Rules for selecting the cement strength grade (32.5 / 42.5) when batching masonry and plastering mortars on site, and the cement grade basis of quota mix tables

Topic Positioning

How to select the cement strength grade (32.5 / 42.5) by mortar strength grade when batching masonry and plastering mortars on site, and the cement grade basis actually used by quota / consumption standard mix tables and the order of decision.

Core Conclusions

1. The code rules (split by mortar type)

1. Masonry mortar - split at M15 (JGJ/T 98-2010 clause 3.0.2): "The cement is preferably common Portland cement or masonry cement complying with GB 175 Common Portland Cements and GB/T 3183 Masonry Cements. The cement strength grade is selected per the mortar type and strength grade. For masonry mortars of M15 and below, 32.5 common Portland cement or masonry cement is preferred; for grades above M15, 42.5 common Portland cement is preferred." Note 1 to table 5.1.2-1 of JGJ/T 98-2010, same position: "for cement mortars of M15 and below, the cement strength grade is 32.5; above M15, 42.5." (That table gives trial-batch cement contents for site-mixed cement mortar under clause 5.1.2; with fly ash, table 5.1.2-2 applies, compiled at 32.5 cement with fly ash not exceeding 25% of total binder.)
2. Plastering mortar - split at M20 (JGJ/T 220-2010 clause 3.0.5): "For plastering mortars of strength grade not over M20, 32.5 common Portland cement or masonry cement is preferred; for grades over M20, common Portland cement of not less than 42.5 is preferred. Common Portland cement is preferably delivered in bulk."

2. The cement grade basis of quota mix tables (unlike the "preferably" rules) Most current quota / consumption standard mix tables are compiled uniformly at 42.5 cement, not one-to-one with the JGJ M15 / M20 "preferred" wording:

• Railway basic quota TB/T 10821-2024, chapter 8 cement mortar items (from HT-907) are marked outright "cement 42.5"; even M5 is compiled at 42.5.
• Some provincial consumption standards (e.g. Hunan 2024, 2025 edition in the source data) state in the mix annex: "the cement in the concrete and masonry mortar mixes is taken as 42.5 grade; where the design expressly requires another grade, the material quantities may be adjusted per the design or laboratory mix data."

3. Ready-mixed dry mortar and 32.5 / 42.5 Dry-mix masonry / plastering / flooring mortars are finished products supplied by strength grade Mxx and priced per m3 (or t); the quota material column lists "dry-mix masonry mortar M10.0", "dry-mix plastering mortar M15.0" outright - no site-batching 32.5-versus-42.5 conversion arises; the 32.5 / 42.5 selection applies only to site-batched mortar.

4. Order of decision First read the quota volume notes plus the cement grade marked against that strength grade in the mix table (mostly 42.5); where the design specifies another grade, adjust the material quantities along the path the volume notes allow (design or laboratory mix data) - the JGJ "preferably" wording cannot directly overturn the quota mix table.

Applicability Boundaries

• The M15 / M20 splits of JGJ/T 98-2010 and JGJ/T 220-2010 are "preferred selection" batching guidance, not a quota conversion basis; quota conversions follow the express volume notes.
• Ready-mix producers may add admixtures, fly ash and water-retaining thickening agents for workability, and may use 42.5 ordinary Portland or Portland cement provided cement is not wasted (commentary to JGJ/T 98-2010 clause 3.0.2).
• This card gives the prevailing sector / quota positions; a specific project still follows the quota volume notes in force and the design documents.

Further Reading

• JGJ/T 98-2010 Specification for Mix Proportion Design of Masonry Mortars, clause 3.0.2, clause 5.1.2 and tables 5.1.2-1, 5.1.2-2.
• JGJ/T 220-2010 Technical Specification for Plastering Mortar, clause 3.0.5.
• GB 175 Common Portland Cements; GB/T 3183 Masonry Cements.
• TB/T 10821-2024 Railway Works Basic Quota, chapter 8 mixes, section 5 cement mortar (HT-907 to HT-931).