Showing papers on "Ettringite published in 2011"

Synergy between fly ash and limestone powder in ternary cements

Abstract: The interaction between limestone powder and fly ash in ternary composite cement is investigated. Limestone powder interacts with the AFm and AFt hydration phases, leading to the formation of carboaluminates at the expense of monosulphate and thereby stabilizing the ettringite. The effect of limestone powder on OPC may be restricted due to the limited amount of aluminate hydrates formed by the hydration of OPC. The additional aluminates brought into the system by fly ash during its pozzolanic reaction amplify the mentioned effect of limestone powder. This synergistic effect between limestone powder and fly ash in ternary cements is confirmed in this study and it translates to improved mechanical properties that persist over time.

Influence of the calcium sulphate source on the hydration mechanism of Portland cement–calcium sulphoaluminate clinker–calcium sulphate binders

Abstract: Two different binders composed of Portland cement, calcium sulphoaluminate clinker and calcium sulphate were studied from early hydration to 28 days, one containing gypsum and one containing anhydrite at equimolar CaSO4 amount. Sodium gluconate was used as retarder to obtain a sufficient fluidity to cast the samples. Solids were analyzed by X-ray diffraction, scanning electron microscopy and thermogravimetric analysis and quantified by Rietveld refinement, while pore solutions were measured by ion chromatography. Thermodynamic modelling was used to model the hydration process of the ternary binders. This combined study allowed a precise understanding of the hydration process over time and the determination of the composition of the crystalline and of the X-ray amorphous hydrates present in the binders, which cannot be determined by conventional methods. Results show that the hydration mechanisms are similar in presence of gypsum or anhydrite, the difference being in the kinetics of reactions due to the slower dissolution of anhydrite compared to gypsum in the presence of sodium gluconate. The hydration starts with the formation of ettringite and of some X-ray amorphous hydrates. In the anhydrite-bearing system, the ettringite-forming reaction is stronger delayed by the addition of the retarder compared to the gypsum-bearing system. This stronger delay results in the formation of a significant amount of X-ray amorphous hydrates. The hydrates amorphous fraction is composed of different phases and its chemical composition is changing over time. During early hydration, it is mainly composed of aluminium hydroxide and stratlingite, while in the anhydrite-bearing system it can additionally contain some monosulphoaluminate. At later ages, the aluminium hydroxide content decreases and additional monosulphoaluminate and a C-S-H type phase are formed.

NMR, XRD, IR and synchrotron NEXAFS spectroscopic studies of OPC and OPC/slag cement paste hydrates

Abstract: This work aims to determine the fundamental similarities and/or differences between OPC and OPC/slag paste hydrates. OPC and 35% slag pastes are investigated using five techniques: 29Si NMR, 27Al NMR, X-ray diffraction (XRD), infrared (IR) and synchrotron near edge X-ray absorption fine structure (NEXAFS) spectroscopy. 29Si NMR provides valuable information related to the formation of the C–S–H gel, the main hydrated phase of the cement paste. 27Al NMR is a useful tool to characterize calcium aluminates and aluminate hydrates such as ettringite and monosulphate hydrate. XRD identifies polycrystalline phases of the hardened cement paste, including ettringite, monosulphate and CaOH2. Vibrational frequencies in IR assist in identifying the silicate, sulphate and carbonate phases of the cement paste. As far as we are aware, Si K-edge NEXAFS has never been applied in cement research and its advantages and disadvantages are discussed. Using these techniques, a comparison between OPC and 35% slag paste hydrates is made, shedding light on differences in the amount and form of hydrated phases present, especially the absence of ettringite in the 35% slag paste.

Coupled Thermochemical Effects on the Strength Development of Slag-Paste Backfill Materials

Abstract: In this paper, the coupled effects of sulfate (chemical factor) and curing temperature (thermal factor) on the strength development of cemented paste backfill (slag-CPB) that contains ground granulated blast furnace slag (slag) as a mineral admixture is studied. Almost 200 slag-CPB samples, cured at temperatures of 2, 20, 35, and 50°C at 28, 90, and 150 days with sulfate concentrations of zero ppm, 5,000 ppm, 15,000 ppm, and 25,000 ppm are tested for uniaxial compressive strength (UCS). The results show that the effect of sulfate on the strength of slag-CPB is significantly dependent on the curing temperature, initial sulfate content, and curing time. The coupled effects of sulfate and temperature can lead to an increase or decrease of the slag-CPB strength and significantly influence the type and amount of minerals formed within the cemented matrix of slag-CPBs. The strength-increasing factors are (1) refinement of the pore structure of slag-CPBs due to the precipitation of ettringite within the empty po...

Analysis of concrete in a vertical ventilation shaft exposed to sulfate-containing groundwater for 45 years

Abstract: The concrete lining of a vertical ventilation shaft exposed to sulfate-bearing groundwater for 45 years was investigated. The concrete was characterized at several sites with different degrees of damage. Apart from strength measurements, optical and electron microscopy were the main tools used to assess the degree of damage and alteration of the concrete. The occurrence of damage appears to be related to the availability of sulfate-bearing groundwater on one side and the presence of poor-quality concrete on the other side. Most of the damage in the form of severe spalling is caused by ettringite formation. The impact of thaumasite formation with disintegration of the cement paste seems to be minor as it occurs after the concrete is already severely damaged. The mineral assemblages and the sequence of mineral formation caused by sulfate attack agree with the findings of laboratory studies and thermodynamic modeling.

Influence of limestone content, gypsum content and fineness on early age properties of Portland limestone cement produced by inter-grinding

Abstract: This paper examines the behavior of Portland limestone cements (PLCs) made by inter-grinding clinker, gypsum and limestone in a full size-cement plant, while varying the content of limestone filler (LF – 0% and 24%), content of gypsum (GC – 2.5% and 5.0%) and PLC fineness, measured as that fraction retained on a 45 μm sieve (R45 – 5% and 18%). The influence of the combined action of these variables on the particle size distribution (PSD) and early age properties of blended cement was studied using a 23 experimental design. Fineness evaluated by the parameters of the Rosin–Rammler–Springer–Bennett (RRSB) distribution function shows that the uniformity index (n-parameter) depends strongly on LF content; while the characteristic diameter (x′-parameter) depends on R45 and LF content. As for the inter-grinding process, water demand is reduced by incorporation of LF and increased by reduction of R45, producing a compensation. Setting time is mainly affected by R45; LF produces a few modifications and the influence of gypsum content and gypsum–limestone interaction are not as obvious. Calorimetry studies show that LF decreases the height of the main peak and the total heat released, while gypsum content modifies the time of acceleration and the post-peak hydration, specifically for fine cements. In accordance, chemical shrinkage decreases when R45, LF content and gypsum content increase. Up to 2 days, strength is mainly governed by the R45 and the LF in PLC, which act inversely. At early ages, the influence of gypsum content on early properties increases up to 48 h. XRD analysis shows its stimulation of calcium silicate hydration during the first hours and ettringite formation after 24 h.

Micro-analysis of “salt weathering” on cement paste

Abstract: Normally, concrete technologists attribute salt weathering, salt crystallization or physical attack to the deterioration of concrete that is partially exposed to sulfate environment. However, there are few convincing evidences supporting this view. The purpose of this paper is to check by means of extensive micro-analysis if traces of sulfate crystals are present in the paste. This would enable to verify in a direct way whether salt weathering really causes cement paste damage or not. In this research, cement paste and cement–fly ash paste specimens were partially exposed to sodium sulfate and magnesium sulfates solution under a constant storage condition (20 ± 2 °C, and 60 ± 5% RH) and a sharply fluctuating environment (40 ± 2 °C and 35 ± 5% RH for 24 h, then 10 ± 1 °C and 85 ± 5% RH, also for 24 h) respectively. ESEM and SEM images, combined with EDS and XRD analysis, were employed for micro-analysis. The experimental results indicate that Na2SO4 crystals or MgSO4 crystals could not be identified in the cement paste or the cement–fly ash paste, both under constant and fluctuating exposure conditions. Instead, large amounts of ettringite, gypsum and brucite were found and identified as probably one of the determining factors causing failure of cement paste. On the other hand, salt crystallization could be observed in the calcium carbonate crystals, the carbonated products of hydrated cement paste.

Hydration of C3A with Calcium Sulfate Alone and in the Presence of Calcium Silicate

Abstract: Tricalcium aluminate (C3A) is one of the main constituents of Portland cement. Even though it represents less than 10% of the total composition, its strong reaction with water can lead to a rapid setting, called flash set. Gypsum is added to regulate this reaction and preserves the workability of the cement paste at early ages. The understanding of the C3A-gypsum reaction is therefore crucial for the comprehension of the early hydration of cement. The role of the amount of C3A and the sulfate balance of cement hydration are of major interest since two important routes for the development of new cementitious materials are the increasing rate of substitution materials and the increasing level of aluminate in clinker. This thesis aimed to investigate the C3A-gypsum reaction alone and in the presence of alite in order to provide basic knowledge on the C3A-gypsum reactions and study the interactions that occur between the cement phases when hydration occurs in alite-C3A-gypsum systems. Alite and C3A as well as clinkers of controlled composition were synthesized. Model systems composed of C3A with different gypsum additions and alite-C3A-gypsum systems were studied in terms of hydration kinetics, phase assemblage and microstructural development. This work confirmed the findings of previous works on the mechanism that controls C3A-gypsum hydration when sulfate ions are present in solution and gave new results on the reaction when gypsum is depleted. It was shown that AFm phases do not crystallize only as platelets that fill the space between the C3A grains but also form an "inner" product within the original C3A grain boundaries and that hydrogarnet (for which the presence depends on the gypsum addition) crystallizes as a rim around C3A grains. Moreover the influence of the gypsum addition on the morphology of the AFm platelets and the role of their morphology on the hydration rate were highlighted. In the presence of alite the hydration kinetics of C3A-gypsum systems was subject to change due to the adsorption of sulfate ions on C-S-H and the reduction of the space available for the reaction. In addition, with the correlation of calorimetric, XRD and SEM analyses it was possible to observe a second formation of ettringite from C3A and sulfate ions released from C-S-H after the depletion of gypsum. Finally, the rate of alite hydration related to the growth of C-S-H was shown to be modified in the presence of C3A and gypsum.

Regeneration aggregate made of carbonized maintaining castoff

Abstract: The invention relates to a recycled aggregate made by pre-adding water to carbonated curing wastes for forming balls which absorb carbon dioxide gas in industrial waste gas, which belongs to the building material technique field. The recycled aggregate is made by adopting the following processes of: utilizing steel slag or cement containing carbonization compositions as a raw material, adding water for even mixture, and implementing the granulation for spheroidizing so as to make the recycled aggregate under the curing of carbonization gas; and the carbonization compositions include one or more than one of calcium oxide, calcium hydroxide, dicalcium silicate, tricalcium silicate, hydrated calcium silicate, hydrated calcium aluminate, hydrated calcium ferrite, ettringite, magnesium hydroxide and magnesium oxide, and the carbonization compositions have a weight percentage of no less than 30 percent in materials. The recycled aggregate has the main point that: the accelerated carbonization technique is utilized to treat industrial wastes such as steel slag, sludge and waste concrete, solidify and store carbon dioxide of greenhouse gas, and make the recycled aggregate. The pollution problem of wastes such as steel slag and waste gas is released; meanwhile, the light recycled aggregate with excellent performance is prepared.

Activation of Ground Granulated Blast Furnace Slag Cement by Calcined Alunite

Abstract: To enhance the early strength of grounded granulated blast furnace slag (GGBFS) blended cement, the activation characteristics of GGBFS were examined by a potassium aluminum sulfate (PSA) clinker, consisting of KAl(SO4)2 and amorphous Al2O3 by calcining alunite (K2SO4� Al2(SO4)3� 4Al(OH)3) at 650 � C for 30 min. The PSA clinker reacted with calcium hydroxide and gypsum to form ettringite (3CaOAl2O3� 3CaSO4� 32H2O, AFt) by following reaction: 2KAl(SO4)2 þ 2Al2O3 þ 13Ca(OH)2 þ 5(CaSO4� 2H2O) þ 74H2O ! 3(3CaOAl2O3� 3CaSO4� 32H2O) þ 2KOH. Mortar was prepared by mixing a blended cement of GGBFS and ordinary Portland cement (OPC) with PSA clinker as activator. The compressive strength of the GGBFS blended cement mortar was compared with that of OPC mortar. When the PSA clinker and gypsum activator was added to the blended cement of GGBFS and OPC, the hydration products investigated by DTA and X- ray diffraction were mainly ettringite and calcium silicate hydrate(C-S-H) gel. The early and long-term strengths of the GGBFS blended cement were higher than those of OPC. Therefore, PSA clinker as activator was shown to improve the early and long-term strengths of GGBFS blended cement. (doi:10.2320/matertrans.M2010350)

Ettringite surface chemistry: Interplay of electrostatic and ion specificity

Abstract: This paper presents a detailed experimental study combined with Monte Carlo (MC) simulations within the primitive model of the physical chemistry at the ettringite-water interface over a wide range of pH and bulk conditions for which ettringite exists thanks to its solubility in aqueous solutions. Ettringite, which is an important phase in hydrated cement-based systems, bears a permanent and positive structural charge. In contrast with previous studies, electrokinetic measurements together with the careful chemical analysis of the equilibrium solutions of the dispersions have brought strong support to designate sulfate as being the ion determining the potential. Simulations showed that electrostatics, through ion-ion correlations, are not strong enough to explain the charge reversal of ettringite immersed in sulfate salt solutions. However, an excellent agreement between simulated and experimental data was obtained including a short-range nonelectrostatic adsorption potential for the sulfate ion. This result strongly suggests the existence of a chemical specificity of sulfate ions for an ettringite surface.