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                                                                     EXHIBIT 5.1



   
                                  July 6, 1998
    


Lafarge Corporation
11130 Sunrise Valley Drive
Suite 300
Reston, Virginia  20191

Ladies and Gentlemen:

         We have acted as counsel for Lafarge Corporation, a Maryland
corporation (the "Company"), in connection with the preparation of the Company's
Registration Statement on Form S-3 (No. 333-57333), as amended (the
"Registration Statement"), filed with the Securities and Exchange Commission
(the "Commission") under the Securities Act of 1933, as amended (the "Securities
Act"), relating to the proposed offering by the Company of $650,000,000
aggregate principal amount of the Company's __% Senior Notes due ____ (the
"Senior Notes"). The Senior Notes are to be issued under an indenture, as
supplemented (the "Indenture"), between the Company and Citibank, N.A., as
Trustee, in the form previously filed with the Commission. The Senior Notes are
proposed to be sold by the Company to Donaldson, Lufkin & Jenrette Securities
Corporation, SBC Warburg Dillion Read Inc. and Citicorp Securities, Inc. (the
"Underwriters") pursuant to and subject to the terms and conditions of an
Underwriting Agreement among the Company and the Underwriters (the "Underwriting
Agreement"), the form of which is filed as Exhibit 1.1 to the Registration
Statement.

         In connection with the foregoing, we have examined the originals or
copies, certified or otherwise authenticated to our satisfaction, of the
Registration Statement, the form of the Underwriting Agreement, the Indenture,
the form of Supplemental Indenture included as Exhibit 4.2 to the Registration
Statement and pursuant to which the Senior Notes will be issued (the
"Supplemental Indenture"), and such corporate records of the Company,
certificates of public officials and of officers of the Company, and other
agreements, instruments and documents as we have deemed necessary to require as
a basis for the opinions hereinafter expressed. Where facts material to the
opinions hereinafter expressed were not independently established by us, we have
relied upon the statements of officers of the Company, where we deemed such
reliance appropriate under the circumstances.

         Based upon the foregoing and in reliance thereon, and subject to the
assumptions and qualifications herein specified, it is our opinion that:

         1. The Company has been duly incorporated and is validly existing and
in good standing under the laws of the State of Maryland.

         2. The Senior Notes have been duly authorized for issuance by the
Company and, upon the effectiveness of the Registration Statement under the
Securities Act, the due execution and delivery of the Supplemental Indenture and
the Underwriting Agreement by the respective parties thereto in substantially
the forms filed as exhibits to the Registration Statement, the issuance,
execution and authentication of the Senior Notes in accordance with the
provisions of the Indenture, and the delivery to and payment for the Senior
Notes by the Underwriters in accordance with the Underwriting Agreement, and
subject to any applicable state securities or Blue Sky laws, will be valid and
binding obligations of the Company and will be entitled to the benefits of the
Indenture.



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Lafarge Corporation
   
July 6, 1998
    
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         3. The Indenture has been duly authorized by the Company and, when
qualified under the Trust Indenture Act of 1939, as amended, and executed and
delivered by the Company, will be the valid and binding agreement of the
Company.

         The opinions expressed above are limited by and subject to the
following qualifications:

         (a) We express no opinion other than as to the federal securities laws
of the United States of America, the laws of the State of New York and the
corporate laws of the State of Maryland; provided, however, (i) we have assumed,
without investigation, that the laws of the State of New York are identical in
all respects to the laws of the State of Texas and (ii) insofar as the opinions
expressed herein relate to matters governed by Maryland law, we have relied with
their permission upon an opinion of even date herewith of Piper & Marbury,
L.L.P.

         (b) In rendering the opinions expressed herein, we have assumed that no
action heretofore taken by the Board of Directors of the Company in connection
with the matters described or referred to herein will be modified, rescinded or
withdrawn after the date hereof.

         (c) The opinions expressed in Paragraphs 2 and 3 above are subject to
the qualification that the validity and binding effect of the Senior Notes and
the Indenture may be limited or affected by (i) bankruptcy, insolvency,
reorganization, fraudulent transfer or conveyance, receivership, moratorium and
other similar laws relating to or affecting creditors' rights generally, (ii)
general principles of equity, regardless of whether applied in a proceeding in
equity or at law and (iii) an implied covenant of good faith and fair dealing.

         We hereby consent to the filing of this opinion with the Commission as
an exhibit to the Registration Statement and to the reference to us under the
caption "Experts" in the prospectus forming a part of the Registration
Statement. In giving this consent, we do not thereby admit that we come within
the category of persons whose consent is required under Section 7 of the
Securities Act or the rules and regulations of the Commission thereunder.

                                   Respectfully submitted,

                                   THOMPSON & KNIGHT,
                                   A Professional Corporation


                                   By: /s/ Peter A. Lodwick
                                      -----------------------------------
                                      Peter A. Lodwick, Attorney