Effect of Portland cement addition on initial dissolution of coal gangue based alkali-activated cement

Release of silicon and alumina from aluminosilicate occurs at early stages of alkaline reaction. Alkalinity and different aluminosilicate could influence dissolution of particles and modify microstructure of alkali-activated systems. However, there is a lack of study regarding the effects of Portland cement (OPC) addition on alkali-activated systems. This paper investigates alkaline dissolution of aluminosilicate by leaching experiments, varying Portland cement content. Chemical stability of AA-hybrid pastes was determined by water immersion and compressive strength at 28 days. Hybrid alkali-activated cement was produced with coal sludge (CS) and coal gangue (CG). Portland cement was added in 10 wt% and 20 wt% of precursor, and the water to binder ratio was of 0.75. Compressive strength increased in hybrid alkali-activated materials after chemical stability test, and leached solutions have a peak of pH and electrical conductivity at seven days. Chemical analysis of leached solutions detected contents of SiO2, SO3, K2O, Fe2O3 and CaO. Portland cement addition increased alkaline dissolution of Si and Al from aluminosilicate at initial stages of chemical reaction with a synchronized behaviour. Dissolution efficiency was according to the material reactivity, following CST>CG>CS

Microstructure and properties of hybrid coal gangue-based alkali-activated cement

Addition of Portland cement in low-calcium aluminosilicate-based alkali-activated materials can provide rapid hardening at room temperature, thus modifying microstructure and mechanical properties. In the present study, the effect of Portland cement addition on alkali-activated materials, cured at room temperature, was analysed by mechanical strength test, FT-IR and electrochemical impedance spectroscopy. The results showed samples hardening at room temperature after 24 hours and compressive strength of 26 MPa (7 days) for 5% of OPC addition. Hybrid alkali-activated cements presented higher sorptivity and lower electrical resistance than alkali-activated cement without OPC addition which can be related to more connected pores. The analysis of EIS spectrum highlights continued formation of microstructure over the ages of alkali-activated cement and can be related to mechanical properties and sorptivity

Effect of alkaline salts on calcium sulfoaluminate cement hydration

This work analyzes the effect of the presence of 5 wt.% of solid sodium salts (NaSO, NaCO, and NaSiO) 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 AH, 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. NaSO induces cracking in the ≥28-d pastes due to posthardening gypsum and ettringite formation from the excess SO present. Anhydrite dissolves more rapidly in the presence of NaCO, prompting carbonation. NaSiO raises compressive strength and exhibits strätlingite as one of its reaction products.This study was partially supported by the Brazilian National Council for Scientific and Technological Development (CNPq) under projects 208380/2017‐5 and 151890/2020‐0, and CAPES. Financial support was also furnished by the Spanish Ministry of the Economy and Competitiveness and FEDER under research project BIA2016‐76466‐R and funding from BES‐2017‐082022. PhD. student Pilar Padilla‐Encinas thanks the Autonomous University of Madrid for the opportunity to complete her thesis in its Applied Chemistry program. The cement supplied by Heidelberg Cement Hispania is gratefully acknowledge