Properties and acceleration mechanism of cement mortar added with low alkaline liquid state setting accelerator

Abstract

Low alkaline liquid state setting accelerator(LSA) for Portland cement was prepared in laboratory from aqueous solution of several inorganic sulfate salts and some organic chemical substances. Properties of cement with addition of LSA relating to its setting time and strength development as well as its resistance to sulfate attack for short and long term exposure were experimentally examined. The experimental results showed that 5%–7% addition of LSA significantly accelerated the initial and final setting of Portland cement in the presence or absence of the blending of mineral admixtures, the initial and final setting time being less than 3 min and 6 min respectively. Meanwhile, the early 1 day curing age compressive strength increased remarkably by 20%, while the late 28th day curing age compressive strength remained almost unchanged as compared with that of the reference accelerator free cement mortar specimen. Furthermore, mortar specimens of cement added with LSA and exposed to 5% Na₂SO₄ solution showed their excellent resistance to sulfate attack, with their short and long term curing age resistance coefficient to sulfate attack being around 1.04 to 1.17, all larger than 1.0. XRD analysis on hardened cement paste specimens at very early curing ages of several minutes disclosed the existence of more ettringite in specimens added with LSA than that of the reference specimens, meanwhile SEM observation also revealed the existence of well crystallized ettringite at very early hydration stage, suggesting that the accelerated setting of Portland cement can be attributed to the early and rapid formation of ettringite over the whole cement paste matrix due to the introduction of LSA. MIP measurement revealed that hardened cement paste specimens with the addition of LSA presented less medium diameter pores, more proportion of small pores and less proportion of large capillary pores, which is in a very good coincidence with the improvement of strength development of cement mortars added with LSA.

Keywords

low alkaline / cement / setting accelerator / sulfate attack / ettringite

References

[1] Pan Z, Cheng Jiankun Present Status and Future Trend of the Research on the Cement Setting Accelerator[J]. Mining Technology, 2005, 26(2): 22-27.

[2] Zeng K, Liu Bo Present Status and Future Trend of the Research on the Chemical Admixture for Shotcrete[J]. China Coal, 1998, 24(5): 18-24.

[3] Xiang X, Ding Qingjun Review on the Research on Cement Setting Accelerator and Its Mechanism[J]. J. Wuhan Univ. Tech., 1999, 21(1): 28-30.

[4] Buerge T, Bodenmann Eurgen Alkali-free Accelerator for a Hydraulic Binder[P], 1983

[5] Allemann K, Deneke K, Hass H, . Liquid Setting Accelerator for Concrete Mixtures[P], 1984

[6] Helmboldt O, Potencsik I, Breker J, . Use of Basic Aluminum Sulfates as Low-alkali Set Accelerators for Cement[P], 1994

[7] Paglia C, Wormbacher F, Bohni H The Influence of Alkali-free and Alkaline Shotcrete Accelerators Within Cement Systems. I: Characterization of the Setting Behavior[J]. Cem. Concr. Res., 2001, 31(6): 913-918.

[8] Pan Z, Lv W, Chen Jiankun Properties of Alkali Free Liquid State Cement Setting Accelerator and Its Mechanism for Setting Acceleration[J]. Mine Construction Technology, 2006, 27(5): 25-29.

[9] Lv W, Li F, Pan Zhihua Modification of the Alkali Free Cement Setting Accelerator and Its Accelerating Mechanism[J]. J. Nanjing Univ. Tech., 2007, 19(2): 34-39.

[10] Pan Z, Li F, Lv Wen Properties of Low Alkaline Liquid State Cement Setting Accelerator and Its Accelerating Mechanism[J]. Chemical Materials for Construction, 2008, 24(2): 39-43.

[11] Li F, Pan Z, Fu X, . Properties of High Efficiency Low Alkaline Liquid State Cement Setting Accelerator and Its Accelerating Mechanism[J]. Mine Construction Technology, 2008, 29(6): 18-20.