Effect of temperature on the pore solution, microstructure and hydration products of Portland cement pastes

The effect of temperature on the hydration products and the composition of the pore solution are investigated for two Portland cements from 5 to 50 °C. Increased temperature leads to an initially fast hydration and a high early compressive strength. At 40 and 50 °C, the formation of denser C-S-H, a more heterogeneous distribution of the hydration products, a coarser porosity, a decrease of the amount of ettringite as well as the formation of very short ettringite needles has been observed. At 50 °C, calcium monosulphoaluminate has formed at the expenses of ettringite. In addition, the amount of calcium monocarboaluminate present seems to decrease. The composition of the pore solution mirrors the faster progress of hydration at higher temperatures. After 150 days, however, the composition of the pore solution is similar for most elements at 5, 20 and 50 °C. Exceptions are the increased sulphate concentrations and the slightly lower Al and Fe concentrations at 50 °C.     

Effects of an early or a late heat treatment on the microstructure and composition of inner C-S-H products of Portland cement mortars

The influence of both early and late heat treatments on the microstructure and on the hydration products of Portland cement mortars has been investigated. The mortars were given either a 4-h or 28-day precure at 20 °C before heating at 90 °C for 12 h and were subsequently stored in distilled water at 20 °C. The microstructure, studied by backscattered electron (BSE) imaging, shows the formation of distinct rims of inner C-S-H with different grey levels during the different stages of the curing cycles. The grey levels and corresponding BSE coefficients of these different rims were determined by image analysis and their chemical compositions by EDS microanalysis. It was found that the compositions depend on the temperature and time at which the rims had developed. The lighter C-S-H formed at 90 °C was denser and contained much more sulfate than the darker C-S-H formed at 20 °C, especially when the heat cure took place at early ages. The sulfate incorporated within the lighter C-S-H was released gradually over time.     

Composite systems fluorgypsum-blastfurnance slag-metakaolin, strength and microstructures

The hydration and properties of composite cementitious pastes with 75% fluorgypsum were investigated; blastfurnace slag and metakaolin were the complementary cementitious materials. The pastes were cured under water at 20 °C for 360 days. All pastes developed and maintained strength under water, except those of commercial gypsum. The addition of metakaolin had a positive effect, after 360 days compressive strengths of 13.4, 13.8 and 14.6 MPa were registered for systems with 0%, 5% and 10% of metakaolin, respectively. The microstructure of the composite pastes was formed of a framework of gypsum crystals, which formed in the initial stages; the matrix was later densified by the formation of C-S-H and ettringite, as a result of the slag and metakaolin reactions. The fluorgypsum reacted rapidly in the first days, however it was still present after one year; the slag reacted in a slower fashion, and the metakaolin was very reactive and contributed with the ettringite since the early ages, which enhanced the strength.     

Composition, morphology and nanostructure of C-S-H in white Portland cement pastes hydrated at 55 °C

The C-S-H present in water- and alkali-activated hardened pastes of white Portland cement hydrated at 55 °C has been characterized. The mean length of the aluminosilicate anions in the C-S-H was similar in both systems and increased with age. Inner product C-S-H generally had a fine scale, homogeneous morphology. Outer product C-S-H was generally fibrillar with water, and foil- or lath-like with alkali. There were some regions of C-S-H with coarse morphology. It was not possible to determine the chemical composition of C-S-H using the SEM; TEM-EDX was necessary. The C-S-H formed in the alkali-activated paste had a lower mean Ca/(Al + Si) ratio than that formed with water, which was offset by a larger quantity of calcium hydroxide. The potassium in the KOH-activated paste was present either within the C-S-H structure charge balancing the substitution of Al³⁺ for Si⁴⁺, or adsorbed on the C-S-H charge balancing sulfate ions.     

Formation and stability of gismondine-type zeolite in cementitious systems

Microcrystalline zeolites of the gismondine family are often reported in alkali-activated and blended cement systems. However, little is known about gismondine's compatibility with other cementitious phases to determine stability in long-term phase assemblage. Experimental studies were conducted to investigate the compositional field of gismondine stability in the lime-alumina-silica-hydrate systems, with a particular focus on understanding the compatibility of gismondine with other cement phases such as C-S-H, ettringite, monosulfate, strätlingite, katoite, gypsum, calcite, portlandite, alkali, silica, and aluminosilicate phases. Results show that gismondine-Ca forms readily at ~85°C in high aluminosilicate compositions; and persists in the presence of calcite, gypsum, ettringite, katoite solid solution, low Ca tobermorite-like C-S-H, silica and aluminosilicate phases, at 20-85°C. However, gismondine-Ca reacts with: (a) monosulfate, producing ettringite-thaumasite solid solution; (b) portlandite, forming tobermorite-like C-A-S-H gel and siliceous katoite at >55°C; (c) aqueous NaOH, generating gismondine-(Na,Ca), a garronite-like zeolite P solid solution; and (d) strätlingite leading to the conversion of strätlingite to gismondine indicating the metastability of strätlingite with respect to gismondine at 55°C. The outcomes are discussed to provide insights into the long-term phase assemblage of relevant cement systems such as lime-calcined clay, alkali-activated materials, and potentially ancient Roman concrete.     

Thermal stability of ettringite in alkaline solutions at 80 °C

The thermal stability of synthetic ettringite was examined in NaOH solutions up to 1 M after 12 h of heat treatment at 80 °C, with or without the coexistence of C₃S in the system. Ettringite was found to convert to the U phase, a sodium-substituted AFm phase, over the heat treatment in the absence of C₃S. The presence of C₃S, leading to C-S-H formation, prevents the U phase formation and results in the conversion of ettringite to monosulfate. Sulfate ions generated from ettringite decomposition mostly remain in the solution, but some is incorporated into C-S-H. During subsequent storage at room temperature, the majority of monosulfate slowly converts back to secondary ettringite under moist conditions, using the supply of sulfate ions from the solution and C-S-H. The observations support the current mechanism of delayed ettringite formation (DEF).     

Impact of prolonged warm (85°C) moist cure on Portland cement paste

A commercial Portland cement paste was fabricated as 200-g cylinders to a water/cement weight ratio of 0.50. After 30 days cure at 20°C, cylinders were additionally cured at 20°C and 85°C, both ±2°C, in sealed, vapour-saturated systems for 8.4 years. Thereafter, cylinders were allowed to stand, still in sealed state, at 20° for 1.5 to 2.0 years. The 20°C cure mineralogy and microstructure is essentially normal: only a little unhydrated clinker persists and the matrix consists of relatively coarse, blocky Ca(OH)₂ crystals embedded in a groundmass of C-S-H together with some AFt (ettringite). However, prolonged 85°C cure significantly alters the microstructure and mineralogy. Clinker hydration progressed only slowly between 28 days and 8.4 years, with the result that 30% cement clinker persists. Subsequent prolonged storage at 20°C has apparently not allowed hydration to restart. Ca(OH)₂ is present in approximately unchanged amounts, comparing the two cures, provided allowance is made for the presence of unhydrated clinker. Paste porosity is, however, significantly increased at 85°C relative to 20° cure. The 85°C mineralogy consists of four solid hydrate phases: Ca(OH)₂, C-S-H gel, with a Ca/Si mole ratio close to 1.52, katoite (a siliceous hydrogarnet) and a hydrotalcite-like phase. The amounts of these phases are determined. The compositions of the C-S-H gel and hydrogarnet have been estimated by transmission electron microscopy and microprobe analysis. The amount and composition of the mineral phases can be recalculated to yield a bulk composition of the cement that agrees with a batch analysis.     

Thermodynamic modelling of the effect of temperature on the hydration and porosity of Portland cement

The composition of the phase assemblage and the pore solution of Portland cements hydrated between 0 and 60 °C were modelled as a function of time and temperature. The results of thermodynamic modelling showed a good agreement with the experimental data gained at 5, 20, and 50 °C. At 5 and at 20 °C, a similar phase assemblage was calculated to be present, while at approximately 50 °C, thermodynamic calculations predicted the conversion of ettringite and monocarbonate to monosulphate.Modelling showed that in Portland cements which have an Al₂O₃/SO₃ ratio of > 1.3 (bulk weight), above 50 °C monosulphate and monocarbonate are present. In Portland cements which contain less Al (Al₂O₃/SO₃ < 1.3), above 50 °C monosulphate and small amounts of ettringite are expected to persist. A good correlation between calculated porosity and measured compressive strength was observed.     

A thermodynamic and experimental study of the conditions of thaumasite formation

The formation of thaumasite was investigated with the progressive equilibrium approach (PEA). This approach experimentally simulates the conditions of various levels of sulfate addition in hardened cement pastes. The influence of limestone, time, C₃A content, temperature and leaching on thaumasite formation was investigated. The results show that thaumasite formation is favoured at lower temperatures (8 °C) independently of the type of cement clinker (high or low C₃A content) used. Thaumasite was found to form only in systems where limestone was present and where sufficient sulfate had been added. Thaumasite precipitated only in systems where the Al present has already been consumed to form ettringite and the molar SO₃/Al₂O₃ ratio exceeded 3. In leached samples (reduction of portlandite and alkalis) slightly less thaumasite was formed whereas gypsum and ettringite are favoured under these conditions. The PEA, used to investigate the chemical aspects of sulfate attack was found to be a good tool for simulating external sulfate attack. Generally, thaumasite was detected were it was modelled to be stable in significant amounts. However, in this study equilibrium conditions were not reached after 9 months.     

Implications of pre-formed microcracking in relation to the theories of DEF mechanism

Microcracks were induced in cementitious systems by freeze-thaw action and by alkali-silica reaction. These mechanisms often co-exist with delayed ettringite formation in concretes. Mortars and concretes were subjected to a heat treatment cycle consisting of a pre-set period of 4 h at 23 °C followed by accelerated curing at 95 °C. To isolate the mechanical effects of induced microcracking, heat-cured specimens were subjected to varied prescribed damage induced by freeze-thaw or alkali-silica reaction prior to the onset of delayed ettringite formation. It was found that inducement of pre-formed microcracks led to an earlier onset of expansion due to delayed ettringite formation. Initially, microcracks enhanced ultimate expansion until a certain relatively high extent of microcracking was reached. Thereafter, ultimate expansion decreased with any further increase in microcracking. This report gives support to the paste expansion theory.     

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.     

Thermodynamic properties of Portland cement hydrates in the system CaO-Al2O3-SiO2-CaSO4-CaCO3-H2O

A database is presented for commonly-encountered cement substances including C-S-H, Ca(OH)₂, selected AFm, AFt and hydrogarnet compositions as well as solid solutions. The AFm compositions include strätlingite. The data were obtained for the most part from experiment and many of the predicted reactions were confirmed by focussed experiments. The temperature-dependence of the thermodynamic data for the above phases, determined partly from experiment and partly from thermodynamic estimations, are also tabulated in the range 1 °C to 99 °C. Relative to previous databases, sulfate AFm is shown to have a definite range of stability range at 25 °C thus removing long-standing doubts about its stability in normal hydrated cement pastes. Carbonate is shown to interact strongly with stabilisation of AFm across a broad range of temperatures and, at low temperatures, to substitute into AFt. The new database enables the ultimate hydrate mineralogy to be calculated from chemistry: most solid assemblages, the persistence of C-S-H apart, correspond closely to equilibrium. This realisation means that hydrate assemblages can be controlled. The development of a thermodynamic approach also enables a fresh look at how mineralogical changes occur in response to environmentally-conditioned reactions; several papers showing applications are cited.     

Dual effectiveness of lithium salt in controlling both delayed ettringite formation and ASR in concretes

The influence of lithium nitrate on expansions due to delayed ettringite formation (DEF) and alkali-silica reaction (ASR) has been investigated. Effects of the lithium salt were examined in heat-cured mortars and concretes containing one or both damage mechanisms. The mortars and concretes made using reactive and/or non-reactive aggregates were subjected to heat treatment consisting of a hydration delay period of 4 h at 23 °C followed by steam-curing at 95 °C and then stored in limewater. Results showed that the lithium salt admixture was able to reduce the occurrence of deleterious expansion due to delayed ettringite formation in addition to controlling alkali-silica reaction in cementitious systems containing one or both mechanisms. In concretes made using non-reactive limestone aggregates, incorporation of lithium nitrate in a proportion of 0.74 M ratio of Li to (Na + K) was found to control delayed ettringite formation during the one-year period of this study.By analyzing the leaching properties of lithium and other alkalis from mortars during storage, it was found that a substantial amount of lithium was retained in the cementitious system in a slightly soluble form, and is expected to be responsible for reducing DEF.     

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.     

Thermal treatment of C-S-H gel at 1 bar H2O pressure up to 200 °C

Homogeneous CaO-SiO₂-H₂O gels were prepared at Ca/Si molar ratios 0.83, 1.01, 1.21, 1.50, 1.83 and 2.02. These were aged for 12-24 months at 25 °C and subsequently treated in steam, 1 bar total pressure, at 130 or 200 °C; also in water at 55 and 85 °C. Gels with low Ca/Si ratios partially crystallised at 85 °C. At 130 °C in steam, crystalline products included 11 and 14 Å tobermorite, xonotlite, afwillite, portlandite and another incompletely characterised phase. At 200 °C, the gels retained much water but remained amorphous to X-ray powder diffraction (XRD). However, electron microscopy, coupled with diffraction and analysis, disclosed that the “amorphous” product obtained at 85-200 °C had undergone crystallisation with domains typically 10-1000 nm. At higher bulk Ca/Si ratios, 1.83 and 2.02, much nanoscale precipitation of Ca(OH)₂ occurs, probably by exsolution, such that the residual C-S-H product has a Ca/Si ratio in the range 1.4-1.5. The complex thermal history of the products is reflected in their pH conditioning ability, measured at 25 °C. The results are applied to predict the evolution of pH in a cement-conditioned nuclear waste repository which experiences a prolonged thermal excursion.     

Reactions of fly ash with calcium aluminate cement and calcium sulphate

The hydration processes in the ternary system fly ash/calcium aluminate cement/calcium sulphate (FA/CAC/C$) at 20 °C were investigated; six compositions from the ternary system FA/CAC/C$ were selected for this study. The nature of the reaction products in these pastes were analysed by X-ray diffraction (XRD) and infrared spectroscopy (FTIR). At four days reaction time, the main hydration reaction product in these pastes was ettringite and the samples with major initial CAC presented minor ettringite but calcium aluminates hydrates. The amount of ettringite developed in the systems has no direct relation with the initial components.     

What causes differences of C-S-H gel grey levels in backscattered electron images?

Backscattered electron (BSE) images of heat-cured concretes show alite grains surrounded by inner C-S-H gel of two distinct grey levels (referred to as two-tone inner C-S-H gel). The lighter rim forms at elevated temperature whereas the darker rim develops during subsequent exposure to moisture at 20 °C. This microstructural feature can potentially be used as an indicator to assess the curing history of a concrete. However, microstructural examinations of room-temperature concretes containing silica fume or which have been exposed to severe conditions (external sulfate, carbonation) also show distinct rims of two-tone inner C-S-H gel.The chemical compositions of the rims were determined by EDX microanalysis in the scanning electron microscope (SEM). Our results show that for heat-cured samples, the different grey levels of the two-tone inner C-S-H are caused by relative differences in microporosity and water content and not by ones in chemical composition. However, in silica-fume blended concrete, sulfate attacked or carbonated specimens the different grey levels of the two-tone inner C-S-H gel were associated with significant differences in chemical composition. This difference allows two-tone inner C-S-H gel arising from heat curing to be distinguished from that arising from these other causes.     

Physical and microstructural aspects of sulfate attack on ordinary and limestone blended Portland cements

The consequences of external sulfate attack were investigated by traditional test methods, i.e. length and mass change, as well as by a newly developed, surface sensitive ultrasonic method, using Leaky Rayleigh waves (1 MHz). The macroscopic changes are discussed and compared with thermodynamic calculations and microstructural findings (SEM/EDS). The results show that the main impact of limestone additions on resistance to sulfate degradation are physical — i.e. addition of a few percent in Portland cement reduces the porosity and increases the resistance of Portland cement systems to sulfate; but higher addition of 25% increase porosity and lower resistance to sulfate. The kinetics of degradation were dramatically affected by the solution concentration (4 or 44 g Na₂SO₄/l) and the higher concentration also resulted in the formation of gypsum, which did not occur at the low concentration. However the pattern of cracking was similar in both cases and it appears that gypsum precipitates opportunistically in pre-formed cracks so it is not considered as making a significant contribution to the degradation. At 8 °C limited formation of thaumasite occurred in the surface region of the samples made from cement with limestone additions. This thaumasite formation led to loss of cohesion of the paste and loss of material from the surface of the samples. However thaumasite formation was always preceded by expansion and cracking of the samples due to ettringite formation and given the very slow kinetics of thaumasite formation it was probably facilitated by the opening up of the structure due to ettringite induced cracking.The expansion of the samples showed a steady stage, followed by a rapidly accelerating stage, with destruction of the samples. The onset of the rapidly accelerating stage occurred when the thickness of the cracked surface layer reached about 1–1.5 mm–10–15% of the total specimen thickness (10 mm).     

Microstructural development of early age hydration shells around cement grains

An important microstructural aspect of the early hydration of Portland cement (PC) is the formation of a shell of hydration products around cement grains. There is, at present, limited information on the mechanism of formation of the shell and of the chemistry of the phases that constitute the shells. Through the use of STEM imaging of early age hydrated cement pastes as early as 2 h, the present work shows that the shells correspond to the first C-S-H type product formed which has a distinct morphology compared to C-S-H formed later when the main reaction occurs (nucleation and growth stage at setting time). The shells form only around the silicate part of the grain and are not empty but filled with a fragile fibrous C-S-H which appears to have a lower (packing) density than the rest of the hydration products. The cement grains underneath the shells are seen to react unevenly and the hydration seems to follow a reaction front, leaving striations up to 1 µm deep on the grains. Over the long term, the original fragile product seems to densify and gives rise to the usual inner C-S-H. High resolution EDS chemical analysis and mappings were used to get insight into the chemistry associated with the formation of these early age products. The C/S ratio of all C-S-H (inner and outer shell) is the same (within the limits of the analysis accuracy) and evolves insignificantly over the first 24 h of hydration. High concentrations of sulfate are associated with the C-S-H formed during the early development of the microstructure, but these decrease later, the sulfate being mainly incorporated into ettringite.     

Effect of temperature on sulphate adsorption/desorption by tricalcium silicate hydrates

Sulphate adsorption from internal sources was studied in hydrating systems containing C-S-H gel and gypsum with respect to delayed ettringite formation. The influence of C₃A addition on sulphate desorption was also investigated. Research indicates that C-S-H gel will adsorb sulphate faster at high temperature resulting in quick depletion of the gypsum phase in C-S-H - gypsum mixtures. Sulphate adsorbed at high temperature is desorbed more slowly than that adsorbed at normal temperature. Slower release of sulphate from an internal sulphate source may be a critical condition for delayed ettringite formation in high temperature cured Portland cement paste.