Kamasani Chiranjeevi Reddy

Formulating alkali-activated binders requires a clear understanding of the role of activator concentration and source material composition on product formation and strength gain. Alkali activation of a low-calcium fly ash (FA) binder with slag replacements at 30% and 50% by mass was evaluated. The reactivity and compressive strength gain of the activated binders were evaluated using activating solutions with different sodium hydroxide molarities (SHMs). An increase in SHM produced a higher compressive strength of the activated binder. The primary reaction product formed in the activated binder was calcium silicate hydrate with aluminium substitution (C-(A)-S-H). An increase in SHM led to an increase in C-(A)-S-H content and a reduction in porosity. The slag contributed to early reactivity of the binder and the initial strength development, while the FA contributed to the later age strength by silica enrichment of the C-(A)-S-H. While increasing the SHM produced an increase in early reactivity, the sodium did not directly contribute to the formation of the reaction product. The sodium from the alkaline activator was not chemically bound to the C-(A)-S-H and could be removed by leaching in water. An increase in basicity with SHM enhanced the contribution of the low-calcium FA, leading to higher silica enrichment of the C-(A)-S-H.

The phase changes in alkali-activated slag samples when exposed to supercritical carbonation were evaluated. Ground granulated blast furnace slag was activated with five different activators. The NaOH, Na₂SiO₃, CaO, Na₂SO₄, and MgO were used as activators. C-S-H is identified as the main reaction product in all samples along with other minor reaction products. The X-ray diffractograms showed the complete decalcification of C-S-H and the formation of CaCO₃ polymorphs such as calcite, aragonite, and vaterite. The thermal decomposition of carbonated samples indicates a broader range of CO₂ decomposition. Formation of highly cross-linked aluminosilicate gel and a reduction in unreacted slag content upon carbonation is observed through ²⁹Si and ²⁷Al NMR spectroscopy. The observations indicate complete decalcification of C-S-H with formation of highly cross-linked aluminosilicates upon sCO₂ carbonation. A 20–30% CO₂ consumption per reacted slag under supercritical conditions is observed.

The assessment of the extent of carbonation and related phase changes is important for the evaluation of the durability aspects of concrete. The phase assemblage of Portland cements with different clinker compositions is evaluated using thermodynamic calculations. Four different compositions of cements, as specified by ASTM cements types I to IV, are considered in this study. Calcite, zeolites, and gypsum were identified as carbonation products. CO₂ content required for full carbonation had a direct relationship with the initial volume of phases. The CO₂ required for portlandite determined the initiation of carbonation of C-S-H. A continual decrease in the pH of pore solution and a decrease in Ca/Si is observed with the carbonation of C-S-H. Type II cement exhibited rapid carbonation at relatively less CO₂for full carbonation, while type III required more CO₂ to carbonate to the same level as other types of cement. The modeling of carbonation of different Portland cements provided insights into the quantity of CO₂ required to destabilize different hydrated products into respective carbonated phases.

The X-ray diffraction (XRD) signature of the glassy phase in blast furnace slag undergoing alkaline dissolution is evaluated. The intensity signature of the glassy phase present in slag can consistently be decomposed into three underlying pseudo-Voigt (PV) peaks. It is shown that the fundamental underlying characteristics of the XRD signature of the undissolved glassy phase of slag in terms of the underlying PV peaks do not change after dissolution in an alkaline solution. The stability of the calcium ions depends on the [OH⁻] concentration in the solution. An intensity-based procedure is developed for quantifying the unreacted glassy phase content in alkali-activated slag. The XRD profile information of the glassy phase in raw slag is suitable for fitting the intensity profile of the dissolved glassy phase. The mass percentage of the unreacted glassy phase of slag within alkali-activated slag is validated with selective acid dissolution. A procedure for determining the degree of reaction in alkali-activated slag is established. The procedure developed here could be used to determine the activity of slag in an alkaline environment.

An X-ray diffraction (XRD)-based evaluation of the crystalline and amorphous phases in slag hydrating in an alkaline environment is presented. A method is developed for the quantification of the amorphous phases present in hydrating slag in a sodium hydroxide solution. In hydrating slag, the amorphous reaction product is identified as calcium aluminosilicate hydrate. A water-soluble sodium-based amorphous reaction product is also produced. The XRD-based quantification method relies on the direct decomposition of the XRD intensity pattern of the total amorphous phase present in partially hydrated slag into the intensity patterns of the amorphous unreacted slag, the hydrate and the sodium-based product. The unreacted slag content in partially hydrated slag is also determined from the decomposition of the intensity signature of the total amorphous phase. An independent verification of the amorphous unreacted slag content in hydrating slag is obtained from measurements of blends of unhydrated and partially hydrated slag. The XRD-based phase-quantification procedure developed here provides a basis for evaluating the extent of reaction in hydrating slag.