Issue 41

L. Zhang et alii, Frattura ed Integrità Strutturale, 41 (2017) 356-368; DOI: 10.3221/IGF-ESIS.41.47 358 Functional mechanism of liquid accelerator As a kind of compound aluminate accelerator, this liquid accelerator has the mole ratio of potassium to aluminum below 1.2 and its active ingredient is mainly aluminate ion. Increasing the content of aluminate ion in the solution can give better play to accelerating effects. Meanwhile, reducing the mole ratio of potassium to aluminum can reduce the alkalinity of the accelerator and inhibit the alkali-aggregate reaction of shotcrete. The main component of this liquid accelerator is KAl(OH) 4 . Given Ca(OH) 2 , it reacts with gypsum in cement to produce calcium sulphoaluminate hydrates (ettringite) as well as potassium hydroxide, reduces the concentration of soluble gypsum for delayed coagulation in cement mortar. Now, C 3 A, a cement mineral component, is quickly dissolved into the solution and hydrated to hexagonal C 3 AH 6 plates, thus accelerating the setting of cement mortar. A large amount of heat of hydration produced by the preceding reactions will also promote the reaction process and strength development. In addition, in the initial hydration stage, those ingredients produced in the solution such as Ca(OH) 2 , SO 4 2- , and Al 2 O 3 combine to produce high-sulfur calcium sulphoaluminate hydrate (ettringite) that is not only conducive to the development of early strength but also reduces the concentration of Ca(OH) 2 , thus facilitating the hydration of C 3 S. The produced calcium silicate hydrate gel interlaps to form crystals with the grid structure, thus boosting condensation [7-16]. E XPERIMENTAL STUDIES ON MATERIAL PROPERTIES Experimental material he test cement adopts 425# ordinary Portland cement, with the main components listed in Tab. 1. Components Chemical formula Shorthand notation Mass fraction/% Tricalcium silicate 3CaO·SiO 2 C 3 S 17.83 Dicalcium silicate 2CaO·SiO 2 C 2 S 55.24 Tricalcium aluminate 3CaO·Al 2 O 3 C 3 A 10.12 Tetra-calcium aluminoferrite 4CaO·Al 2 O 3 ·Fe 2 O 3 C 4 AF 7.81 Calcium sulphate dihydrate CaSO 4 ·2H 2 O CSH 2 8.0 Table 1 : Main components of cement. Test method (1) Refer to the JC477-2005 Flash setting admixtures for shotcrete standard (hereinafter referred to as the Standard) in the building material industry. (2) Get 400g cement with the water-cement ratio of 0.30 to 0.50 (The additive amount of water needs to deduct the water content in the liquid accelerator) to stir evenly, then add the recommended dosage of liquid accelerator, rapid mixing 25 s and 30 s, immediate loading mode, several times of artificial vibration, pare off excess water mud, make the surface clean. From adding liquid accelerator operating time should not exceed 50 s. The standard consistency and setting time of cement paste is adopted to measure the initial and final setting time of neat cement paste. Different reagents are added with compound it and then the initial and final setting time is measured. (3) Get 900g cement and 1350g standard sand, with the water-cement ratio of 0.5, mix the mortar uniformly, add the liquid accelerator, and quickly stir the cement mortar for 40 to 50 seconds. Make a 40 mm x 40 mm x 160 mm trial model with cement mortar, maintain it in the standard curing room at the temperature of 20 ℃ ±2 ℃ and with humidity above 95%, measure its strength after one day or 28 days, and calculate the strength ratio. Test results and analysis (1) Measurement of the setting time of cement with different dosages According to the requirements of the Standard, keep water-cement ratio of 0.4 unchanged, the dosages of liquid accelerator is 1.5% to 4% of the mass of cement. Fig. 1 shows the initial and final setting time of neat cement paste with 1.5% to 4% dosages of this liquid accelerator. T