughness, and fatigue resistance.     

Microsphere-filled lightweight calcium phosphate cements

The incorporation of inorganic and organic microsphere fillers into calcium phosphate cement (CPC) to produce lightweight cementitious materials that could be used under hydrothermal conditions at high temperatures between 200 and 1000 °C was investigated. An aluminosilicate based hollow microsphere, with a density of 0.67 gcm^–3 and a particle size of 75–200 m, was the most suitable having a low slurry density of 1.3 gcm^–3, and a compressive strength greater than 6.89 M Pa. This microsphere-filled lightweight CPC exhibited the following characteristics: 1. after autoclaving at 200 °C, amorphous ammonium calcium orthophosphate (AmCOP) salt and Al₂O₃·xH₂0 gel phases, formed by the reaction between calcium aluminate cement and an NH₄H₂P0₄ based fertilizer, were primarily responsible for the development of strength; 2. at a hydrothermal temperature of 300 °C, the microsphere shell moderately reacted with the CPC to form an intermediate reaction product, epistilbite (EP), while crystalline hydroxyapatite (HOAp) and boehmite (BO) were yielded by the phase transformations of AmCOP and Al₂O₃·xH₂O, respectively; 3. at an annealing temperature of 600 °C, the HOAp phase remained in the cement body, even though an EP anorthite (AN) phase transition occurred; 4. at 1000 °C, the phase conversion of HOAp into whitlockite was completed, while the AN phase was eliminated; and 5. the microsphere demonstrated excellent thermal stability up to temperatures of 1000 °C.This work was performed under the auspices of the US Department of Energy, Washington, DC, under Contract No. DE-AC02-76CH00016.     

High-T c superconducting Bi(Al)-Ca-Sr-Cu-O glass-ceramic fibres drawn from glass preforms

Bi(Al)-Ca-Sr Cu-O glass-ceramic fibres over 100 cm in length were successfully drawn from a glass preform above the crystallization temperature,T _x. The diameters of the uniformly drawn fibres with circular cross-section could be controlled in the range from 25–200 m and the drawing speed was as high as 200 cm min^–1. In this work Al₂O₃ was used to modify the properties of the glass. It increased the glass transition and crystallization temperatures but did not significantly increase the glass working range. Shrinkage and increase of density during heat treatment of the glass fibres were observed. The annealed (825°C/12 h in air) Bi₄Al_0.1Ca₃Sr₃Cu₄O_y glass-ceramic fibre showed aT _c(onset) of 82 K and aT _c(zero) of 71 K.