The residual flocculant in manufactured sand produces adverse impact on properties of concrete. The effects of three types of flocculants, which are anionic polyacrylamide (APAM), nonionic polyacrylamide (NPAM) and polyaluminium chloride (PAC), and four dosages (0%, 0.015%, 0.030%, 0.050%, mass fraction) on the fluidity, setting time and mechanical properties of Portland cement were examined. X-ray diffraction (XRD) detection and scanning electron microscope (SEM) examination were conducted to explore the affecting mechanism of flocculants. In additions, the effects of water reducing agent, retarder and dispersant on the fluidity and mechanical strength reduced by flocculant were studied. The results show that APAM affects the fluidity of paste most significantly, and NPAM secondly, while PAC the least. Both APAM and NPAM slightly shorten the setting time, while PAC slightly prolongs the setting time of paste. Three flocculants all reduce the strength of mortar, and the higher the dosage is, the greater the decline is. Three flocculants all hardly change the hydration products of Portland cement, while APAM and PAC promote cement hydration, and NPAM inhibits the hydration. The combined use of water reducing agent and retarder significantly improves the fluidity and compressive strength of mortar mixed with flocculant.