Advances in understanding alkali-activated materials

Paulo J. M. Monteiro, Sabbie A. Miller and Arpad Horvath provide an overview of the challenges and accomplishments in reducing the environmental burden of concrete production.

Towards sustainable concrete.

The relation between the chemical composition of calcium–silicate–hydrate (C–S–H) phases and its influence on the compressive strength of cementitious materials is investigated by a new approach where prisms of C–S–H phases have been synthesized directly by the reaction of calcium hydroxide, amorphous SiO2 and water. The synthesized samples employ molar Ca/Si ratios of 0.83, 1.0, 1.25 and 1.50, and the compressive strengths and age-depending changes of the pastes have been followed for up to three months of hydration, after which the microstructures were characterized. The model pastes feature most characteristics of hydrated cements, including zones of different density but similar chemical composition. The experimental data demonstrates that the compressive strengths of the C–S–H pastes increase for decreasing Ca/Si ratio for all synthesized samples and testing ages. The molar volumes of the C–S–H phases decrease with decreasing Ca/Si ratio, which along with the related higher surface areas may partly explain the differences in strength development.

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Influence of the Ca/Si ratio on the compressive strength of cementitious calcium–silicate–hydrate binders

The past and future of sustainable concrete: A critical review and new strategies on cement-based materials

Recently, China roughly yields 60% of global production of cement. In this review, the driving force, environment impact and sustainable development of China’s cement industry are discussed basing database from society, economy and industry. At 2014 the production of China’s cement is 2.48 billion tons, equivalently 1.77 ton per capita and 137.4 gram per USD of GDP, the driving forces of China’s cement industry include urbanization, industrialization and the economic stimulation, the production of cement might come to a plateau after decades of rapid increasing. By closing down massive outdated capability, restricting the industries to denitration, desulfuration, recovering waste heat and slashing carbon emission, the pollutants from the cement industry reduced significantly during the last decades; the cement industry completed a great deal of various solid wastes and contributed to the cleaner production. Various approaches to increase the recycling degree are reviewed and an efficient way is put forward. The alternative cements must be based on high abundance raw materials, low carbon emission, low energy consumption, low pollutants and waste byproducts. Cleaner production, recycling and alternative cement are three effective approaches for the sustainable development of cement industry.

Cement industry of China: Driving force, environment impact and sustainable development

Author(s): Jackson, MD; Mulcahy, SR; Chen, H; Li, Y; Li, Q; Cappelletti, P; Wenk, HR | Abstract: Pozzolanic reaction of volcanic ash with hydrated lime is thought to dominate the cementing fabric and durability of 2000-year-old Roman harbor concrete. Pliny the Elder, however, in first century CE emphasized rock-like cementitious processes involving volcanic ash (pulvis) "that as soon as it comes into contact with the waves of the sea and is submerged becomes a single stone mass (fierem unum lapidem), impregnable to the waves and every day stronger" (Naturalis Historia 35.166). Pozzolanic crystallization of Al-tobermorite, a rare, hydrothermal, calcium-silicate-hydrate mineral with cation exchange capabilities, has been previously recognized in relict lime clasts of the concrete. Synchrotron-based X-ray microdiffraction maps of cementitious microstructures in Baianus Sinus and Portus Neronis submarine breakwaters and a Portus Cosanus subaerial pier now reveal that Al-tobermorite also occurs in the leached perimeters of feldspar fragments, zeolitized pumice vesicles, and in situ phillipsite fabrics in relict pores. Production of alkaline pore fluids through dissolution-precipitation, cation-exchange and/or carbonation reactions with Campi Flegrei ash components, similar to processes in altered trachytic and basaltic tuffs, created multiple pathways to post-pozzolanic phillipsite and Al-tobermorite crystallization at ambient seawater and surface temperatures. Long-term chemical resilience of the concrete evidently relied on water-rock interactions, as Pliny the Elder inferred. Raman spectroscopic analyses of Baianus Sinus Al-tobermorite in diverse microstructural environments indicate a cross-linked structure with Al3+ substitution for Si4+ in Q3 tetrahedral sites, and suggest coupled [Al3++Na+] substitution and potential for cation exchange. The mineral fabrics provide a geoarchaeological prototype for developing cementitious processes through low-temperature rock-fluid interactions, subsequent to an initial phase of reaction with lime that defines the activity of natural pozzolans. These processes have relevance to carbonation reactions in storage reservoirs for CO2 in pyroclastic rocks, production of alkali-activated mineral cements in maritime concretes, and regenerative cementitious resilience in waste encapsulations using natural volcanic pozzolans.

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Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete

The pyroclastic aggregate concrete of Trajan’s Markets (110 CE), now Museo Fori Imperiali in Rome, has absorbed energy from seismic ground shaking and long-term foundation settlement for nearly two millenia while remaining largely intact at the structural scale. The scientific basis of this exceptional service record is explored through computed tomography of fracture surfaces and synchroton X-ray microdiffraction analyses of a reproduction of the standardized hydrated lime–volcanic ash mortar that binds decimeter-sized tuff and brick aggregate in the conglomeratic concrete. The mortar reproduction gains fracture toughness over 180 d through progressive coalescence of calcium–aluminum-silicate–hydrate (C-A-S-H) cementing binder with Ca/(Si+Al) ≈ 0.8–0.9 and crystallization of stratlingite and siliceous hydrogarnet (katoite) at ≥90 d, after pozzolanic consumption of hydrated lime was complete. Platey stratlingite crystals toughen interfacial zones along scoria perimeters and impede macroscale propagation of crack segments. In the 1,900-y-old mortar, C-A-S-H has low Ca/(Si+Al) ≈ 0.45–0.75. Dense clusters of 2- to 30-µm stratlingite plates further reinforce interfacial zones, the weakest link of modern cement-based concrete, and the cementitious matrix. These crystals formed during long-term autogeneous reaction of dissolved calcite from lime and the alkali-rich scoriae groundmass, clay mineral (halloysite), and zeolite (phillipsite and chabazite) surface textures from the Pozzolane Rosse pyroclastic flow, erupted from the nearby Alban Hills volcano. The clast-supported conglomeratic fabric of the concrete presents further resistance to fracture propagation at the structural scale.

https://www.pnas.org/content/pnas/111/52/18484.full.pdf

Heng Chen

Papers

Phillipsite and Al-tobermorite mineral cements produced through low-temperature water-rock reactions in Roman marine concrete

Mechanical resilience and cementitious processes in Imperial Roman architectural mortar.