IntroductionMagnesium sulfide oxide cement (MOSC) is an air-hardening cementing material, which is produced by the reaction of light calcined MgO with a certain concentration of MgSO₄ aqueous solution. Light calcined MgO raw materials have large differences in the aspect of MgO activity due to different calcination processes, and the MgO activity has a large impact on the performance of MOSC. For the preparation of MOSC, the increased MgO activity usually accelerates the hydration reaction speed of cement paste, leading to the increased exothermic amplitude of the system and the accelerated growth of Mg(OH)₂ crystals in MOSC. As such, the process of condensation and hardening of MOSC is accelerated, which is unfavorable to the growth of the 5·1·7 phase, leading to an inferior compressive strength of MOSC. Therefore, efficiently inhibiting the hydration process of high-activity MgO plays a crucial role in the preparation of high-performance MOSC. In this work, a series of organic acids and high-activity MgO were used to prepare MOSC. The effects of organic acids on the hydration process of high-activity MgO, the setting time, the compressive strength, the phase composition, the micro-morphology, the pore structure, and the exothermic temperature of hydration of MOSC were investigated in detail.MethodsThe high-activity MgO powder from Hebei Magnesium Sheng Chemical Technology Co., Ltd. was used, and the content of active MgO was 85% as determined by the hydration method, with an average particle size of 20.82 μm and a specific surface area of 47.45 m²/g. The MgSO₄·7H₂O was of industrial grade with a purity of 99%, and the citric, tartaric, oxalic, lactic, salicylic, lauric, and stearic acids were all analytically pure, purchased from Tianjin Hengxing Ltd.Hydration experiments of high-activity MgO: The hydration reaction was carried out under the conditions involving the temperatures of 45, 60, 75, 90 ℃, the reaction times of 0.5, 1.0, 1.5 h and 2.0 h, and stirring rate of 300 r/min, with a fixed solid-liquid ratio (m_MgO: m_H2O) of 1:10. The additions of organic acids were 0.25%, 0.50%, 0.75%, 1.00% and 3.00% at the MgO mass fraction. The pH value of the slurry during the hydration of high-activity MgO was monitored by a pH meter. The white solid obtained by filtration at the end of the experiment was calcined at 500 ℃ for 2 h. Then, the hydration rate of high-activity MgO was calculated.Preparation of MOSC: The raw material ratio for preparing MOSC was fixed to be n(MgO):n(MgSO₄):n(H₂O)=8:1:16. Organic acid was added at 1%, 2%, 3% and 4% of the mass fraction of high-activity MgO. MgO powder and MgSO₄ aqueous solution were mixed together at a rate of 300 r/min at room temperature (25 ℃) and low temperature (ice water bath, 0–5 ℃), respectively. After the raw materials were mixed uniformly, the fluidity and setting time of MOSC paste were determined at room temperature (25 ℃) by referring to the national standards GB/T 8077—2012 "Test Methods for Homogeneity of Concrete Admixtures" and GB/T 1346—2011 "TestMethodsfor Standard Consistency of Cement in terms of Water Consumption, Setting Time, and Stability", respectively. The prepared MOSC slurry was placed in a curing box with constant temperature ((25 ± 2) ℃) and constant humidity ((60% ± 5%) relative humidity). The hydration temperature of the slurry was monitored by a RC-4HC-type temperature sensor for 12 h. The specimens (40 mm×40 mm×40 mm) cured in the curing box until 7 d and 28 d were tested for compressive strength. X-ray diffractometer, scanning electron microscope and mercury injection apparatus were employed to test the physical composition, microstructure and pore structure of the specimens cured for 28 d, respectively.Results and discussionAll the tested organic acids such as citric, tartaric, oxalic, lactic, salicylic, lauric, and stearic acids inhibited the hydration process of high-activity MgO, of which the hydration rate decreased with the increase of organic acid addition. Tartaric acid, citric acid and oxalic acid can effectively stabilize the hydration layer of high-activity MgO and inhibit its hydration process. The inhibition ability of organic acids on the hydration process of high-activity MgO can be summarized as: tartaric acid > citric acid > oxalic acid. The addition of tartaric acid, citric acid and oxalic acid as modifiers in the preparation of MOSC can effectively inhibit the hydration reaction of high-activity MgO in the slurry during the prehydration period. Thus, the generation of Mg(OH)₂ was suppressed, and the growth of the 5·1·7 phase was promoted, resulting in the extended setting time and improved compressive strength of MOSC. The low-temperature preparation of MOSC significantly prolonged its solidification time and improved the fluidity of the slurry. Among them, the modification effect of tartaric acid reached the optimal performance with adding 2% (mass fraction to MgO) at room temperature. The addition of tartaric acid could effectively prolong the time required for the early hydration reaction to reach the maximum exothermic temperature, reduce the maximum exothermic temperature, stabilize the hydration layer of MgO and form an organic-magnesium complex to impede the generation of Mg(OH)₂,. As a result, the growth of the 5·1·7 phase was promoted, with reducing the most available pore diameter of the product. Finally, the setting time and the compressive strength of the MOSC were significantly improved.ConclusionsTartaric acid could effectively inhibit the hydration process of high-activity MgO. Tartaric acid showed a good effect on suppressing the hydration of MgO, for which the the hydration rate of MgO was reduced from 58.7% to 19.2%. The addition of tartaric acid effectively prolonged the initial and final setting time of the slurry, inhibited the generation of Mg(OH)₂, and promoted the formation and growth of the 5·1·7 phase in the preparation of MOSC with high-activity MgO. The compressive strength of the MOSC at 7 d and 28 d were 48.4 MPa and 54.8 MPa, respectively, which were increased by 111.4% and 83.9% compared with that of the MOSC prepared without tartaric acid.