Valorization of secondary raw materials through alkali activation: study of binders based on sulfate-bearing clay and Pietra Serena sewage sludge

Alkali activation is the chemical process through which precursors sufficiently rich in reactive silica and alumina and with amorphous or vitreous structures interact with a strongly alkaline medium to give rise, under mild processing conditions, to materials with good binding and cementing properties. Nowadays, Alkali Activated Materials (AAMs) have emerged as alternative to traditional construction materials, such as mortars or Ordinary Portland Cement (OPC), with the aim to become primary components in the development of environmentally sustainable building products. This Ph.D. study has assessed the possibility to valorize through the alkali activation process two different Italian raw materials: a sulfate-bearing kaolinitic clay and Pietra Serena sewage sludge for the synthesis of AAMs. The former is currently unused resource and the latter simply discarded in landfills. Considered the need to have a long-term, secure supply of the materials for the cement production, different raw materials need to be evaluated. The results are promising, as both resources demonstrated to be reactive in the alkali activation process giving an aluminosilicate gel binder as main product. Beside the first more applicative objective, a second, fundamental objective of this thesis has underpinned the study of the effect of the sulfate deriving from L02-K clay in the alkali activation process. In the system Na2O–Al2O3–SiO2–H2O(–SO4), achieved by activating sulfate-bearing kaolinitic clay with sodium silicate, the amorphous N-A-S-H gel is the main reaction product in presence or in absence of sulfate ions. In all samples containing sulfate, no new crystalline phases have been found apart from thenardite, which forms as by-product after setting time. It is leached in water, but part of sulfate remains in the samples. These findings indicate that sulfate may be trapped in amorphous insoluble phases, likely the aluminosilicate gel itself or nanozeolites. The amount of unsoluble sulfate is higher in samples with the highest SiO2/Al2O3 ratio. The system Na2O–CaO–Al2O3–SiO2–H2O–SO4 has been achieved by blending sulfate-bearing kaolinitic clay and residual sludge of Pietra Serena cultivation. The use of heat-treated sludge, sPS_800, add reactive CaO to the investigated system. The behavior of sPS_800 is similar to pozzolanic cement, the interaction between the different precursors occurs and gives rise to a mixture of N,(C)-A-S-H / C-(A)-S-H gels, similar to those produced in so-called mixed alkali cements or hybrid cements. In this system, calcium sulfo-aluminate compounds such as U-phase or ettringite are formed depending on the alkalinity of the activation solution. Sulfate-bearing cancrinite has been found in all samples prepared at 8M and cured at 85°C. Formation of cancrinite and ettringite is considered desirable, as both structures can trap sulfate, in fact they can be used for nuclear waste encapsulation via cementation. The crystalline products of the reaction of the two kaolinitic clays with alkali have been evaluated. Zeolitation of Sl-K was carried out under molten condition in the absence or presence of sulfate by adding thenardite to the mixture. In the former case, it gave A and Y zeolite, as expected. On the other hand, when sulfate is contained in the reacting mixtures sulfate bearing cancrinites are the main reaction products. Therefore, it can be concluded that sulfate ion could drive the nucleation and precipitation of CAN-type zeolites, likely acting as a ‘structure director’ during crystallization. From these findings, one may speculate that sulfate may have the same effect also in alkali activation process and that the local structure of the N-A-S-H gel may resemble that of cancrinite. A possible continuation of this work could focus on the experimental verification of this hypothesis.