Rheological and hydration characterization of calcium sulfoaluminate cement pastes

Calcium sulfoaluminate (CSA) cements are currently receiving a lot of attention because their manufacture produces less CO₂ than ordinary Portland cement (OPC). However, it is essential to understand all parameters which may affect the hydration processes. This work deals with the study of the effect of several parameters, such as superplasticizer (SP), gypsum contents (10, 20 and 30 wt.%) and w/c ratio (0.4 and 0.5), on the properties of CSA pastes during early hydration. This characterization has been performed through rheological studies, Rietveld quantitative phase analysis of measured X-ray diffraction patterns, thermal analysis and mercury porosimetry for pastes, and by compressive strength measurements for mortars. The effect of the used SP on the rheological properties has been established. Its addition makes little difference to the amount of ettringite formed but strongly decreases the large pore fraction in the pastes. Furthermore, the SP role on compressive strength is variable, as it increases the values for mortars containing 30 wt.% gypsum but decreases the strengths for mortars containing 10 wt.% gypsum.     

Contribution of limestone to the hydration of calcium sulfoaluminate cement

Calcium sulfoaluminate (CSA) cements can be blended with mineral additions such as limestone for properties and cost optimization. This study investigates the contribution of limestone to the hydration of a commercial CSA clinker regarding the hydration kinetics, hydrate assemblage and compressive strength. Nine formulations were defined at M-values of 0, 1.1 and 2.1 (M = molar ratio of anhydrite to ye’elimite) without and with medium and high limestone contents.Calorimetric results indicate that limestone accelerates the hydration reaction especially at M = 1.1, probably due to the filler effect. The phase assemblages were calculated by thermodynamic modeling using Gibbs Energy Minimization Software (GEMS). With increasing limestone content the formation of ettringite and calcium monocarboaluminate is predicted at the expense of calcium monosulfoaluminate. With increasing M-value more ettringite is predicted at the expense of the monocarbonate and less calcite takes part in the hydration reactions.The modeled results compare well with the experimental data after 90 d of hydration, except that calcium hemicarboaluminate was found instead of monocarbonate, which is assumed to be due to kinetics considerations.The lowest compressive strength occurs in ternary formulations, whereas in the absence of calcium sulfate, strength is significantly higher.The results presented here indicate that in CSA cements, limestone accelerates early hydration kinetics, takes part in the hydration reactions at M < 2, and has a positive effect on strength development in systems without anhydrite.     

Micro-mechanical properties of calcium sulfoaluminate cement and the correlation with microstructures

To investigate the micro-mechanical properties of calcium sulfoaluminate cement and the correlation with the microstructures, we apply a variety of advanced techniques of microstructural and micro-mechanical characterization, including scanning electron microscopy with backscattered electron and energy-dispersive X-ray spectroscopy detectors, X-ray fluorescence, X-ray diffraction and nanoindentation. For the first time, the micro-mechanical properties of material microstructures present in a calcium sulfoaluminate cement are estimated. In the calcium sulfoaluminate cement used in this research, two type of hydration product microstructures with the differentiable microstructural morphologies, compositions and micro-mechanical properties are identified and investigated. The correlation of the micro-mechanical properties with the microstructures shows that the hydration product microstructure containing more ettringite has lower indentation modulus and hardness than that containing more aluminum hydroxide.     

Effect of calcium sulfate source on the hydration of calcium sulfoaluminate eco-cement

The availability of cements, including eco-cements, with tailored mechanical properties is very important for special applications in the building industry. Here we report a full study of the hydration of calcium sulfoaluminate eco-cements with different sulfate sources (gypsum, bassanite and anhydrite) and two water/cement ratios (0.50 and 0.65). These parameters have been chosen because they are known to strongly modify the mechanical properties of the resulting mortars and concretes. The applied multi-technique characterization includes: phase assemblage by Rietveld method, evolved heat, conductivity, rheology, compressive strength and expansion/retraction measurements. The dissolution rate of the sulfate sources is key to control the hydration reactions. Bassanite dissolves very fast and hence the initial setting time of the pastes and mortars is too short (20 min) to produce homogeneous samples. Anhydrite dissolves slowly so, at 1 hydration-day, the amount of ettringite formed (20 wt%) is lower than that in gypsum pastes (26 wt%) (w/c = 0.50), producing mortars with lower compressive strengths. After 3 hydration-days, anhydrite pastes showed slightly larger ettringite contents and hence, mortars with slightly higher compressive strengths. Ettringite content is the chief parameter to explain the strength development in these eco-cements.     

A hydration study of various calcium sulfoaluminate cements

The present work studies the hydration process and microstructural features of five calcium sulfoaluminate (CSA) cements and a ternary mixture including also ordinary Portland cement (OPC). The pastes were studied with simultaneous differential thermal-thermogravimetric (DTA-TG) analysis, mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), and expansion/shrinkage tests. The DTA-TG analysis confirmed the role of the hydration reactions involving the main CSA clinker constituent, tetracalcium trialuminate sulfate, which produced (i) ettringite when combined with lime and calcium sulfate, (ii) ettringite and aluminum hydroxide in the presence of calcium sulfate alone, and (iii) monosulfate and aluminum hydroxide in the absence of both lime and calcium sulfate. The MIP and SEM were able to discriminate between expansive (ternary mixture and CSA cement containing 50% gypsum) and non-expansive cements. Expansive cement pastes had (i) a nearly unimodal pore size distribution shifted toward higher radii and (ii) ettringite crystals smaller in size during the first day of curing. In a SEM image of a hardened paste of the CSA cement containing 50% gypsum, a stellate ettringite cluster was observed.     

Identification of hydration stage of calcium sulfoaluminate cement at an early age with helium pycnometry

This work is to study the mechanism of the hydration of calcium sulfoaluminate (CSA) cement at an early age by identifying the hydration stage with helium pycnometry, heat evolution test, X-ray diffraction, thermo-gravimetric/differential scanning calorimetry and scanning electron microscope. The results show that the hydration process of CSA cement has four stages: rapid dissolution stage, dissolution-crystallization stage, crystal growth stage and stable stage. In the rapid dissolution stage, exothermic wetting and fast hydration result in the shrinkage occurring from 0 to 15 min. A significant volume decrease happens in the dissolution-crystallization stage. In the crystal growth stage, ettringite prefers to crystalize at the neighboring places of pores at the nano scale due to the combined action of crystallization pressure and size effect. The volume increase considerably slows down throughout the stable stage indicating the deceleration of CSA cement hydration. In addition, the influence of the proportions of clinker and gypsum in the CSA cement on its hydration is studied as well.     

Influence of curing temperatures on the hydration of calcium aluminate cement/Portland cement/calcium sulfate blends

In this paper, the effects of curing temperature on the hydration of calcium aluminate cement (CAC) dominated ternary binders (studied CAC: Portland cement: calcium sulfate mass ratio were 22.5: 51.7: 25.8) were estimated at 0, 10, 20 and 40 °C, respectively. Both α-hemihydrate and natural anhydrite were employed as the main source of sulfate. The impacts of temperature on the phase assemblages, morphology and pore structure of pastes hydrated up to 3 days were determined by using X-ray diffraction (XRD), backscattered electron imaging (BEI) and mercury intrusion porosimetry (MIP). Results reveal that the main hydration products are firmly related to calcium sulphoaluminate based phases. Increasing temperature would result in a faster conversion from ettringite to plate-like monosulfate for both calcium sulfate doped systems. When the temperature increases to 40 °C, an extraordinary formation of strätlingite (C₂ASH₈) and aluminium hydroxide is observed in anhydrite doped pastes. Additionally, increased temperature exerts different effects on the pore structure, i.e. the critical pore diameter shifts to finer one for pastes prepared with α-hemihydrate, but changes to coarser one for those made with anhydrite. From the mechanical point of view, increased temperature accelerates the 1-day strength development prominently, while exerts marginal influence on the development of 3-day strength.     

甲酸钙对硫铝酸盐水泥早期水化过程的影响

利用维卡仪、水化放热速率、XRD、TG-DSC和SEM等测试手段,研究了甲酸钙(Ca(HCOO)2)对硫铝酸盐水泥凝结时间、水化历程和水化产物及微观形貌的影响。结果表明,Ca(HCOO)2可明显促进硫铝酸盐水泥的凝结,并缩短初凝和终凝时间间隔;显著缩短了硫铝酸盐水泥的水化诱导期,且使水化加速期提前,使第一水化热峰值提高32%,但对水化稳定期的水化放热速率无明显影响;Ca(HCOO)2可以提高硫铝酸盐水泥水化环境的碱度,在早期提高了水化产物钙矾石(AFt)的结晶度,水化早期生成的水化产物结构致密,但并不改变水化稳定期的水化产物和微观形貌。     

Ettringite and calcium sulfoaluminate cement: investigation of water content by near-infrared spectroscopy

Calcium sulfoaluminate (CSA) cement is a sulfate-based binder whose high-performance hydraulic behavior depends on the rapid formation of ettringite, when grinded clinker is hydrated in presence of gypsum. Ettringite is a calcium aluminum sulfate mineral characterized by high water content, estimated as 32 water molecules per formula unit. Three examples of utilization of near-infrared (NIR) spectroscopy are here shown. First of all, information on water distribution in pure ettringite was deduced and compared with infrared analyses. Then its thermal behavior has been followed up to 400 °C, allowing to improve the knowledge about water loss and thermal decomposition of this hydrated phase. Finally, the obtained results have been employed in order to follow hydration of CSA cement sample, demonstrating thus that NIR spectroscopy, being highly sensitive to water amount and distribution, can be an extremely useful tool for hydration studies.     

Effect of crystal seeding on the hydration of calcium phosphate cement

In this paper, the effect of crystal seeding on the hydration of calcium phosphate cement (CPC) has been carefully investigated. The setting time of the CPC slurry not containing any crystal seeds was 150 min, while the setting time for the specimen containing 5 wt% low crystallinity hydroxyapatite used as a crystal seed was 7 min. This improvement in the setting time was due to HAP serving as a substrate for heterogeneous nucleation which accelerated nucleation. In addition, the compressive strength of the specimen containing the crystal seeding was deduced and we report values different from those previously reported in the literature. The calorimetric curve indicated that crystal seeding could reduce the induction period. A.c. impedance spectroscopy revealed that at the beginning of hydration, the rate of reaction increased and also that the mean diameter and porosity decreased as the seed content increased. At the end of the hydration reaction the situation was changed with the mean diameter and porosity in the sample without any seeds being a minimum, which indicated that the compressive strength was a maximum. This result could be explained by the dissolution and reprecipitation of small hydration products produced by the high rate of reaction produced by the introduction of the crystal seeds.     

Influence of finely ground limestone on cement hydration

Some work has been carried out on the effect of calcium carbonate on cement paste, but there is no general agreement on the relative effects of different amounts of calcium carbonate on cement paste properties. The objective of the present work is to assess the effect of various amounts of calcium carbonate on the hydration of tricalcium silicate in order to explain the physico-chemical changes occurring during Portland cement hydration. It is shown that calcium carbonate has an accelerating effect on C₃S and cement hydration and leads to the precipitation of some calcium carbosilicate hydrate.     

An alternative to Portland Cement for waste encapsulation--the calcium sulfoaluminate cement system

Currently, Portland Cement (PC) is used extensively in the solidification/stabilisation of a wide variety of wastes. In the nuclear industry, low and intermediate level radioactive wastes are encapsulated or immobilised within composite PC cement systems based on high replacement with blast furnace slag or fly ash. However, the high alkalinity of these PC-based systems will corrode reactive metals found in some wastes releasing hydrogen and forming expansive corrosion products. Alternative cement systems could provide a different hydration chemistry, which would allow wastes containing these metals to be encapsulated with lower reactivity. Calcium sulfoaluminate (CS A) cement is one such cement. It combines economy of cost and low emission of CO(2) with rapid strength gain and compatibility with other construction materials. Hydration provides an internal pore solution where the pH is considerably lower than that of PC. The main hydration product, ettringite, can incorporate a number of ions into its crystal structure, making it an ideal candidate for waste immobilisation. This paper details some results from a commercial CS A system that examines aspects of mixing, hydration of different formulations and aluminium corrosion behaviour. The fluidity of mixes can be adjusted by changing the formulations. All designed mixes were set within 24 h with little bleeding and the pH values were in the range of 10-11.5. In addition, a significant reduction in Al corrosion was observed compared to a composite OPC system. Although these results provide encouragement for the idea that CS A cement can provide a possible alternative to PC in the immobilisation of difficult and reactive wastes, further investigation is needed.     

Influence of cement kiln dust on the physical properties of calcium lime mortars

This work investigates the influence of cement kiln dust (CKD) on the properties of mortars made with a non-hydraulic binder of high available-lime content (calcium lime—CL), in order to further recycle industrial waste. Physical properties of CKD-CL90 mortars with increasing CKD content were compared to those of feebly-hydraulic lime (NHL2) and CL90 mortars. This paper concludes that, despite the CKD in this study being partially inert, the abundant reactive, free lime provided by the CL90 binder has enabled formation of hydration products. The strength development, rising proportionally to the amount of CKD when addition is over 5%, and the reduction in porosity/suction of the CKD/CL90 mixes, support the occurrence of hydraulic set. The high alkalinity of the CKD/CL90 system; the high specific surface of the CKD particles and the presence of amorphous reactive silica further support the presence of hydraulic set. Results evidenced that CKD addition significantly increased the mortar’s water demand simultaneously enhancing compressive strength and bulk density, and decreasing porosity and capillary suction. These effects can be ascribed to both the gain of packing density induced by the CKD particles, and the formation of hydration phases within pores and the space originally filled with water. Finally, this work concludes that the physical properties of CKD/CL mortars including at least 20%CKD are comparable to those of feebly hydraulic lime mixes, however, fracturing by shrinkage (due to high water demand) and damage related to sulphur, chlorine and alkali content need to be investigated before CKD/CL mixes are advised for application.     

Development of self-leveling screed based on calcium sulfoaluminate cement: Modelling of curling due to drying

The development of cement-based screed unbound to its support is still limited because of the curling that occurs at the corners and perimeter of the screed. This phenomenon is mainly due to the moisture gradient that appears within the thickness of the screed: the upper surface dries and shrinks, whereas lower regions dry less and stay wetter. This paper demonstrates this phenomenon can be mitigated through the use of calcium sulfoaluminate cement instead of ordinary Portland cement. Experiments utilizing an original, specially designed device, show that curling is 3.5 times lower when calcium sulfoaluminate cement is used compared to ordinary Portland cement. The moisture gradient within the thickness of the screed is also lower.A model based on simplified poroelasticity theory describes both fluid transfer and hydro-mechanical coupling. The comparison between experimental and calculated results shows that the model gives a good estimation of the kinetics of the mass loss, and that the numerical simulation is an effective tool to predict curling due to drying.     

Carbonation of calcium sulfoaluminate mortars

This study investigated potential physical and chemical parameters that could govern the carbonation rate of calcium sulfoaluminate (CSA) mortars and endeavored to elucidate the microstructural and chemical factors that govern CSA cement's carbonation rate. Experiments included: water absorption, oxygen diffusion, mercury intrusion porosimetry, quantitative X-ray diffraction, thermogravimetric analysis, accelerated carbonation, compression and flexure tests. Additionally, the carbonation process was investigated using thermodynamic modeling. The results show that CSA mortars carbonate much faster than Portland cement mortars and at approximately the same rate as calcium aluminate cement mortars. Additionally, CSA mortars carbonate slower with decreasing w/c, and the anhydrite content of the CSA mortars strongly affects the ye'elimite reaction kinetics which plays an important role in imparting carbonation resistance in CSA mortars. Finally, calcium sulfate additions to CSA clinker to produce CSA cement dilutes the clinker content and reduces the amount of CO₂ that the CSA cement can ultimately bind.     

Dielectric and electrical properties of ordinary Portland cement and slag cement in the early hydration period

The dielectric constant and electrical conductivity of ordinary Portland cement (OPC) with water-cement ratios (w/c) of 0.30, 0.35 and 0.40 were measured for the first 30 h hydration, using a microwave technique in the frequency range 8.2–12.4 GHz. It was found that both the dielectric constant and electrical conductivity of the cement paste are sensitive to the water-cement ratio, the higher the w/c value, the greater the dielectric constant and electrical conductivity, and the longer the hydration time. We also found that the higher the frequency the greater the electrical conductivity but the smaller the dielectric constant. The dielectric constant and electrical conductivity of high- and low-slag cement with water-solid ratio (w/s) of 0.40 were measured in the first 30 h after mixing. The changes in dielectric constant and electrical conductivity of low-slag cement with time are similar to that of OPC, but the high-slag cement shows very different dielectric and electrical properties compared with OPC and low-slag cement. The relationship between the dielectric and electrical properties of cement paste and cement hydration was also discussed.     

Influence of KOH solution on the hydration and carbonation of high alumina cement mortars

The influence of KOH presence on the evolution of hydration and carbonation of high alumina cement mortars at two different curing temperatures (4 and 40 °C) has been studied. It has been confirmed that hydration reactions at both temperatures are accelerated with KOH presence and it has a great influence on hydrated and carbonated species. The massive deposition of carbonation products leads, in all cases to a decrease in mortar porosity which leads to an increase of mechanical strengths.     

双金属催化剂制聚醚多元醇在聚氨酯软泡中的应用

介绍双金属催化剂(DMC)制备普通软泡聚醚多元醇及其合成工艺、产品质量、以及在PU泡沫中的应用,并与KOH制聚醚多元醇、进口3010聚醚多元醇进行了质量与发泡性能比较,得出:DMC制聚醚多元醇可以取代KOH制聚醚多元醇或3010聚醚多元醇用于普通软泡生产。