In the present contribution, the water-cement-density ratio law for the standard curing 28-day compressive strength of cement-based materials including grout, normal concrete, ceramsite concrete, and foamed concrete is proposed. The standard curing 28-day compressive strength of different grouts, normal concrete, ceramsite concrete, and foamed concrete was tested. Simulations for Abrams’ law, Bolomey’s formula, and water-cement-density ratio law were carried out and compared. The water-cement-density ratio law illustrates better simulations for the prediction of the 28-day compressive strength of cement-based materials. The water-cement-density ratio law includes both the water-cement ratio and relative apparent density of the cement-based material. Relative apparent density of the cement-based material is an important one of all the factors determining the compressive strength of the cement-based material. The water-cement-density ratio law will be beneficial for the precise and generalized prediction of the 28-day standard curing compressive strength of cement-based materials.
The expansion mechanism of magnesium oxide expansive hydraulic cement as a novel expansive hydraulic cement was reviewed. Anisotropic crystallization results in crystal growth pressure, causing volume expansion while also increasing the porosity of the whole system. The theoretical relationship between porosity and expansion was analyzed. A basic method is given for predicting the expansion rate considering the expansive agent content in MgO expansive hydraulic cement. A concise equation is proposed for calculating the ultimate expansion. A theoretical relationship between porosity and expansion is presented. The compressive strength and durability of magnesium oxide expansive hydraulic cement were analyzed considering porosity changes and compared with hydraulic cement. If the expansion rate exceeds 0.8%, the mechanical properties and durability changes caused by porosity should be considered. If magnesium oxide expansive concrete is used with restraining in real structure, extra compressive stress is generated and the porosity decreases, compared with that during free expansion. In particular, for strain-hardening cementitious composites, expansion confined with the fibers present in the composite is beneficial for refining cracks and improving the self-healing ability of these materials whenever exposed to humid environments. This paper describes the expansion mechanism and properties of magnesium oxide expansive hydraulic cement for engineering applications.
The use of ultra-fine dredged sand instead of natural sand in construction 3D printing materials can significantly reduce the cost. However, ultra-fine dredged sand has fine particles and high angular morphology, which can hinder the buildability and continuous printability of construction 3D printing materials. The addition of polycarboxylate superplasticizer (PCE) can effectively solve this problem. Considering that the change of PCE (content of 0, 0.1%, 0.2%, 0.3%) content has a great influence on the printing performance of mortar, in order to make up for this deficiency, nano clay (content of 0,1%) is added to the mortar. The experimental results showed that the addition of nano clay can significantly reduce the negative effects of PCE on the yield stress and apparent viscosity of 3D printing materials (3DPM). When the content of PCE is 0.2%, the addition of 1% NAC could increase the static yield stress and viscosity growth rate of 3DPM by 111.8% and 115.3% respectively. In addition, unconfined compressive strength test, isothermal calorimetry, mercury invasion porosity method and thermogravimetric analysis were used to characterize the hardening properties of 3DPM. The results of heat of hydration showed that the addition of nano clay reduced the hydration exothermic peak of 3DPM, but increased the total heat release. The results of pore structure analysis showed that the addition of nano clay reduced the macropore (>1000 nm) of 3DPM from 19.31% to 18.82%. Thermogravimetric analysis showed that the addition of nano clay increased the hydration products. Therefore, the compressive strength of 3DPM was kept within an acceptable range. Finally, the laboratory's printing results indicated that the 3DPM can print up to 20 layers continuously.
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