In this work, the results of preparing a novel hydraulic binder material on a-C 2 SH basis using both chemically pure and natural raw materials are presented, as well as hydration and strength properties of such binders are described. a-C 2 SH was hydrothermally synthesized at 200°C for 8 h from reagents (CaO, SiO 2 ÁH 2 O) and for 4 h from natural rocks (decalcinated chalk marl, opoka). DSC and XRD analysis showed that remaining portlandite and part of a-C 2 SH decompose during 5 min of intensive tribochemical activation of the synthesis products and quartz mixture. Unfortunately, according to TG data, water released from these compounds remains in the sample. Applying different types of mills as well as the adding water adsorbents (CaO, CaCl 2 and BaO) has no effect on the hydraulic activity of the binder: The results of microcalorimetric analysis indicate that heat flow values of the main hydration reactions decreased in all cases. Most suitable way of removing water from the samples is lowtemperature thermal treatment at 450°C. Milled a-C 2 SHquartz mixture exhibited good binding properties after burning mixtures at this temperature. The mortar with the highest compressive and bending strength values (20.43 and 5.17 MPa after 28 days of hydration) was obtained using mixture which consisted of 25 % (by mass) synthesis product (obtained from hydrothermal synthesis of opoka and chalk marl), 25 % together with a-C 2 SH-milled quartz sand (during the tribochemical activation) and 50 % standard sand.

There are approximately 35 million t of carbonate opoka and 21 million t of chalk marl in Lithuania. These materials are suitable for the production of various hydraulic binders. In this work, the influence of burning temperature (700 °C -950 °C) and duration (0.5 h -2 h) on the calcination of isomorphic Ca-Si rocks was investigated. The goal was to prepare a mixture of opoka and chalk marl that would be reactive enough for the hydrothermal synthesis of various calcium silicate hydrates, as this would suggest another potential use of these stocks. It was determined that the largest amount of free CaO (59.09 %) is obtained after burning opoka for 2 h at 750 °C and after burning chalk marl for 2 h at 800 °C (84.22 %). The remaining CaO is bound to the structure of poorly crystalline pseudowollastonite, which transforms to wollastonite when the temperature of the thermal treatment is increased. Because the calcination temperature of both materials is similar, they can be burned simultaneously; this would significantly simplify the preparation procedure of these mixtures. The optimal temperature for burning the opoka -chalk marl mixture with a molar ratio of CaO/SiO 2 = 2 (62.8 % chalk marl and 37.2 % opoka) was determined to be 825 °C for 2 h. In this case, the amount of free CaO is the largest and the crystals of formed wollastonite are poorly crystalline. This blend was treated hydrothermally at 200 °C for 24 h. The dominant product of the synthesis was α-C 2 S hydrate, which in literature was reported to be the main constituent in the production of new hydraulic binder.

In this study, parameters of α-C2SH synthesis from limestone and quartz sand were determined and compressive strength values of the hydraulic cementitious binder, manufactured on its basis, were established. The results show that slow hydrothermal reactions in the calcium oxide–quartz–water system can be intensified using sodium oxide additive. In this case, α-C2SH was already the dominant compound in the product after 8 h of hydrothermal synthesis at 200°C in stirred suspensions. It was found that tribochemical activation must be combined with thermal treatment in order to obtain a hydraulically active cementitious binder from the mixture of synthesised α-C2SH and quartz sand. The heat flow value of such binding material during its main hydration reaction varied from 0·0019 W/g to 0·0028 W/g when the temperature of the thermal treatment was increased from 400°C to 450°C. Strength measurements showed that the addition of sieved quartz sand increased the compressive strength of the mortars, compared with a pure hydraulic cementitious binder, from 10·3 MPa to 13·5 MPa after 3 d, from 12·9 MPa to 16·7 MPa after 7 d and from 19·4 MPa to 22·5 MPa after 28 d of hydration.

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