The content of calcium oxide in ordinary portland cement is about 47%, and waste concrete may be composed of different types (labels) of concrete. In order to improve the quality of waste concrete, it is necessary to classify different types of concrete. CS Poon and Shuihe et al. [3] tested the properties of several kinds of waste concrete in Hong Kong, and some results are listed in table 1. The apparent density and water absorption of the three aggregates are quite different. The density of natural aggregate is the highest, followed by HPC of high-strength concrete, and the density of NC aggregate of ordinary concrete is the lowest. The pore distribution of three aggregates was analyzed by mercury intrusion method, and the results were consistent with the above properties. The porosity of three kinds of aggregates are: natural aggregate 1.6%, ordinary concrete NC recycled aggregate 16.8%, and high strength concrete HPC recycled aggregate 7.86%. From the pore distribution of two kinds of recycled aggregates, the pores of NC aggregate are mainly concentrated in the range of 0.0 1 ~ 1 micron; However, most of the pores of HPC aggregate are below 0. 1 micron.

The mixture of lime and pozzolan used by ancient Romans in construction projects is very similar to modern lime-pozzolan cement. After hardening, the concrete made of crushed stone cemented with it not only has high strength, but also can resist the erosion of fresh water or salt water. For a long time, as an important cementing material, it has been widely used in construction projects. 1756, British engineer J. Smeaton found that in order to obtain hydraulic lime, limestone containing clay must be used for firing; The ideal composition of masonry mortar for underwater structures is made of hydraulic lime and volcanic ash. This important discovery laid a theoretical foundation for the research and development of modern cement. 1796, Englishman J. Parker burned a kind of cement with marl, which was brown in appearance, very similar to the mixture of lime and volcanic ash in ancient Rome. Therefore, it was named Roman cement. It is also called natural cement because it is made of natural marl and contains no ingredients. Roman cement has good hydraulic and quick-setting characteristics, which is especially suitable for projects in contact with water except general building projects. Until 1850, Roman cement was widely used in the prosperous period of civil engineering, and then it was gradually replaced by silicate cement. 1824, on the basis of predecessors' work, British construction worker J. asp Ding first obtained the patent right of portland cement through continuous experiments and practice. He used limestone and clay as raw materials, mixed them in a certain proportion, calcined the mature materials in a vertical kiln similar to lime burning, and then made cement by grinding. Because the color of hardened cement is similar to the stone used for building in Portland, England, asp named it Portland cement. Because of its excellent building performance, Portland cement has gradually replaced other kinds of cementitious materials, such as hydraulic lime and Roman cement, and has been widely used. The invention of Portland cement is of epoch-making significance in the history of cement. Since then, the development of cement has entered a new historical period.