Composition and microstructure of CaCO3·MgO-MgSO4-H2O cement paste

The composition and microstructure of the dolomite-derived CaCO 3 ·MgO-MgSO 4 -H 2 O cement paste are characterized by interlaced framework units of “porous CaCO 3 skeletons rooted by 517 phase crystals”, forming a homogeneous framework matrix containing a high proportion of capillary pores. The interlaced framework units provide the primary source of strength of the cement paste, enabling high performance and superior environmental sustainability. • CaCO 3 ·MgO derived from dolomite is a green clinker. • MgO nanospheres are uniformly dispersed in the porous CaCO 3 skeleton. • A porous CaCO 3 skeleton rooted by 5 Mg(OH) 2 ·MgSO 4 ·7H 2 O crystals forms the framework. • The cement shows higher strength and lower CO 2 intensity than MgO-MgSO 4 -H 2 O cement. MgO-based cement (MBC) is regarded as a low-CO 2 alternative to Portland cement (PC), yet its advantages are challenged by high emissions from MgCO 3 calcination and the limited distribution of magnesite. Herein, a green and globally available CaCO 3 ·MgO-based cement (CMBC) derived from abundant dolomite was proposed, using the CaCO 3 ·MgO (CM)-MgSO 4 -H 2 O (CMOS) system as a representative case. The composition and microstructure of CMOS cement paste were systematically investigated, with MgO-MgSO 4 -H 2 O (MOS) and MgO + limestone powder-MgSO 4 -H 2 O (MCOS) systems as references. CM particles consisted of MgO nanospheres uniformly dispersed on the nano- to micro-scale pore walls of a micron-scale CaCO 3 skeleton. This promoted the formation of framework units described as “porous CaCO 3 skeletons rooted by 517 phase crystals.” The intergrowth and aggregation of these units produced a homogeneous framework matrix dominated by capillary pores (10 100 nm) in MOS and MCOS pastes. Consequently, compressive strength of CMOS paste was governed by reinforced framework units rather than 517 crystals, leading to a 56.9% increase at 28 d compared with MOS paste. Moreover, CMOS cement qualified as a CaCO 3 -clinker-based green cement, and reduced carbon emissions by 22.9% and 50.9% per unit volume and strength.