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A practical method for Fourier analysis of a periodic waveform using only low-cost equipment is described. The method is suitable for any periodic voltage signal whose frequency can be continuously varied. By employing multistage tuning it can realize an accuracy that is limited only by that of the waveform of the test signal. The method is didactic and has the aesthetic appeal of the myriad variations of Lissajous figures. It is also suitable for mass duplication as a standard classroom experiment in the undergraduate laboratory. Exemplary results are shown for the square and triangular waves generated with simple circuit designs.

In order to study what the effect of superplasticizers on the setting time, fluidity and compressive strength of calcium sulfoaluminate cement (CSA) a naphthalene-based superplasticizer (BNS) and a polycarboxylic acid superplasticizer (PC) were selected to interact with CSA pastes and ye’elimite, respectively. X-ray diffraction (XRD), thermogravimetric (TG) analysis and scanning electron microscopy (SEM) analytical methods were used to investigate the class, amount and microstructure of the CSA pastes and ye’elimite pastes hydration products under the effect of the superplasticizers. The results showed that the addition of BNS can promote ettringite generation and thus improve the early compressive strength. As the addition of BNS increased from 0.8 wt% to 2.0 wt%, the initial setting time was prolonged 10 min, the final setting time was prolonged 7 min, the 5 min fluidity was improved from no fluidity to 220 mm. However, as the addition of PC increased from 0.08 wt% to 0.20 wt%, the setting time of the PC just changed within 3 min; the 5 min fluidity increased from 110 mm to 195 mm and no 15 min fluidity at all was observed. AS seen by SEM, it can be stated that generated ettringite under the addition of PC was layered and lacking bonding, and its morphology changed from rod-like to flake-like, leading to a decrease in early compressive strength.

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The self-healing capability of cracks plays an important role in improving the durability of concrete. This study investigated the the self-healing capability of mortar with crystalline admixture and fly ash under three conditions. The compressive strength, water absorption, and surface crack pattern were used to evaluate self-healing capability of mortar, and the composition of the healing products was analyzed by x-ray diffraction and scanning electron microscopy. The results indicated that the crystalline admixture could improve the self-healing capability of cementitious materials but caused a negative effect if adding too great an amount of fly ash (≥20%). The specimen that was immersed in water showed favorable self-healing capability of cracks. The healing products in cracks are mainly composed of CaCO3.