Sodium carbonate ( soda ash ) and sodium bicarbonate are 2 principal categories of accelerators that can be applied in sprayed concrete and paste cement. In this study, the application 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 outcomes 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 distinct. After one day, soda ash at 3% had the hardest strength while soda ash at 1% had the highest strength. After seven days, both of the two accelerators at 1% had the highest compressive strength. After 28 days, the compressive strength fell with the increase of the two. The improved strength at 1 and 7 days was caused by the accelerated formation of ettringite (Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O) 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.
quick hardening ability and high early strength are fundamental properties for shotcrete or sprayed concrete. Diverse accelerators were used in order to meet these requirements. The mostly used accelerators include alkali carbonates, alkali aluminate, alkali hydroxide, and alkali silicate. For instance, the sodium silicate was found to be capable of modifying the ITZ between the cement paste and aggregates and decrease the porosity of mortar. Potassium aluminate and sodium aluminate can accelerate the formation of ettringite in the cement paste, and thereby cause a rapid hardening effect. Sodium aluminate was reported to be capable of modifying the pore structure of cement paste at an early age, improve the resistance to chloride ingress, and increase early-age compressive strength. Carbonates and bicarbonates with alkali were also among the most-used accelerators, such as soda ash (Na2CO3) and (NaHCO3). The experts reported that the NaHCO3 can improve the strength and porosity of cement paste. They also found that the sodium carbonate could enhance the compressive strength and durability of cement paste. However, it has been reported that the both of Na2CO3 and NaHCO3 could decrease the compressive and tensile strength of concrete regardless of the content added or test age, and they also reported a serious reduction of strength after 28 days. However, to summarize, it is reported that either Na2CO3 or NaHCO3 could increase the early age strength at 3 and 7 days but decrease significantly the strength after 28 days, besides, they reported that sodium carbonate accelerated the setting time whereas the sodium bicarbonate retarded the setting time. In addition, the structure and shape of the interface transition zone between the slurry and the aggregate in the cement composite material is a complicated problem. It has been well accepted that the cement-based interface transition zone of the coarse aggregate is the weakest unit in the concrete, and the fly ash as mineral additive has a positive impact on the performance improvement of the interface transition zone.
It can be seen that there exist conflicted findings on the influence of Na2CO3 and NaHCO3 on the setting time and physical properties of cementitious materials. It is necessary to carry out a comprehensive study on the effects of the two accelerators on the properties of cement paste and compare the two of them. In order to investigate and compare the effects of the two accelerators on the properties of OPC paste, the same amount of Na2CO3 and NaHCO3 with 0%, 1%, 2%, 3%, and 4% weight of OPC were added into different mixes and the setting time and compressive strength at ages of 1, 7, and 28 days were studied.
- Materials and Methods
P.O. 42.5 ordinary Portland cement (OPC) in accordance with a Chinese standard GB175-2007 was used. The Na2CO3 and NaHCO3 used were in powder form and the purity was >99.5% and >99.8%, respectively. A superplasticizer used was polycarboxylate. The mixing water was deionized water.
3.1. Effect of soda ash and sodium bicarbonate on the Compressive Strength of OPC Paste
For the pastes with sodium carbonate, the compressive strength at ages of 1 and 7 days firstly increased and then decreased with the increase of Na2CO3 content, and the paste with 1% Na2CO3 had the highest compressive strength. The compressive strength of paste with 1% Na2CO3 was 7.2% higher at age of 1 day and 7.7% higher at age of 7 days compared to that of OPC paste. Similarly to NaHCO3, the compressive strength of pastes with soda ash at age of 28 days decreased continuously with the increase of Na2CO3. The reason could be that the formation of caustic soda caused the decrease of compressive strength. It can be seen that the NaHCO3 had the similar beneficial effect as Na2CO3 on the early age strength when the addition was below 1%, but much worse effect than sodium carbonate on the strength development when the addition was above 1%.
At the age of 1 day, with the increasing content of NaHCO3, the compressive strength of OPC paste increased initially and then decreased. The highest compressive strength happened in the mix with 3% NaHCO3. The strength of the mix with 3% NaHCO3 at the age of 1 day was 14% higher than that of the paste with no NaHCO3. At the age of 7 days, with the increase of NaHCO3, the compressive strength of OPC paste increased initially and then decreased with the highest strength happened in the mix with 1% NaHCO3. The strength of the mix with 1% NaHCO3 at the age of 7 days was 6% higher than that of OPC. At 28 days, the compressive strength of cement paste deceased continuously with the increase of NaHCO3. It can be seen that below 1% NaHCO3 can increase the early age strength but higher content of could decrease the later age strength significantly. This can be caused by the formation of caustic soda, which is a strong alkali and could react with the silica sand in the paste specimen.
3.2. Effect of NaHCO3/Na2CO3 on the Setting Time of OPC Paste
The initial setting time of OPC paste with 1%, 2%, 3%, and 4% Na2CO3 decreased by 90.44%, 90.80%, 91.18%, and 91.91% respectively compared to that of pure OPC paste. The final setting time of OPC paste with 1%, 2%, 3%, and 4% sodium carbonate decreased by 39.95%, 53.69%, 54.96%, and 64.38% respectively compared to that of pure OPC paste. It can be seen that the influence of Na2CO3 on the initial setting time was more significant than the final setting time. Soda ash showed similar effect as NaHCO3 on the initial setting time but its influence on the final setting time was less than the NaHCO3.
The results show that both of the initial and final setting time of the OPC paste decreased with the increase of NaHCO3 or Na2CO3 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% respectively compared to that of pure OPC paste. The final setting time of OPC paste with 1%, 2%, 3%, and 4% sodium bicarbonate decreased by 43.51%, 68.70%, 85.50%, and 86.01% respectively compared to that of pure OPC paste. It can be seen that the addition of 1–2% NaHCO3 significantly deceased the initial and final setting time of OPC paste. Further increase of NaHCO3 beyond 1% up to 4% showed little influence on the initial setting time, and further increase of NaHCO3 beyond 2% up to 4% showed little influence on the final setting time.
4.1. Effect of soda ash and sodium bicarbonate on the PH of OPC Paste
It is clear that the both soda ash and sodium bicarbonate are soluble and their main difference is that the Na2CO3 dissolves into Na+ and CO32− and the NaHCO3 dissolves into Na+ and HCO3− in water. Solutions of Na2CO3 or NaHCO3 have a PH > 7, and the PH of Na2CO3 solution is higher than that of bicarbonate solution when the same content of the two are added. For example, under the same concentration 1 mmol/L (25 °C and 1 atm), the pH values of sodium carbonate and sodium bicarbonate solutions are 10.52 and 8.27 respectively. When they are added in cement paste, both of them can react with the portlandite, which is a hydration product of cement, and form CaCO3. Cement slurry was prepared for pH measurements with a water-cement ratio of 0.5, a water reducing agent of 0.5%, and soda ash and NaHCO3 of 0%, 1%, 2%, 3%, and 4%. The pH meter was initially calibrated with a neutral solution (pH = 7) and then with an alkaline solution with a known pH. After the calibration is completed, the electrode of the pH meter was immersed into the cement slurry and the slurry was gently vibrated to reach a uniform state during the measurements. It can be seen the pH of the OPC paste increased with the increase of Na2CO3 but it decreased with the increase of NaHCO3. This was caused by the different pH of the solutions with the same amount of Na2CO3 and NaHCO3. There could be a risk of alkali silica reaction in the concrete with a high amount of sodium carbonate because of the increased pH. There could be a decay of the C-S-H gel in the concrete with a high amount of NaHCO3 because of the decreased pH.
The influence of soda ash and sodium bicarbonate as additional additives on compressive strength and the setting time of OPC paste was investigated and the related effect on the hydration mechanism was studied through TG-DTA, XRD, and SEM tests. The following conclusions can be drawn.
I- The addition of either Na2CO3 or NaHCO3 could increase the early age compressive strength (1 and 7 days) depending on the content added but they could decrease the compressive strength at later ages, such as 28 days, with the increase of content added.
II- The initial and final setting time of OPC paste decreased with the increase of either NaHCO3 or Na2CO3.
III- As an accelerator, the optimum content of soda ash and sodium bicarbonate were found to be in the same level as 1% of the weight of OPC. The addition 1% of either of the two accelerators could significantly shorten the setting time, increase the early age strength and did not have an obvious detrimental effect on the later age strength.
IV- Further increase of sodium carbonate and sodium bicarbonate above 1% could decrease the compressive strength of OPC paste although the ettringite formation was accelerated and increased. This decay was mainly caused by the Na+ ions introduced and the Na+ could partly replace the Ca2+ in the C-S-H gel and cause the discontinuity of the C-S-H gel.