Autogenous and drying shrinkage of alkali-activated slag mortars

Shrinkage of alkali-activated slag (AAS) cement is a critical issue for its industrial application. This study investigated the mechanisms and effectiveness of shrinkage-reducing agent (SRA) and magnesia expansive agent on reducing autogenous and drying shrinkage of AAS mortars that were activated by liquid sodium silicate (LSS) solution with modulus (SiO₂/Na₂O molar ratio) of 0-1.5. The results showed that the autogenous shrinkage of AAS mortars increased with the increase of LSS modulus from 0 to 0.5, then decreased as modulus increased up to 1.5. The drying shrinkage consistently increased with the increase in the modulus of LSS. The oxyalkylene alcohol-based SRA could significantly reduce the autogenous and drying shrinkage of AAS mortars while the magnesia expensive agent was comparatively less effective. The autogenous shrinkage of AAS mortars was inversely proportional to the internal relative humidity, while the drying shrinkage was more related to the mass loss of samples. Mathematical models were established to describe the autogenous and drying shrinkage behavior of AAS mortars.     

Examining the relationship between the microstructure of calcium silicate hydrate and drying shrinkage of cement pastes

Cement paste undergoes a volumetric contraction called drying shrinkage when placed in a low relative humidity (RH) environment. Only a portion of this shrinkage is reversible upon rewetting. In order to understand better the mechanisms responsible for the irreversible portion of drying shrinkage, a quantitative comparison was made between shrinkage values and microstructural properties of cement pastes. Drying shrinkage, surface area and pore volume were manipulated using curing temperature and chemical admixtures. It was observed that total and irreversible drying shrinkage increase with surface area and pore volume as measured by nitrogen (1-40 nm pore radius range), when degree of hydration and water-to-cement ratio (w/c) are held constant (0.55 and 0.45, respectively).     

Formation of magnesium silicate hydrate (M-S-H) cement pastes using sodium hexametaphosphate

Magnesium silicate hydrate (M-S-H) gel is formed by the reaction of brucite with amorphous silica during sulphate attack in concrete and M-S-H is therefore regarded as having limited cementing properties. The aim of this work was to form M-S-H pastes, characterise the hydration reactions and assess the resulting properties. It is shown that M-S-H pastes can be prepared by reacting magnesium oxide (MgO) and silica fume (SF) at low water to solid ratio using sodium hexametaphosphate (NaHMP) as a dispersant. Characterisation of the hydration reactions by x-ray diffraction and thermogravimetric analysis shows that brucite and M-S-H gel are formed and that for samples containing 60 wt.% SF and 40 wt.% MgO all of the brucites react with SF to form M-S-H gel. These M-S-H cement pastes were found to have compressive strengths in excess of 70 MPa.     

Development of low pH cement systems forming magnesium silicate hydrate (M-S-H)

This work aimed to develop novel cement systems for waste encapsulation that would form with a pH of around 10. The approach taken was to investigate the formation of brucite by hydration of a light burned periclase (MgO). Commercially available MgO powders often contain some CaO, and therefore silica fume was added to form C-S-H gel. Identification of the hydrated phases in MgO/silica fume samples showed that brucite formed in substantial quantities as expected. However, brucite reacted with the silica fume to produce a magnesium silicate hydrate (M-S-H) gel phase. After 28 days, the pH of systems rich in MgO tended towards the pH controlled by residual brucite (~ 10.5), whereas when all brucite reacts with silica fume a cement with an equilibrium pH just below 10 was achieved.     

Synthesis and characterisation of magnesium silicate hydrate gels

Magnesium silicate hydrate gels (M-S-H) have been prepared by precipitation. The range of gel compositions lie between Mg/Si molar ratios 0.67-1.0. The gels were subject to short cure, approximately 24 h at approximately 22 °C and longer cure, 180 days at 85 °C, following which they were characterised by XRD, FT-IR and solid-state ²⁹Si NMR. Ageing at longer times and higher temperatures somewhat improves the local ordering. The nature of the partially ordered structures is related to those of M-S-H mineral phases. The structures and compositions of M-S-H gels differ from those of C-S-H gels and partly on that account, C-S-H gels contain little magnesium while M-S-H gels in blended cements coexist with C-S-H but contain little calcium.     

Models for the composition and structure of calcium silicate hydrate (C---S---H) gel in hardened tricalcium silicate pastes

Models for the structure of C---S---H gels occuring in hardened C₃S cement pastes are considered and compared to some examples in which composition and silicate anion structure have been investigated experimentally.     

Formation of magnesium silicate hydrate and its crystallization to talc

Mixtures of Mg(OH)₂ and colloidal silica with Mg/Si = 0.75 were treated hydrothermally at 180° to 600°C for 4 h ? 8 weeks. Reaction seemed always to proceed through the formation of magnesium silicate hydrate to talc, which had random displacements of layers parallel to b of nb/3, and gave a decrease of the basal spacing with increase of crystallinity. The magnesium silicate hydrate gave X-ray powder patterns indicative of two-dimensional crystals without basal spacing, had a specific area of about 300 m²/g, and gave an exotherm at 830°C. Dehydration and infrared spectra for products are also described.     

Alkali uptake in calcium alumina silicate hydrate (C-A-S-H)

Uptake of the alkalis K and Na by calcium silicate hydrate (C-S-H) and calcium alumina silicate hydrate (C-A-S-H) of molar Ca/Si ratios = 0.6 to 1.6 and molar Al/Si ratio = 0 or 0.05 has been studied at 20 °C. Alkalis are thought to be bound in the interlayer space of C-A-S-H and show preferred uptake by lower Ca/Si ratios and by higher alkali concentrations. A consequence of alkali uptake into C-A-S-H is a rearrangement of the C-A-S-H structure. Less calcium is present in the interlayer and shorter silica chains are observed for the same molar Ca/Si ratio.No significant difference was observed between sodium and potassium uptake. Equilibration times of 91 days to 1 year or the solid phase being either C-S-H or C-A-S-H had seemingly no effect on alkali uptake.     

Properties of magnesium silicate hydrates (M-S-H)

Investigations of synthetic magnesium silicate hydrate (M-S-H) samples have shown that M-S-H aged for 1 year can exhibit variable compositions with molar Mg/Si ratios in the range 0.7 ≤ Mg/Si ≤ 1.5. At lower Mg/Si ratio, additional silica is present whereas brucite is observed for Mg/Si ≥ 1.3. FT-IR and ²⁹Si NMR data reveal a high degree of silicate polymerisation, indicating the formation of silicate sheets. TGA shows the presence of bound water and of hydroxyl groups bound to Mg and as silanol groups in the M-S-H, in accord with ²⁹Si{¹H}CP/MAS and high-speed ¹H NMR measurements. Raman and XRD data suggest that the M-S-H structure is related to a disordered talc precursor at low Mg/Si and to a serpentine precursor at high Mg/Si ratio. Solubility products for M-S-H phases were calculated on basis of the compositions of the aqueous solutions and a solid solution model was suggested.     

Calcium silicate hydrate (II) (“C-S-H(II)”)

This semicrystalline phase, originally named ‘calcium silicate hydrate(II)’ by Taylor (1950), has been studied with X-rays, electron optics, chemical investigation of silicate anion type, infrared spectra, and thermal methods. It is structurally related to jennite (C₉S₆H₁₁) and probably also to the fibrous CSH of cement pastes, the three phases forming a sequence of decreasing crystallinity. The specimen studied had approximate composition C₂SH_3.2 after standing over saturated CaCλ₂ at about 15°C. CSH(II) contains metasilicate chains and pyrosilicate groups and has a disordered layer structure. Much of the water can be lost reversibly without significant change in lattice parameters.     

Creep and drying shrinkage of calcium silicate pastes IV. Effects of accelerated curing

The dependent deformations and evidence of structural changes were measured on pastes of C₃S containing CaCl₂, and on pastes of C₃S or a C₃S/C₂S blend cured at 65°C. It was concluded that the addition of CaCl₂ enhances the role of the “pore component” in controlling irreversible strains even when well-hydrated pastes are dried. The formation of ore stable CSH at 65°C can explain the reduction in time-dependent deformations observed for these pastes. Even though a change in pore size distribution occurs at 65°C, it is not considered to affect irreversible strains in well-hydrated pastes.     

Chemical alteration of calcium silicate hydrate (C-S-H) in sodium chloride solution

The effect of sodium chloride on the chemical alteration of calcium silicate hydrate (C-S-H) was measured and discussed. The release of calcium from C-S-H was increased as the concentration of sodium chloride in the solution increased. It was observed that sodium sorbed onto the C-S-H phases and some sodium replaced calcium in C-S-H so that the release of calcium was enhanced. An integrated modelling approach employing an ion-exchange model and an incongruent dissolution model of C-S-H is developed. It reasonably and accurately predicted the release of calcium from C-S-H in sodium chloride solution by considering cation exchange and the effect of the ionic strength on the solubility of C-S-H.     

Creep and drying shrinkage of calcium silicate pastes III. A hypothesis of irreversible strains

Irreversible strains, measured on pastes of pure calcium silicates which were loaded without drying, dried to 53% rh without loading, or were loaded and simultaneously dried to 53% rh, are correlated with indications of structural changes occurring within the hydrated pastes. These changes are interpreted in terms of microstructural models of the hydrated pastes. It is assumed that irreversible strains are caused only by changes in the “pore component” and the “CSH component.” In young pastes irreversible shrinkage can be explained by a reduction in pores in the range 40–100 Å diameter. In well-hydrated pastes structural changes in the silicate framework of the “CSH component” dominate irreversibility. Microshearing between CSH particles is thought to occur under load. An increase in degree of silicate polymerization bonding occurs on drying, and this increase is greater when drying takes place under load.     

Creep and drying shrinkage of calcium silicate pastes II. Induced microstructural and chemical changes

Structural changes occurring during creep and shrinkage of pure calcium silicate pastes were investigated. The tests included determination of pore structure and surface area by adsorption of H₂O and N₂. Helium inflow tests were also carried out. The degree of silicate polymerization was analyzed using the trimethylsilyl method. Penetration of He and H₂O into the pore system is unaffected by the creep and shrinkage of the pastes. At low degrees of hydration drying decreases the N₂ surface area S(N₂), and increases the amount of highly polymerized silicate, while loading does not affect S(N₂) but inhibits the additional polymerization that can occur on drying. At high degrees of hydration, drying causes a slight reduction in S(N₂) while loading increases it. Drying, and especially loading and drying, promote silicate polymerization while loading without any drying does not induce significant changes.     

Creep and drying shrinkage of calcium silicate pastes I. Specimen preparation and mechanical properties

In this first paper of a series dealing with the creep and shrinkage of calcium silicate pastes, the materials, specimen preparation methods, mechanical test procedures and results are discussed. Pastes were prepared with a w/s ratio of 0.4 using C₃S, β-C₂S or a C₃S/C₂S blend. Thin-wall, hollow-cylinder specimens were cast and subjected to various conditions of load and drying. The structural and chemical modifications resulting from these treatments will be covered in subsequence papers.     

Effect of temperature on the microstructure of calcium silicate hydrate (C-S-H)

Temperature affects the properties of concrete through its effect on the hydration of cement and its associated microstructural development. This paper focuses on the modifications to C-S-H induced by isothermal curing between 5 and 60 °C. The results show that as the temperature increases (within the range studied) the C/S ratio of C-S-H changes only slightly, with a higher degree of polymerisation of silicate chains, but there is a significant decrease in its bound water content and an increase of apparent density of 25%. This increase seems to come from a different packing of C-S-H at the nanoscale. As a consequence of these changes, the microstructure of the cement paste is much coarser and porous, which explains the lower final strengths obtained by curing at elevated temperatures.     

水玻璃在硅凝胶制造的应用

本文介绍了水玻璃在硅凝胶制造中的应用     

The coexistence of geopolymeric gel and calcium silicate hydrate at the early stage of alkaline activation

Scanning electron microscopy was used to study the effects of the addition of ground granulated blast furnace slag (GGBFS) on the microstructure and mechanical properties of metakaolin (MK) based geopolymers. It was found that it is possible to have geopolymeric gel and calcium silicate hydrate (CSH) gel forming simultaneously within a single binder. The coexistence of these two phases is dependent on the alkalinity of the alkali activator and the MK / GGBFS mass ratio. It has been found that the formation of CSH gel together with the geopolymeric gel occurs only in a system at low alkalinity. In the presence of high concentrations of NaOH (> 7.5 M), the geopolymeric gel is the predominant phase formed with small calcium precipitates scattered within the binder. The coexistence of the two phases is not observed unless a substantial amount of a reactive calcium source is present initially. It is thought that voids and pores within the geopolymeric binder become filled with the CSH gel. This helps to bridge the gaps between the different hydrated phases and unreacted particles; thereby resulting in the observed increase in mechanical strength for these binders.     

Reactive molecular simulation on the ordered crystal and disordered glass of the calcium silicate hydrate gel

Modifying the properties of modern concrete highlights the decoding the molecular structure of C–S–H gel, which is the main binding phase in the cementitious materials. In this paper, the structural, dynamical and mechanical properties were investigated by using C–S–H glassy model and its crystal analog tobermorite 11 Å to represent the disordered and ordered molecular structure. By using reactive force field molecular simulation, the structural discrepancy for ordered and disordered phase was illustrated in respect of silicate chain skeletons, local structure of the calcium oxygen octahedrons and hydroxyl distribution. In the glassy model, the local structure of C–S–H gel, with defective silicate chains and distorted calcium sheet, is similar to the silicate glass phase of metallic ions. Furthermore, to predict the mechanical properties of the C–S–H gel and tobermorite, uniaxial tension testing by the reactive force field coupled with both the mechanical response and chemical response during the large tensile deformation process. During the tensile process, water molecules, attacking the Si–O and Ca–O bond, are detrimental to the cohesive force development in the C–S–H gel.