Hydration Characteristics of Portland Cement after Heat Curing: I, Degree of Hydration of the Anhydrous Cement Phases

The degree of hydration of the four major anhydrous cement phases in three U.K. portland cement mortars has been observed during the period of water storage at room temperature after an initial short-term heat cure. Such a heat cure at 85° or 100°C for 12 h generally accelerated the initial hydration of the four major anhydrous minerals in portland cement. Subsequent retardation of the degree of hydration of the alite, tricalcium aluminate, and ferrite phases was observed when these heat-cured mortars were stored at ambient temperature. General similarity but some differences in hydration behavior were observed between the three cements. The hydration of belite in the heat-cured mortars during storage at room temperature produced porous inner products that favored deposition of ettringite and reduced the risk of expansive ettringite formation. The substantial retardation in hydration of the aluminate-bearing phases, especially the ferrite phase, during the storage at room temperature raised the overall SO₃/Al₂O₃ ratio of the cement hydrates formed, bringing about a potential for ettringite formation and hence the risk of expansion through delayed ettringite formation.     

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).     

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.     

Studies of Early Stages of Paste Hydration of Different Types of Portland Cements

The early stages of hydration of four different types of portland cements were studied by electron-optical and X-ray diffraction techniques. It was observed that, except for low-heat cement, very little ettringite formed up to 3 hours of hydration and that the alite present in the cements was more reactive than the laboratory form. Ettringite formed earlier in the low-heat cement than in other cements. Ettringite was found to be the stable sulfate-bearing phase in sulfateresistant cement, at least up to 30 months, although in other cements ettringite began to change to monosulfate by 14 days. Direct evidence was found for the formation of gypsum from either CaSO₄±0.5H₂O or soluble anhydrite in some cements.     

Formation of Ettringite from Monosubstituted Calcium Sulfoaluminate Hydrate and Gypsum

The formation of ettringite (3CaO·Al₂O₃·3CaSO₄·32H₂O) from monosulfate (3CaO·Al₂O₃·CaSO₄·12H₂O) and gypsum (CaSO₄·2H₂O) was investigated by isothermal calorimetry and X-ray diffraction (XRD) analyses. Hydration was carried out at constant temperatures from 30° to 80°C using deionized water and 0.2 M , 0.5 M , and 1.0 M sodium hydroxide (NaOH) solutions. Ettringite was found to be the dominant crystalline phase over the entire temperature range and at all sodium hydroxide concentrations. A sodium-substituted monosulfate phase was formed as a hydration product in the 1.0 M sodium hydroxide solution regardless of temperature. XRD and calorimetry demonstrate that hydration in increasing sodium hydroxide concentrations decreases the amount of ettringite formed and retards the rate of reaction. The apparent activation energy for the conversion of the monosulfate/gypsum mixture to ettringite was observed to vary depending on the sodium hydroxide concentration. Ettringite formation was observed to depend upon the concentration of calcium in solution; thus the formation of calcium hydroxide and sodium-substituted monosulfate phase competes with ettringite formation.     

Delayed ettringite formation symptoms on mortars induced by high temperature due to cement heat of hydration or late thermal cycle

Cases of delayed ettringite formation (DEF) have mainly been detected on mortars or precast concretes steam-cured according to a predefined temperature cycle during hydration. The present study shows that other situations in which the material is submitted to a temperature cycle can induce DEF expansions. Mortar bars were made with three different cements (types 10, 20M, and 30). As a first heat treatment, the mortar bars were steam-cured to reproduce the temperature cycle they would undergo if they were at the center of a large mortar member. The dimensional variations of these specimens were studied for 1 year. After 1 year, half of the specimens were steam-cured for 1 month at 85 °C. The expansions were followed for two more years. The early-age steam-cure-induced expansions for mortar types 10 and 30. Late steam-curing induced expansions for the three cements tested. In one case (cement type 20M), the early-age steam cure has suppressed or delayed the expansion induced by the late steam cure. A scanning electron microscopy (SEM) study showed that typical DEF symptoms are associated with the expansions.     

The effect of pozzolans and slag on the expansion of mortars cured at elevated temperature: Part II: Microstructural and microchemical investigations

The microstructural and microchemical development of heat-cured Portland cement mortars containing silica fume, metakaolin, blast-furnace slag, and fly ash were analysed using pore solution analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive X-ray analysis (EDX). Incorporation of these materials into the mixture modifies the composition of the C-S-H gel, the quantities of the hydration products, and the microstructure. Ettringite was formed during moist storage in all specimens, but was not accompanied by expansion where a sufficient amount of metakaolin, blast-furnace slag, or a suitable fly ash replaced a proportion of the Portland cement; replacement with silica fume was not as effective at eliminating expansion. The different behaviour of silica fume from the other supplementary cementing materials is believed to reflect a difference in the way ettringite is formed in the presence of Al₂O₃-bearing mineral admixtures.     

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.     

Some factors affecting delayed ettringite formation in heat-cured mortars

Although more than 10 years of studies on delayed ettringite formation (DEF) have led to consensus in numerous areas of past disagreements, some questions remain experimental work is needed to complete the knowledge of this pathology. Following this objective, this paper studies the influence of pre-existing microcracking, wetting/drying cycles and the type of sulfated addition on DEF in steam cured mortars. The mortar specimens were prepared using an Ordinary Portland Cement and two types of sulfate were added to the mixtures: calcium sulfate (CaSO₄) or sodium sulfate (Na₂SO₄). The results confirm the well-known effect of temperature: no expansion was observed in any of the mixtures cured at room temperature. Moreover, no expansion was observed after 800 days for the reference mortar or for the mortar containing calcium sulfate but all the specimens of heat-cured mortars containing sodium sulfate expanded markedly after about 50 days whatever the supplementary treatments applied (thermal shrinkage or wetting/drying cycles). These results show the significant role played by alkalis in the occurrence of delayed ettringite. The supplementary treatments intended to cause prelimiray microcracking of the specimens did not promote expansion but contributed to a slight acceleration of the reaction. The ultimate values of expansion were similar to those obtained with sound mortars.     

Slurry Consistency and In Situ Synchrotron X-Ray Diffraction During the Early Hydration of Portland Cements With Calcium Chloride

Class A and H oil well cements are compared at 25° and 50°C with 0%, 1%, 2%, and 4% CaCl₂. Up to 4% CaCl₂ accelerated Class A thickening, but 4% led to slower thickening than 2% for Class H. C₃S hydration in the two cements responded differently to CaCl₂. CaCl₂ always accelerated aluminate hydration. For Class A, CaCl₂ accelerated early Ca(OH)₂ precipitation, but sometimes reduced the amount at longer times. This may be coupled to C–S–H gel composition changes. For Class H, Ca(OH)₂ precipitation changes nonlinearly with CaCl₂ concentration. Ettringite to monosulfate conversion and Friedel's salt formation were sometimes seen.     

Hydration of fly ash-portland cements

Hydration products of fly ash-portland cements were studied with x-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) as part of a continuing research effort to understand the pozzolanic activity of fly ashes. It was found that the amount of calcium hydroxide crystals in the cement pastes is diminished due to the addition of fly ash to the cement. Ettringite was produced in the early age, and the consumption of sulfate by the formation of ettringite was accelerated by the addition of fly ash. A partial conversion of ettringite to monosulfate within the first 7 days of hydration in the fly ash-portland cement pastes, but the formation of ettringite continued to form up to at least 28 days of hydration in the pastes without fly ash. Examination of the fly ash bearing pastes showed, in all cases, varying amounts of calcium hydroxide and unreacted portland cement, with minor quartz and gehlenite hydrate. It appears that hydration reactions actually occur in the fly ash cement pastes more or less on a particle-by-particle basis.     

Suppression of Sulfate Attack on a Stabilized Soil

A soil containing calcium sulfates was stabilized at 40°C, with ample moisture, by cementitious mixtures producing a range of calcium hydroxide upon hydration. Maximum expansion occurred when the stabilizing agent was lime, whereas a mixture containing portland cement, Class C fly ash, and an amorphous silica stopped the expansion. X-ray diffraction peak width and scanning electron microscopy (SEM) based image analysis showed that maximum expansion correlated with crystallization of colloidal ettringite. Ettringite was seen by SEM within an hour of mixing the soil with lime. Colloidal ettringite was not observed when portland cement, with supplementary cementing materials, was used for stabilization.     

The behavior of silicocarbonatite aggregates from the Montreal area

In a previous paper, it was concluded that silicocarbonatite aggregates from the Francon quarry, Montreal contributed to durability problems in Portland cement concrete. Results show that, at 2 days after casting, concrete made with silicocarbonatite aggregates contained over 1.5% more Na₂O than similar bars made with Exshaw limestone aggregates. A reaction involving the rare mineral dawsonite in the silicocarbonatite is thought responsible for the higher Na₂O content. In turn, this caused increased expansion of concrete bars made with alkali expansive aggregates. Also, concrete made with alkali-carbonate reactive Pittsburg aggregate showed more expansion when cured at 80 °C than bars cured at 23 °C. Concrete bars made with Exshaw limestone aggregates cured for 4 h at 85 °C showed late-stage expansion, which is attributed to delayed ettringite formation (DEF). However, no expansion was shown by heat-cured concrete prisms or mortar bars made with silicocarbonatite aggregates. Release of alkalis, aluminates and carbonates by the dawsonite reaction may have inhibited DEF. Concrete bars made with nonreactive Nelson dolostone and 10% silicocarbonatite cured at 80 °C for 4 h showed up to 0.15% expansion after several years at 23 °C and 100% relative humidity (R.H.), indicating that a deleterious reaction did occur.     

Influence of a thermal cycle at early age on the hydration of calcium sulphoaluminate cements with variable gypsum contents

Hydration of a belite calcium sulphoaluminate cement was investigated over one year as a function of its initial gypsum content (variable from 0 to 35%). Particular attention was paid to the influence of the thermal history of the material at early age on its subsequent evolution. Pastes and mortars (w/c 0.55) were either cured at 20 °C or submitted for one week to a thermal treatment simulating the temperature rise (up to 85 °C) and fall occurring in drums of cemented radwastes. The thermal cycle accelerated the early stages of hydration and mainly decreased the proportion of AFt versus AFm hydrates, especially at low initial gypsum contents (≤ 20% by weight of cement). It also strongly reduced the compressive strength of gypsum-free specimens (by 35% after one year), and doubled their expansion under water. These results were explained by mineralogical evolutions towards a more stable phase assemblage which included retarded ettringite formation.     

Influence of Gluconate, Lignosulfonate, and Glucose Admixtures on the Hydration of Tetracalcium Aluminoferrite in the Presence of Gypsum with or without Calcium Hydroxide

The influence of 0.3 wt% gluconute, lignosulfonate, or glucose on the hydration of 4CaO·A1₂O₃-Fe₂O₃ in the presence of gypsum with or without Ca(OH)₂, was examined. In the absence of Ca(OH)₂ all the admixtures retard both ettringite production and subsequent conversion of ettringite into the monosulfate in the decreasing order glucose >lignosulfonate >gluconate. In the presence ofCa(OH)₂ all the admixtures accelerate early ettringite production but do not affect subsequent conversion of ettringite into the monosulfate, at least up to 28 d.     

Expansion associated with ettringite formation at different temperatures

Expansion of hydrated mixtures made with C₃A, CaSO₄ · 2H₂O, Ca(OH)₂ and SiO₂, at 22, 30, 40, 50, 60°C, was studied to verify if expansion is associated with colloidal ettringite formation or with the solid state conversion of C₄AH₁₃ to monosulfate hydrate in presence of calcium hydroxide. From the results of our investigation it can be drawn the conclusion that mortars expansion is in connection with colloidal ettringite formation and the monosulfate hydrate is formed only when the greatest expansion is ended. The increase in hydration temperature seem to be favorable to the formation of colloidal ettringite.     

Kinetics of Tricalcium Aluminate and Tetracalcium Aluminoferrite Hydration in the Presence of Calcium Sulfate

Hydration reactions of C₃A and C₄AF with calcium sulfate hemihydrate and gypsum were investigated and the kinetics of the reactions compared. The rates of C₃A and C₄AF hydration, as determined by heat evolution, vary depending on whether the sulfate-containing reactant is gypsum or calcium sulfate hemihydrate. The following sequence of reactions involving C₄AF occurs when hemihydrate is the reactant: gypsum formation during the first hour, ettringite formation between 20 and 36 hours, and the conversion of ettringite to monosulfate over a period of about 12 hours. Monosulfate formation initiates prior to the complete consumption of gypsum. The onset of this conversion occurs at a shorter hydration time when hemihydrate is a reactant and the total amount of heat evolved is lower. The hydration reactions in saturated calcium hydroxide solution occur more slowly than those in water. Based on heat liberation, C₄AF reacts at a much higher rate than C₃A. Ettringite formation occurs during the first 8 to 9 days of C₃A hydration. Once the gypsum is consumed, ettringite converts to monosulfate during two additional days. Compared to gypsum, hemihydrate decreases the rates of hydration of both C₃A and C₄AF. The effects on the hydration characteristics of C₄AF are significant. The hydration of C₃A with gypsum in water, in saturated Ca(OH)₂ solution, and in 0.3 M NaOH solution were compared. Heat evolution is the lowest for hydration in 0.3 M NaOH. The onset of monosulfate formation occurs prior to the complete reaction between gypsum and C₃A in the NaOH solution.     

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.     

Delayed ettringite formation in Swedish concrete railroad ties

A petrographic examination of cracked Swedish concrete railroad ties identified delayed ettringite formation (DEF) as the damaging mechanism. This was unexpected because the concrete railroad ties were steam-cured with a maximum concrete temperature below 60 °C.The consensus in the published literature is that DEF only occurs in concrete subjected to heat curing above 70 °C. However, DEF is not only influenced by the curing temperature, but also by various other factors, such as cement composition (alkalis, C₃S, C₃A, SO₃, and MgO), fineness, etc. If an unfavorable combination of these parameters exists, delayed ettringite may occur at lower temperatures than 70 °C.In this paper, the influence of various parameters on DEF is discussed with reference to the investigated concrete.     

Effect of hydration temperature on the solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pastes

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement and white Portland cement pastes were measured during the first 28 d of curing at temperatures ranging from 5–50 °C. Saturation indices with respect to solid phases known to form in cement paste were calculated from a thermodynamic analysis of the elemental concentrations. Calculated saturation levels in the two types of paste were similar. The solubility behavior of Portlandite and gypsum at all curing temperatures was in agreement with previously reported behavior near room temperature. Saturation levels of both ettringite and monosulfate decreased with increasing curing temperature. The saturation level of ettringite was greater than that of monosulfate at lower curing temperatures, but at higher temperatures there was effectively no difference. The solubility behavior of C-S-H gel was investigated by applying an appropriate ion activity product (IAP) to the data. The IAP_CSH decreased gradually with hydration time, and at a given hydration time the IAP_CSH was lower at higher curing temperatures.