Chemical activation of calcium aluminate cement composites cured at elevated temperature
01 Nov 2012-Cement & Concrete Composites (Elsevier)-Vol. 34, Iss: 10, pp 1187-1193
TL;DR: In this article, the influence of sodium sulfate on the hydration of CAC-fly ash-silica fume composites was investigated, and it was shown that Na2SO4 accelerated hydration reactions of calcium aluminate cement as well as the reactions of FA and SF with CAH10 and C2AH8 to form the stratlingite.
Abstract: The influence of sodium sulfate, as an activator, on the hydration of calcium aluminate cement (CAC)–fly ash (FA)–silica fume (SF) composites was investigated. Different mixes of CAC with 20% pozzolans (20% FA, 20% SF and 10% FA + 10% SF) were prepared and hydrated at 38 °C for up to 28 days. The hydration products were investigated by XRD, DSC and SEM. The results showed that sodium sulfate accelerated the hydration reactions of calcium aluminate cement as well as the reactions of FA and SF with CAH10 and C2AH8 to form the stratlingite (C2ASH8). The later reactions prevent the strength loss by preventing the conversion of CAH10 and C2AH8 to the cubic C3AH6 phase. The acceleration effect of Na2SO4 on the reactivity of fly ash was more pronounced than on the reactivity of silica fume with respect to reaction with CAH10 and C2AH8 phases.
TL;DR: In this article, the microstructural and chemical changes of calcium aluminate cement (CAC)-based UHPC exposed to high temperatures were investigated, which led to a significant increase in micro-pores.
Abstract: This study investigated the microstructural and chemical changes of calcium aluminate cement (CAC)-based UHPC exposed to high temperatures. Upon exposure to 100 °C, C3AH6 was formed by the dehydration of CAH10. A further increase in the exposure temperature to 450 °C resulted in the formation of a new phase C12A7, which is attributed to the dehydration of C3AH6 and AH3. The compressive strength of UA50 and UA70 increased significantly due to the formation of C-A-(S)-H gel resulting from the further hydration of anhydrous CAC and silica fume upon exposure to 450 °C. The hydration reaction of CAC in UHPC led to a significant increase in the micro-pores (
TL;DR: The present review study aimed to investigate the activation methods to improve the properties of tailings resulted from minerals processing in order to be used as a replacement for cement, to reduce the pollution caused by cement production, as well as to reduced the volume of unused mineral tailings.
Abstract: Over the past few decades, as demand for minerals and metals has increased, the amount and volume of wastes and tailings has also increased dramatically. The management and reuse of mineral wastes and tailings not only help protect the environment but also are properly associated with economic benefits. As a result, mineral processing wastes disposal and storage has become a global issue. Along with the use of cemented paste as a backfill in underground spaces, the use of mineral processing wastes in the construction industry or as a substitute for cement is one of the new approaches to mineral tailings management. It is worth noting that the cement industry is facing crucial environmental issues. Portland cement production in industries increases the greenhouse effect and creates acidic rain. In fact, it generates greenhouse gases directly through carbon dioxide emission during clinker production as well as through energy consumption. In addition, the increasing stringency of environmental regulations has forced the mining industries to make efforts in order to manage tailings. One of the new and attractive techniques to reduce environmental problems and to obtain economic and technological benefits is to increase the use of tailings, for example, the use of mineral tailings as a substitute for cement. It is important to note that mineral tailings are not normally cemented and are accompanied by reduced strength of cement and concrete mortars; thus, there is a need for methods to increase their cementitious properties. Activation is one of the methods improving cementitious/pozzolanic properties of mineral tailings. Therefore, the present review study aimed to investigate the activation methods to improve the properties of tailings resulted from minerals processing in order to be used as a replacement for cement, to reduce the pollution caused by cement production, as well as to reduce the volume of unused mineral tailings. Different physical, chemical, and thermal activation methods were examined, and criticisms and research gaps of previous studies were presented.
TL;DR: In this article, a thermal energy storage (TES) in concrete as solid media for sensible heat storage is proposed to improve the cost and efficiency of solar thermal electricity (STE) plants.
Abstract: A concept for thermal energy storage (TES) in concrete as solid media for sensible heat storage is proposed to improve the cost and efficiency of solar thermal electricity (STE) plants. Mortar and concrete mixes were designed with calcium alumina cement (CAC) blended with blast furnace slag (BFS), using aggregates of different sources and size for stability performance after long term at high temperature. Seventy-five thermal cycles of 24 h length, within the temperature range 290 °C to 550 °C, have been used to simulate the expected operating conditions of TES. The dehydration processes at microstructural level have been evaluated and correlated with mechanical properties. Dehydration processes and consecutive heat/cool cycles induce changes in concrete at micro- and macro-level. The stabilization of damage with the charge/discharge heat cycles for thermal fatigue depends significantly on the aggregate type used. CAC is a suitable binder to use in thermal energy storage systems able to maintain its properties under repetitive heat cycles.
TL;DR: In this article, the effect of nano-silica on the hydration and conversion of calcium aluminate cement (CAC) was investigated, and it was shown that adding nano silica in CAC mitigated the formation of C3AH6 and AH3.
Abstract: This study investigated the effect of nano-silica on the hydration and conversion of calcium aluminate cement (CAC). The specimens were exposed to 60 °C to accelerate the transformation of hydrates in CAC after initial curing at 20 °C. At an early stage of curing, adding nano-silica in CAC mitigated the formation of C3AH6 and AH3. At 1 day after exposure to 60 °C, the compressive strength reduction was observed in all specimens, and it was closely related to the conversion of CAH10. That is, the conversion of all specimens occurred at 28 days after exposure, which resulted in a reduction in compressive strength. However, adding nano-silica mitigated the conversion from metastable phases (CAH10 and C2AH8) to stable phases (C3AH6 and AH3). As a result, the extent of compressive strength loss could be reduced by the formation of stratlingite (C2ASH8) in the case of 4% addition of nano-silica. In conclusion, adding nano-silica in CAC was effective regarding structural integrity by delaying the conversion of metastable hydrates, as well as maintaining the long-term strength of CAC by forming stratlingite.
TL;DR: In this paper, a calcium aluminate cement paste is exposed to a CO2 concentration of 10%. X-ray diffractometry, thermogravimetry and mercury intrusion porosimetry were conducted on the carbonation-cured and on reference samples at 28 days.
Abstract: The hydration of monocalcium aluminate in calcium aluminate cement forms metastable products such as CAH10 and C2AH8, which undergo phase transformations into stable products, inducing significant volumetric instability. The present study adopted carbonation curing, in which a calcium aluminate cement paste is exposed to a CO2 concentration of 10%. X-ray diffractometry, thermogravimetry and mercury intrusion porosimetry were conducted on the carbonation-cured and on reference samples at 28 days. The results showed that the metastable phases in the carbonation-cured sample were converted into stable phases and calcium carbonate polymorphs without inducing any significant change in the pore structure. This study may provide new insight into innovative means of avoiding the unstable conversion of hydrates in calcium aluminate cement by utilizing CO2 collected from industrial sources.
TL;DR: In this article, a database of commonly-encountered cement substances including C-S-H, Ca(OH)2, selected AFm, AFt and hydrogarnet compositions as well as solid solutions is presented.
Abstract: 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 stratlingite. 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.
TL;DR: In this paper, the authors discuss the use of calcium aluminate cements in hydraulic structures, including resistance to acid attack and particularly to biogenic corrosion and abrasion resistance.
Abstract: Calcium aluminate cements have a radically different chemistry to Portland cements. Due principally to their higher cost, they do not compete directly with Portland cements. Nevertheless, concretes based on these cements have very high performance in specific applications. Two of these are discussed in this article: resistance to acid attack and particularly biogenic corrosion and abrasion resistance in hydraulic structures. Such applications extend the range of applications for cementitious materials.
15 Jun 2002
01 Jan 2003
TL;DR: In terms of the length of time it has been produced, the volume produced and the breadth of applications, calcium aluminate cements are by far the most important class of non-Portland cements.
Abstract: In terms of the length of time it has been produced, the volume produced and the breadth of applications, calcium aluminate cements are by far the most important class of non-Portland cements. Calcium aluminate cements are a relatively large family with a range of compositions which varies much more widely than Portland cement. Today the two major markets for calcium aluminate cement are in castable refractories and in dry mix mortars for special construction applications, which together account for around 80% of consumption. The use in technical concretes, for example, sewer linings, rapid repair, etc. is rather small. Calcium aluminate cements are known for their rapid strength gain, especially at low temperatures, superior durability across several categories and high temperature resistance. Their ability to consume water rapidly during hydration makes them a preferred component in building chemistry applications as this contributes to construction expediency. CACs are highly versatile materials that can be used as the full binding material, or as is more common, a component of a blended system where the contribution is based on the final desired properties.
TL;DR: In this paper, a study was carried out comparing silica fume (SF) and dealuminated kaolin (DK) as pozzolanic materials in blended cements.
Abstract: A study was carried out comparing silica fume (SF) and dealuminated kaolin (DK) as pozzolanic materials in blended cements. Ten, 20 or 30 wt% of SF or DK were substituted for Portland cement. The kinetics of hydration up to 45 h were studied using isothermal conduction calorimetry. Blends containing pozzolanic materials usually have decreased heats of hydration compared to pure cement during the period of C3S hydration, i.e. during the main hydration peak. Depending on the chemical composition and the activity of the pozzolan, the reaction taking place with the lime typically contributes to the heat output after the main hydration peak. The pozzolanic activity of DK is the principal factor and heat evolution increases with respect to pure PC mortar, during the first 15 h. The presence of hydrated silica (silanol groups) in DK increases the pozzolanic activity especially before and during induction period. The acidic silanol sites are capable of a fast acid–base reaction with the alkalis and with any Ca(OH)2 present in cement during the induction period.