【正文】 rticularly in the case of Portland cement with a high C3A content. Keywords: gypsum, heat of hydration, Portland cements, silica fume Cement studies range from the individual analysis of each of the ponent phases, to research into highly plex systems with all their variables. Joint studies of Portland clinker ponents and their interaction with the gypsum (CaSO4_2H2O) added to regulate setting time, for instance, have found that: ? The C3A and C4AF in ordinary Portland cement pete for gypsum, but as it is more reactive, C3A consumes higher quantities of the addition more , in conjunction with constraints on theamount of gypsum used to ensure that only the setting time is regulated, leads to the formation of less ettringite than might otherwise be expected . ? Gypsum also accelerates the hydration rate of calcium silicates, which likewise pete for sulphate ions during hydration, given the significant amount of sulphates included in the CSH gel. The inclusion of 2 to 6% of gypsum has been found to accelerate alite hydration, and from 2 to 4%, belite hydration . A high gypsum content contributes to the formation of large amounts of ettringite, however, which retards paste setting and hardening and prompts substantial changes in volume as a result of micro structural expansion and cracking。 their fineness, on the contrary, is parable. SF contains: over 90% SiO2, % SiO2 r – (reactive silica) [19, 20] and yet has a much lower density than quartz () and a very high specific surface. Its diffractogram, shown in Fig. 1, reveals the presence of cristobalite (C)。 the rate dropped to W kg–1 after 2 h, accounting for the first trough on the calorimetric curve. This was followed by the acceleration of hydration reactions with the initial precipitation of the CSH gel, primarily from C3S –, until the rate of heat release peaked for a second time at W kg–1 at reaction time 11:12. During acceleration, PC1 began to set (Table 2). Thereafter the rate of the hydration reactions declined through the 13th hour, overlapping in this stage with the aluminous phase transformation reactions, which take place when the SO3/Al2O3 molar ratio falls to under 3 (in this case, ). But the most prominent effect of the reaction was the enormous amount of heat released, which rose to W kg–1, forming the third peak on the calorimetric curve, 17 h and 24 min into the reaction. Finally, the hydration reactions slowed to and remained at a low rate. Fig. 3 Calorimetric curves for mixes with PC1 The test was considered pleted after 48 h, taking the reading at that time as the third trough on the calorimetric curve. Although when 10 and 20% SF was added to the PC1 the calorimetric phases or stages observed were essentially the same as for the plain PC1, certain differences were noted. During the first stage, for instance (from t=0 to the first trough), the effect of the SF particles was to dilute the Portland cement, with rates of and W kg–1, respectively. At the same time, however, the hydration reactions were enhanced in the fraction of the PC1 with which they were mixed, given that the first trough appeared earlier than in plain PC1 (1:36 and 1:42 h, respectively). This was followed by the acceleration of the hydration reactions – with the initial formation, in this case, of gels of chiefly C3S and SF origin and silanol groups of SF origin – with the second trough recorded at reaction times of 10:12 and 7:54, respectively. That is to say, both minima appeared earlier than in plain PC1, and the interval between the first and second troughs was narrower with both 10 and 20% replacement rates. This confirmed that the hydration reaction was stimulated by the SF, an effect that was further corroborated by the higher rate of heat release values ( and W kg–1) found for the mixes than for plain PC1. Subsequently, the low SO3/Al2O3 molar ratios in the two cement mixes ( and , respectively) prompted aluminous phase transformation. The oute was third peak reaction times of 14:24 and 11:34, respectively, for 10 and 20% replacement, both higher than for plain PC1. Moreover, the rate values at these third peaks were and W kg–1 – higher than for plain PC1 –, again providing dual confirmation (troughs appearing in shorter times and higher heat of hydration release values) of hydration stimulation as a result of replacing PC1 with SF. In short, the first troughs on the heat release curves for the samples containing PC1 appeared earlier and were attenuated for all mixes, and the second and third peaks appeared earlier for all mixes and their intensities were higher than in plain PC1. When gypsum was added to the PC1, the calorimetric curve was substantially attenuated and the third peak disappeared。teborg, Norway 1997, p. 4. 3 A. A. Amer, J. Thermal Anal., 54 (1998) 837. 4 Z. Giergiczny, J. Therm. Anal. Cal., 76 (2020) 747. 5 E. ElShimy, S. A. AboElEnein, H. ElDidamony andT. A. Osman, J. Therm. Anal. Cal., 60 (2020) 549.