Effects of combined accelerating admixtures on mechanical strength and microstructure of cement mortar.

• Two new double-component early strength agents were developed by combined calcium formate and methacrylic acid with cement paste powder respectively. • The mechanical strength of cement mortar was significantly improved by adding double-component early strength agents. • The hydration products increase and porosity decreases of mortar with the addition of double-component early strength agents. • The influences of curing temperature and water-cement ratio on strength development were studied. This paper investigated the effects of calcium formate and methacrylic acid combined with cement paste powder on flowability, setting time, and mechanical strength of cement mortar. The effects of curing temperature and water-cement ratio on the hydration of cement mortar incorporating these two double-component early strength agents were also studied. Experimental results show that the 0.5, 1, 3 and 28-day compressive strength increased by 96.7% (89.3%), 81.7% (75.6%), 74.7% (67.4%) and 50.1% (49.9%) when 1.0% calcium formate (or 0.5% methacrylic acid) combined with 1.0% cement paste powder. These two types of double-component early strength agents presented an excellent synergistic effect. Higher curing temperature can accelerate the early hydration of cement but has a negative impact on the later strength. X-ray diffraction (XRD), Fourier transformation infrared spectroscope (FTIR), mercury intrusion porosimeter (MIP), and scanning electron microscope (SEM) results show that combined accelerating admixtures can accelerate the hydration of cement and decrease the porosity of cement mortar. Finally, the cost evaluation indicates that double-component early strength agents are applicable to manufacture high early-age strength cement-based materials. [ABSTRACT FROM AUTHOR]