Chemical reactivity of lightweight aggregate in cement paste

• All the types of LWA showed chemical activity, which absorb Ca2+ and OH−, and released Si4+ into cement solution.
• The TGA and XRD results showed that a pozzolanic reaction between cement paste and LWA can be expected.
• The chemical reactivity of LWA in cement paste is not significant, but it may also make some contribution to the dense ITZ.

Quantitative investigation on the chemical reactivity of lightweight aggregate (LWA) in cement paste is important to enrich our understanding of the role played by LWA in influencing the microstructure of the interfacial transition zone (ITZ). In this paper, three types of LWA (YT, NT and FT) were chosen, which are based on shale rock, expanded clay and sintered fly ash respectively, and the ion concentration, calcium hydroxide (CH) content and reaction degree of LWA in cement pastes at different curing ages were studied, the fly ash (FA) was also included for comparison. The results show that, all the types of LWA studied showed chemical activity, which absorb very significant amounts of Ca2+ and OH−, and released significant amounts of Si4+ into cement solution. The CH reduction content of the paste also indicates that the LWA have pozzolanic reactivity, especially in the later period the pozzolanic reaction become the main part of the hydration, but the reaction degree of them is slightly lower than that of FA. In addition, with the increase of the water–binder ratio, the reaction degree of FA and LWA has some increase as well. Although LWA in the form of a powder is more reactive than in the form of a coarse aggregate, the XRD results also show that the CH peak height of the ITZ samples around different types of LWA have a small decrease, in contrast to the bulk cement paste. The results obtained confirm that the dense ITZ around LWA was not only characterized by the water absorbing and mechanical interlocking, the chemical reaction at the interface also has some contribution.