Abstract
Carbonates and bicarbonates are two groups of accelerators which can be used in sprayed concrete. In this study, the effects of the two accelerators sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) (0%, 1%, 2%, 3%, and 4% by weight of ordinary Portland cement OPC) on the properties of OPC paste were compared. The results show that both of them could accelerate the initial and final setting time of OPC paste, but the effect of the two accelerators on the compressive strength were different. After 1 day, sodium bicarbonate at 3% had the highest strength while sodium carbonate at 1% had the highest strength. After 7 days, both of the two accelerators at 1% had the highest compressive strength. After 28 days, the compressive strength decreased with the increase of the two. The improved strength at 1 and 7 days was caused by the accelerated formation of ettringite and the formation of CaCO3 through the reactions between the two with portlandite. The decrease of strength was caused by the Na+ could reduce the adhesion between C-S-H gel by replacing the Ca2+. NaHCO3 was found be a better accelerator than Na2CO3.
Highlights
Rapid hardening ability and high early strength are essential properties for shotcrete or sprayed concrete
The results show that both of the initial and final setting time of the ordinary Portland cement (OPC) paste decreased with the increase of NaHCO3 or Na2 CO3 content
The initial setting time of OPC paste with 1%, 2%, 3%, and 4% NaHCO3 decreased by 86.76%, 94.12%, 96.69%, and 97.43%
Summary
Rapid hardening ability and high early strength are essential properties for shotcrete or sprayed concrete. The mostly used accelerators including alkali carbonates, alkali hydroxide, alkali silicate, and alkali aluminate. The sodium silicate was found to be able to modify the ITZ between the cement paste and aggregates and decrease the porosity of mortar [10]. Sodium aluminate and potassium aluminate can accelerate the formation of ettringite in the cement paste, and cause a rapid hardening effect [11]. Sodium aluminate was reported to be able to modify the pore structure of cement paste at an early age, improve the resistance to chloride ingress, and increase early-age compressive strength [12]
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