Effect of Alkaline Salts on Calcium Sulfoaluminate Cement Hydration

Abstract

This work analyzes the effect of the presence of 5 wt.% of solid sodium salts (Na2SO4, Na2CO3, and Na2SiO3) on calcium sulfoaluminate cement (CSA) hydration, addresses hydration kinetics; 2-, 28-, and 90-d mechanical strength, and reaction product microstructure (with X-ray diffraction (XRD), and Fourier transform infrared spectroscopy, (FTIR). The findings show that the anions affect primarily the reactions involved. Ettringite and AH3, are the majority hydration products, while monosulfates are absent in all of the samples. All three salts hasten CSA hydration and raise the amount of ettringite formed. Na2SO4 induces cracking in the ≥28-d pastes due to post-hardening gypsum and ettringite formation from the excess SO42– present. Anhydrite dissolves more rapidly in the presence of Na2CO3, prompting carbonation. Na2SiO3 raises compressive strength and exhibits strätlingite as one of its reaction products.

Thermal stability assessment of calcium monosulfoaluminate 12-hydrate by applying the in-situ X-ray diffraction method at 25–1250 °C

“…Towards the lower energies (below 1200 cm −1 ) are indicated absorptions of the SO www.nature.com/scientificreports/ (bending at 785 cm −1 and 617 cm −1 ) and AlO 6 vibration (at 588 cm −1 , 522 cm −1 ). The interpretation of the FTIR spectrum bands agrees well with the previously published data [29][30][31] . The 27 Al NMR analysis was used to examine the amorphous nature of the sample.…”

Effect of Na versus Ca Sulfate Salts on the Hydration of Calcium Sulfoaluminate Clinker

“…The behavior observed in the KCSA-20NS pastes closely resembles that of the hydrated pastes with high alkali contents (KCSA-4M and KCSA-8M) [36]. This suggests that the high sodium sulfate content releases sulfate ions that react to form AFt, as well as releasing Na + ions that may contribute to the increase in pH level.…”

“…The stability range of ettringite as a function of pH remains under debate. Based on studies conducted on different types of cements, several authors [15,21,36] indicate that ettringite is stable at pH levels between 9 and 13. It has been observed how, in synthetic systems and at high pH levels (>13), the morphology of the AFt changes from needle to spherical form.…”

The Neutralization of Tricalcium Aluminate Hexahydrate and its Spontaneous Transformation into Friedel's Salt, a Layered Double Hydroxide

“…In this respect, the solid-state and aquatic chemistry of tricalcium aluminate, Ca 3 Al 2 O 6 or C 3 A (where C = CaO, A = Al 2 O 3 according to the conventional notation) is of particular interest, since it is the most reactive clinker phase upon hydration, amounting to 4-11 % of Portland cement [8,[17][18][19][20][21][22]. C 3 A is also present in alternative formulations, such as calcium sulfoaluminate [23][24][25][26][27] cements, proposed to be a 'greener' alternative to Portland cement due to its lower CO 2 emission, lower sintering temperature, better chemical resistance, faster setting, etc. [20,[23][24][25][26][27][28][29].…”

“…C 3 A is also present in alternative formulations, such as calcium sulfoaluminate [23][24][25][26][27] cements, proposed to be a 'greener' alternative to Portland cement due to its lower CO 2 emission, lower sintering temperature, better chemical resistance, faster setting, etc. [20,[23][24][25][26][27][28][29]. However, a major drawback of calcium sulfoaluminate cements is their high production cost [20,24], primarily due to bauxite as the main alumina source.…”