The influence of calcined mixture cooling method on hydration products composition of blended cement stone

Complex additives based on combinations of calcined clays, including kaolinite with limestone, due to the synergetic effect play a major role in the creation of blended cements. Usually carbonate rocks contain clay impurities with adverse effects on the properties of the resultant cements and concretes. At the same time calcium carbonate contained in marl clays during calcination allows getting high-quality pozzolanic material. The effective complex additive based on the calcined mixture of clay and limestone was created. The aim of the study is to determine the effect of fast and slow cooling methods of the artificial mixture after calcination on the hydration products composition of blended cement stone with complex additives of calcined mixtures of clays and carbonates. Obtained results allow determining the conditions for obtaining effective complex additives based on calcined mixtures of ubiquitous clays and carbonate rocks for their application in blended Portland cement, and thus to expand the range of the latter. It is found that the preferred method is the fast cooling of the resulting mixture, which contributes to obtaining a complex additive with higher pozzolanic properties, compared with the additive obtained by the slow cooling method.


Introduction
Complex additives differing by the synergetic effect presence in the co-introduction of multiple mineral additives play a major role in the blended cements creation [1][2][3][4][5]. One of the more promising approaches is devoted to the creation of complex additives based on combinations of calcined clays, including kaolinite with limestone [1][2][3].
Usually carbonate rocks contain clay impurities with adverse effects on the properties of the resultant cements and concretes. At the same time calcium carbonate contained in marl clays during calcination allows getting high quality pozzolanic material.
The main products formed during low-temperature solid-phase synthesis are: onecalcium aluminate, which is formed at a temperature of 500 ºС [6] and dicalcium silicate, which is formed at 700-800 ºС [6,7]. At the same time, dicalcium silicate according to N.A. Toropov [8] can exist in five modifications; moreover, three of them are formed only as a result of sintering. For lowtemperature solid-phase synthesis, βС2S and γС2S are characteristic. The first one is active, but unstable and, upon slow cooling, is capable of transforming into the second, which is extremely undesirable, due to the fact that γС2S does not possess hydraulic properties under normal conditions.
The stability of βС2S is determined by both the presence of impurities and the size of the crystals. In the case of impurities, the stabilization of the βС2S transition to γС2S is facilitated by the presence of MgO, Al2O3, Cr2O3, K2O, Na2O, Fe2O3, P2O5, SO3, due to the fact that metal ions can replace calcium ions, and the [SiO4] 4group can replace [SO4] 2and [РO4] 3- [9][10][11]. At the same time, the stability of crystals increases with a decrease in their size [12].
As applied to the production of Portland cement clinker, the method of quenching is used to stabilize βС2S, while the glassy phase covers the βС2S grains, preventing their expansion.
The aim of the work was to determine the effect of fast and slow methods of cooling an artificial mixture after thermal activation on the composition of hydration products of a composite cement stone with complex additives of thermally activated mixtures of clays and carbonates.
The mineral compositions of materials are presented in Table 1. The differential thermal curves of clays are shown in Fig. 1 The specific surface is 500 m 2 /kg.

Methodology
The calcination of mixture was conducted in the laboratory chamber furnace SNOL 7.2/1100 with a vacuum fiber chamber. Calcined mixtures were introduced into the OPC in amount of 20 % of its mass according to the GOST 31108-2016 and the EN 197-1:2000.
The thermal analysis of blended cement stone was carried out using a combined method of thermogravimetry (TG) and differential scanning calorimetry (DSC) when using thermoanalyzer NETZSCH STA 449C under continuous heating (40 to 1000 º C) samples with a mass of about 35-40 mg at a rate of 10 ºC/min in a flow (50 ml/min) of air in alund crucibles. Temperatures of thermal effects are determined with an accuracy of ±1-3 ° C, warmth -±5 %, weight of ±0.01 mg.

Results and discussion
The complex additive was introduced in an amount 20% by Portland cement weight. The specific surface of mixtures was 500 m 2 /kg.
The following compositions of calcined mixtures of clays and limestones were selected for research: 1. Kaolinitic clay and limestone L1; 2. Polymineral clay and limestone L2. The choice of the compositions of mixtures is due to the results of previous studies, presented in printed works [13][14][15][16].

Hydration products composition of blended cements with the complex additive
The research of hydration products composition was conducted on cement stone samples at the 28 days age with following compositions: 1. The control sample of OPC; 2. The blended cement with 20 % of complex additive based on calcined mixture of kaolinitic clay and limestone L1; 3. The blended cement with 20 % of complex additive based on calcined mixture of polimineral clay and limestone L2; In Fig. 3 simultaneous TG-DTA of control sample OPC cement stone is submitted. The observed endothermic effect with the maximum at 70 °C can be bound with loss of free water from a cement stone [17]. The nearby endothermic effect at a temperature of 100-125 ºС is associated with the release of adsorption water from calcium hydrosilicates (CaO • SiO2 • 2H2O) [17]. The endothermic effect in the temperature range of 125-150 ºС is characteristic of dehydration of calcium hydrosilicates of the types 2CaO • 3SiO2 • 2H2O and 5CaO • 6SiO2 • 9H2O and ettringite [18][19][20]. The calcium hydroxide present in the sample decomposes with an endothermic effect at a temperature maximum of 460.65 ºС [18]. On the simultaneous TG-DTA data of the mixture sample obtained by rapid cooling (Fig.  4b), at temperatures of 480.0-668.72 ºС, a decrease in the mass of the sample is observed. At the same time, at temperatures of 668.72-708.04 ºС, the decomposition of low-basic calcium hydrosilicates the C-S-H (I) type formed in the hydration process occurs [18]. Decomposition of highly basic hydrosilicates and metastable products of their carbonization is characterized by an endothermic effect at 708.04 ºС, similar to the sample without additives. Limestone calcium carbonate, which has not entered into the thermal synthesis reaction, decomposes at a temperature of 733.4 ºС.
As can be seen from Fig. 4 for the sample with the additive of slow cooling, a clearly pronounced endothermic effect at 745.0 ºС indicates, first of all, the decomposition of thermally unreacted relict calcium carbonate of limestone, as well as the dehydration of highly basic hydrosilicates and metastable products of their carbonization. At the same time, the weight loss, amounting to 7.23 %, is greater than for sample with additive of a rapid cooling mixture. Based on this, it can be concluded that compounds of the CS and CSA types are more actively formed during the mixtures calcination of polymineral clays and carbonate rocks obtained by rapid cooling. Small exothermic effects at 850 ºС, typical for the vollastonite formation from calcium hydrosilicates [18], are observed for both methods of mixture cooling.
Simultaneous TG-DTA data of cement stone with the second type of additive are shown in Fig. 5, with a slow (a) and fast (b) cooling method, respectively.
For the mixture of slow cooling (Fig. 5a) the endothermic effect at 108.7 ºС characterizes the loss of adsorption water from hydrosilicates and ettringite. With rapid cooling, the endothermic effect of free water loss is observed at a temperature maximum at 73.02 ºС, similar to the control composition. Dehydration of low-basic hydrosilicates of the C-S-H (I) type is observed at temperatures of 100-125 ºС. Endothermic effects at temperatures of 125-200 ºС are associated with the dehydration of ettringite and calcium hydrocarboaluminates. 6 higher than for the rapid cooling sample, therefore, more of it is formed in this case. The latter indicates a high pozzolanic activity of the rapid cooling additive.
Weak endothermic effects (Fig. 5b) in the temperature range 475.02-634.98 ºС characterize the dehydration of low-basic calcium hydrosilicates of the types C-S-H (A) and C-S-H (B) [17].
The calcium carbonate present in the compositions decomposes with an endothermic effect at a temperature maximum of 682.87 ºС -in the sample with the rapid cooling additive, but in the sample of slow cooling at 728.5 ºС. In the first case the decomposition of relict calcium carbonate occurs. In the second case calcium carbonate formed during the samples hydration is similar to the control composition.
Weak exothermic and endothermic effects after 758 and 708 ºС characterize the transition of tobermorite like compounds to vollastonite, as well as the dehydration of hydrosilicates and hydroaluminates of the C2SH2 and C4A3H3 type [16].

Conclusions
Using the method of differential scanning calorimetry, it was found that the most effective method is the rapid cooling of artificial mixture of clays and limestones to obtain a complex additive for blended Portland cement.
It has been shown that the use of the rapid cooling method for complex additives makes it possible to obtain a material with higher pozzolanic properties than the slow cooling method, due to the fact that the rapid cooling method is more conducive to the calcium hydroxide binding by the active centers of the obtained complex additive, in comparison with slow cooling method.
A decrease or complete absence of the reaction peak of endothermic relict calcium carbonate decomposition of limestone in a mixture with rapid cooling is established, which suggests that it completely enters into the reaction of solid-phase synthesis in a mixture with rapid cooling.