Abstract: Abstract: Admixture and water-cement ratio are important factors affecting the development of properties of cement mortar or concrete. In order to study the influence of admixtures and water-cement ratio on the hydration products and the mechanical properties of cement-based materials, the study combined hydration reaction mechanism of pure cement and silicate admixture, derived the formulae of hydration products, theoretical maximum mixing amount and total porosity of composite cement-based materials, and investigated the effects of mixing amount of cement mortar with fly ash, steel slag and lithium slag on total porosity, mechanical properties and hydration products. This paper designed 3 gradients of water-cement ratio (0.50, 0.42 and 0.34), 3 kinds of admixtures (lithium slag, fly ash and steel slag) and 2 contents (20% and 60%); the ratio of cementitious material to sand was 1:2.5, and then, molding specimen accorded with the mix of mortar in the triple mold and the mechanical properties of mortar were tested when specimen was cured to 1, 3, 7, 28 and 90 d. The results showed that, after the same content of fly ash, steel slag and lithium slag incorporation, the contents of hydration products of composite cementitious materials, i.e. calcium hydroxide (CH) and calcium silicate hydrate (CSH), and the total porosity were smaller than those of pure cement; when the water-cement ratio decreased from 0.50 to 0.34, the total porosity of cement mortar decreased from 16.0% to 9.3%, and the contents of CH and CSH increased for the compound cement-based materials with mixing amount of 5%, but the increments were not big; the porosity of fly ash, steel slag and lithium slag cement-based composite materials reduced from 16.6%, 17.2% and 16.0% to 9.9%, 10.9% and 9.3%, respectively. When the admixture amount increased to 10%, the variation of porosity and hydration products of the 3 kinds of mortar was different. The content of hydration products (CH and CSH) of composite cementitious materials was still smaller than that of pure cement. Compared to pure cement, the contents of CH and CSH of lithium slag mortar reduced by about 81.1% and 4.2%, fly ash mortar reduced by about 20.9% and 9.5%, and steel slag mortar reduced by approximately 3.0% and 9.5%, respectively. The porosity of lithium slag, fly ash and steel slag mortar increased by 0.1%, 3.6% and 7.6%, respectively. Among the 3 kinds of admixtures, the theoretical maximum mixing amount of fly ash was the maximum, followed by lithium slag, and the steel slag's theoretical maximum mixing amount was the minimum. It was found that the compressive strength of lithium slag cement-based composite material was prior to fly ash and steel slag cement-based composite materials. In early stage (14 days ago), the compressive strength of fly ash mortar was prior to steel slag mortar; when water-cement ratio was relatively high, the compressive strength of steel slag mortar was close to fly ash. When the mixing amount was 20%, the compressive strength of lithium slag cement mortar was prior to pure cement mortar; if the mixing amount was 60%, the compressive strength of fly ash, steel slag and lithium slag mortar was worse than pure cement mortar. The study provides theoretical basis for material adding in mortar and concrete.