Studies on the influence of different fly ashes and Portland cement on early hydration of calcium aluminate cement

Barbara Pacewska 0 Iwona Wilin ska 0 Mariola Nowacka 0 0 B. Pacewska (&) I. Wilinska M. Nowacka Faculty of Civil Engineering, Mechanics and Petrochemistry, Institute of Chemistry, Warsaw University of Technology , Lukasiewicza 17 St., 09-400 Plock, Poland The influence of three mineral additives, i.e. fly ashes from pulverized combustion and from fluidized combustion of hard coal as well as Portland cement, on early hydration (up to 28th day) of calcium aluminate cement was investigated. Cement pastes containing 0, 5 and 25 wt% of additives were studied by the use of calorimetry, thermal analysis and infrared spectroscopy methods. It was confirmed that hydration of calcium aluminate cement is closely dependent on the type of addition and its amount. The influence of additives of different properties on cement hydration was discussed basing on received results and other literature reports. - Calcium aluminate cements are hydraulic binders of special properties and applications. They are mainly applied in production of fire-resistant materials but this kind of cement is also useful in cases when concreting in temperatures below zero, higher resistance for sulphate aggression or fast increase of strength are required. Moreover calcium aluminate cement may be a component of ready for use mixtures for special applications in building works, for example, quick hardening or expansive binders. Calcium aluminate cements are expensive materials, especially in comparison with Portland cement. That is why, it is aimed at reduction of amount of calcium aluminate cement in composite or its partial substitution by other, cheaper materials. Despite high price of calcium aluminate cements, their application might be potentially greater if one succeed in eliminating danger of strength reduction being a consequence of conversion of aluminate. Calcium aluminate cements are especially sensitive to changes of conditions of hardening. It causes that their properties, such as rate of hardening, strength, etc., change inter alia in presence of additives. Research works concerning mixing of calcium aluminate cement with different fine-grained additives such as waste materials (e.g. silica fume [1, 2], fly ash [13], slag [4], spent aluminosilicate [5]) as well as other types of cements (e.g. Portland cement [6, 7]) have been undertaken. In this way one may obtain a new binder of improved and/or changed properties. It is the effect of action of active additives that influence physical and chemical processes and arising products of hydration. The main products of hydration of calcium aluminate cement at ambient temperature (2025 C) are hydrated hexagonal calcium aluminates type of CAH10 and C2AH8 as well as aluminium hydroxide AH3 (abbreviations used in cement chemistry and thereby in this work: C-CaO, A-Al2O3, H-H2O, S-SiO2, c-CO2). The hexagonal hydrates, CAH10 and C2AH8, are metastable products. They inevitably convert with time or at higher temperature and in presence of humidity into cubic product C3AH6 and AH3. Strength of hardened cement material decreases as the effect of this conversion [8]. Some additives of pozzolanic or hydraulic properties, replacing part of cement in the material, are proposed to reduce hydrate conversion and its negative effects. In cement paste, reactive silica is released from these materials and then it reacts with components of the system. Phase C2ASH8 arises in this process. Some other CAS H phases may also be formed, as it was observed by authors of work [9]. Arising of CASH type phases may be an alternative reaction way for C3AH6 forming during conversion. One of mechanism of additive action is proposed as follow [1]: silica released from the additive reacts initially with calcium aluminates to produce C2ASH8. In this way the formation of C2AH8 and subsequently its conversion to C3AH6 are avoided. However, in literature reports one may find that some researchers do not agree whether the total prevention of conversion in this way is possible. Mechanism of action of pozzolanic and hydraulic additives in aluminate cement system and their role in conversion process is not fully explained yet. Thus, such investigations should be continued. A lot of research works have been devoted to introduction of fly ash into hydrating aluminate cement system [13]. These works mainly relate to the influence of different fly ashes coming from conventional technologies of combustion on hydration of cement and properties of the composite. In literature there are no research works found devoted to introduction of fly ash from fluidized combustion of coal into hydrating aluminate cement system. This type of fly ash exhibits different properties in comparison with the ashes coming from conventional methods of combustion. It is an effect of lower temperature of combustion (on average about 850 C). Fluidized fly ash is characterized by porous grains and, in contrast to conventional fly ash, it usually does not include glass or spherical grains. In composition of fluidized fly ash one may find: mainly dehydrated aluminosilicates, quartz, calcium compounds in the form of CaO, Ca(OH)2 and CaCO3 as well as anhydrite or gypsum from flue-gas desulphurization, and also non-combusted coal [10]. In the presence of Ca2? ions, fluidized fly ash exhibits higher pozzolanic activity than conventional fly ash [11]. One may expect that, for the sake of its chemical composition, fluidized fly ash will react in hydrating aluminate cement system in similar way as Portland cement. Thereby, fluidized fly ash may be considered as potential additive limiting the aluminate conversion (like conventional fly ash) as well as substitute of Portland cement in quickly hardening or expansive binder. The aim of this work was investigation of early hydration (up to 28th day) of calcium aluminate cement in the presence of different additives. The action of fly ash from fluidized combustion of hard coal, as a new type of additive, in aluminate cement paste was studied in comparison with fly ash from pulverized combustion and Portland cement. The results presented in this work concern avoidance of conversion of aluminates and possibilities of usage of industrial waste in binders for special applications. Grey calcium aluminate cement Go rkal 40, produced by Go rka Cement Sp. z o.o (Poland), was used as main component of the samples. For this cement the content of components recalculated into oxides is (mass%): Al2O3 [ 40%, CaO [ 36%, SiO2 \ 4%, Fe2O3 approximately 14% and mineralogical composition is: CA as a basic phase and C4AF, C12A7 and C2AS as accompanying phases (according to product description given by producer [12]). Three types of inorganic additives were used: two kinds of fly ashes from different technology of combustion of hard coal and Portland cement type of CEM I 32.5 R. Fly ash from fluidized combustion (mean temperature of combustion about 850 C, calcium sorbent added for fluegas desulphuriza (...truncated)